United States Environmental Protection Agency
CBP/TRS 3/87
August 1987
Nutrient-Dissolved Oxygen
Dynamics in Chesapeake Bay:
The Roles of Phytoplankton and
Micro-Heterotrophs Under
Summer Conditions, 1985
Chesapeake
|^. Bay
w' Program
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Ref. No. [UMCEES]CBL 86-125a
(This cancels Ref. No.
[UWCEESlCBL 86-125 in
its entirety.)
NUTRIENT-DISSOLVED OXYGEN DYNAMICS IN CHESAPEAKE BAY:
The Roles of Phytoplankton and Micro-Heterotrophs
Under Summer Conditions, 1985
Final Report to
EPA Chesapeake Bay Program
by
Jon H. Tuttle, Thomas C. Malone,
Robert B. Jonas, Hugh W. Ducklow and David G. Cargo
The University of Maryland Center for Environmental & Estuarine Studies
Chesapeake Biological Laboratory
Solomons, Maryland 20688-0038
and
Horn Point Environmental Laboratories
Cambridge, Maryland 21613-0775
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DISCLAIMED
This report has been reviewed by the Chesaneake Bay Program. U.S.
Environmenta1 Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmenta1 Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or recommendation
for use.
ACKNOWLEDGEMENTS
Tlii.s study was funded by grants X 00331 1 OL0 and X-0033 10-C2-C fion
the U.S. Environmental Protection Agency and grant NA84AA-D-0001A from the
National Oceanographic and Atnospheric Administration through the University
of Maryland Sea Grant Collide.- The latter ^r.ir.t funded tin- major [.ore ion of
our 1985 field program which, forms the basis fur much of this report. One
of us (JHT) also wishes to acknowledge the support of grant OCE-8208032 from
the National Science Foundation which supported the sulfur cycling research
discussed briefly in this report.
We wish to thank J. Tyler Bell, Sue Hill, and Brian Wendler for
excellent technical support and the captains and mates of R/V Orion and R/V
Aquarius for much needed help with shipboard work. We are grateful to
Gail Canaday for her typing expertise and for aid in compiling this report
3nd to Margaret Weir for proof reading and reference compilation.
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ABSTRACT
This study focused on the relationships of phytoplank ton and micro-
heterotrophs to the development and maintenance of anoxia in the mesohaline
region of Chesapeake Bay. A series of 14 cruises were made from February to
October, 1985, on which water quality, nutrient concentrations, phyto-
plankton production, and microheterotroph production and metabolism were
assessed .
Phytoplankton production from February to May generates a quantity of
organic matter more than adequate to cause oxygen decline in mid-Bay deep
vaters. The development of the summer maximum in phytoplankton productivity
appears to depend upon the regeneration of nutrients from phytoplankton
carbon produced in the spring which sinks into deep water and is retninera-
li2ed by bacteria. These nutrients are recycled into the euphotic zone via
vertical mixing and oscillations of the pycnocline.
Bacterial biomass, production, and metabolism are tightly coupled to
phytoplankton production during the spring and summer. Phytodetritus is the
source of carbon fueling bacterial oxygen consumption in deep waters. A
relationship for predicting oxygen consumption rates from bacterial abun-
dance estimates has been developed. Water column oxygen consumption by
microheterotrophs is a major contributor to the development of anoxia during
the spring and early summer. Microbial sulfur cycling then becomes an
important mechanism for maintaining anoxia. The potential for the estab-
lishment of anoxia is greatest in the northern regions of the mesohaline
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Chesapeake Bay. However, phy top lankton and bacterial parameters exhibit
greater variation over shorter distances in an east to west direction.
Comparisons of biological measurements made in the summer of 1984, a
high flow year in which anoxia was widespread, with measurements made in
1985, a low flow year in which anoxia was limited and of short duration,
suggest that fundamental changes have occurred in the ecosystem of mid-
Chesapeake Bay such that a significant portion of primary production is
channeled into bacterial production. Thus, the severity of anoxic condi-
tions depends upon climatic differences regulating freshwater flow, pycno-
cline tilting, and wind mixing rather th.in upon changes in biological
parameters.
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CONTENTS
Page
Abstract ii
Figures vi
Tables xv
Abbreviations and Symbols xvi
Acknowledgment xvii
1. INTRODUCTION 1
GENERAL CONSIDERATIONS 1
THE 1984 STUDY 5
THE 1985 STUDY 8
2. CONCLUSIONS 11
3. RECOMMENDATIONS 14
FURTHER RESEARCH . . 14
MANAGEMENT 16
4. MATERIALS, METHODS AND EXPERIMENTAL PROCEDURES 17
CRUISE SCHEDULE AND STATION DESCRIPTIONS 17
SAMPLE COLLECTION 17
DETERMINATION OF PARAMETERS . 20
Phyaica1 Water Quality Parameters 20
Particulate Organic Carbon (POC). Nitrogen (PON) .
and Nutr ient Determinat ions 20
Phvtoplankton Pigments and Cell Dens ities 21
Photosynthet ic Produc t ion 21
Microb ia1 Metabo1 ism and Secondary Product ion -
Dual Labe 1 Technique 22
Microb ia1 Production-Single Labe1 Technique 23
Bacterial Biomass 24
Microbial Oxygen Consumption . 25
Biochemical Oxygen Demand (BOD-5) 25
5. RESULTS AND DISCUSSION 27
THE 1985 ANNUAL CYCLE-HYDROGRAPHY, NUTRIENTS, PHYTOPLANKTON . . 27
Hydrography 27
Dissolved Inorganic Nutrients 31
Dissolved Oxygen . 31
Phvtoplankton Biomass and Productivity 39
La tera 1 Variat ion 46
1984 AND 1985 INTERANNUAL CONTRASTS -
HYDROGRAPHY, NUTRIENTS, PHYTOPLANKTON 52
Dissolved Oxygen and Vertical Stratification ........ 57
Product ion and Fate of Phytoplankton Biomass ........ 60
Nutrients and Phvtoplankton Production 60
THE 1985 ANNUAL CYCLE - MICROBIOLOGY 61
Bacterial Abundance 61
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Temporal Changes along the Main Channel 61
Temporal Changes along Transect 2 63
Differences with Depth 74
North to South Differences 74
East to West Differences 74
Bacterial Size 79
Bacterial Production 83
Temporal Changes along the Main Channel 83
Differences with Depth 85
North to South Differences 92
East to West Differences 96
Amino Ac id Metabo 1 ism 96
Temporal Changes along the Main Channel 96
Differences with Depth 105
North to South Differences 105
East to West Differences 109
Glucose Metabo1 ism 109
Temporal Changes along the Main Channel Ill
Differences with Depth 118
North to South Differences 119
Ox^^gii Conjjump^um 119
Carbon Sources for Bacteria and Oxygen Consumpt ion 127
Chlorophyll 127
Phaeopigment9 133
Particulate Organic Nitrogen 133
Particulate Organic Carbon 136
Biochemical Oxygen Demand 141
1984 AND 1985 INTERANNUAL CONTRASTS - MICROBIOLOGY i 150
Microheterotrophs 151
The Ro le of Sulfur Cyc 1 ing 154
References 156
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FIGURES
Number Page
1 Site map of Chesapeake Bay illustrating transect (T1 , 9
T2, T3) locations { ) and additional deep channel
stations (0) occupied during the 1985 study.
2 Temporal variations in the vertical distribution of 28
temperature.
3 Temporal variations in the vertical distribution of 29
salinity.
4 Temporal variations in the vertical distribution of 30
s igma-t.
5 Temporal variations in the vertical distribution of 32
nitrate (uM).
6 Temporal variations in the vertical distribution of 33
ammonium (uM).
7 Temporal variations in the vertical distribution of 34
phosphate (uM).
8 Frequency distributions of the atomic ratio of 35
dissolved inorganic nitrogen (nitrate + nitrite +¦
ammonium) to dissolved inorganic phosphorus (ortho-
phosphate); open bars-surface, solid bars-near
bottom.
9 Temporal variations in the vertical distribution of 36
dissolved oxygen (mg/1); hatched areas indicate
locations of mid-depth oxygen minima.
10 Variations in the vertical distributions of salinity 38
and dissolved oxygen along the mainstem transect on
cruise 10. Station numbers are indicated by closed
triangles at top.
11
Temporal variations in the vertical distribution of
chlorophyll (ug/1).
40
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Frequency distributions of the ratio of particulate
organic carbon (ug/1) to chlorophyll (ug/1); open
bars-surface, solid bars-near bottom.
Frequency distributions of the ratio of particulate
organic carbon (ug/1) to particulate organic
nitrogen (ug/1); open bars-surface, solid bars-near
bot torn.
Lateral distributions of salinity and oxygen along the
CHOP-PAX transect (from west on the left to east on
the right). Solid triangles at top indicate
stations along the transect.
Lateral distributions of chlorophyll (ug/1) along the
CHOP-PAX transect (from west to east). Solid
triangles at top indicate stations along the
t ransec t .
Temporal variations in surface salinity, vertical
stability across the pycnocline, and the thickness
of the hypoxic layer having oxygen concentrations
less than 1 mg/1 during 1984 (solid line) and 1985
(broken line); circled point is from June 1984.
Variations in the vertical distribution of salinity
along the mainstem transect in June of 1984 and
1985. Numbers above solid triangles indicate CBI
stations (above) and stations from this study
(be low).
Variations in the vertical distribution of dissolved
oxygen along the mainstem transect in June of 1984
and 1985. Stations are the same as in Figure 16.
Temporal variations in the chlorophyll content of the
euphotic zone and chlorophyll specific productivity
of the euphotic zone during 1984 and 1985.
Mean oxygen concentration of the bottom layer in
relationship to vertical stability across the
pycnocline and to bottom water temperature (circle,
station 32; triangle, station 3; diamond,
station 24); regressions run for 12 Feb.-16 May.
Temporal variations in the mean concentration of
dissolved oxygen in bottom layer (solid line) and
vertical stability across the pycnocline (dashed
line).
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Numb
22
23
24
25
26
27
28
29
30
31
32
33
34
35
62
64
65
66
67
68
69
70
71
72
73
75
76
77
Time-dependent vertical variations in bacterial
abundance along a north-south (station 32 to 24)
main channel transect.
Lateral distributions of bacterial abundance along
transect 2 for the period 13 Feb. - 16 Apr. 1985
Lateral distributions of bacterial abundance
along transect 2 for the period 29 Apr. - 23 Hay
1985 .
Lateral distributions of bacterial abundance along
transect 2 for the period 28 May - 19 June 1985.
Lateral distributions of bacterial abundance along
transect 2 for the period 11 July - 15 Aug. 1985.
Lateral distributions of bacterial abundance aLong
transect 2 for the period 11 Sep. - 10 Oct. 1985.
Variations in salinity with depth along transect 2 for
the period 13 Feb. - 16 Apr. 1985.
Variations in salinity with depth along transect 2 for
the period 29 Apr. - 23 May 1985.
Variations in salinity with depth along transect 2 for
the period 28 May - 19 June 1985.
Variations in salinity with depth along transect 2 for
the period 11 July - 15 Aug. 1985.
Variations in salinity with depth along transect 2 for
the period 11 Sep. - 10 Oct. 1985.
Mean bacterial abundances within () and below (0) the
euphotic zone for transects 1-3 during 1985.
Bacterial abundances are expressed in millions of
bacteria 1 ml .
Mean bacterial abundances within the euphotic zone
along transects 1 (&), 2 (0), and 3 (~) during 1985.
Bacterial abundances are expressed in millions of
bacterial ml .
Mean bacterial abundances below the euphotic zone
along transects 1 (A), 2 (0), and 3 (~) during 1985.
Bacterial abundances are expressed in millions of
bacterial ml .
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Mean bacterial abundances along the west flank (A) ,
main channel (0), and east flank (~) of Chesapeake
Bay during 1985. Values are means of data from all
three lateral transects. Bacterial abundances are
expressed in millions of bacterial ml
The percent of total bacterial abundance within the
euphotic zone attributable to bacteria smaller than
1 um along transects 1 (A), 2 (0), and 3 O) during
1985.
The percent of total bacterial abundance below the
euphotic zone attributable to bacteria smaller than
1 um along transects 1 (A) , 2 (0), and 3 flj) during
1985.
The percent of total bacterial abundance attributable
to bacteria smaller than 1 um along the west flank
(A), main channel (0), and east flank (~) of
Chesapeake Bay during 1985. Percentages are calcu-
lated from means of data from all three lateral
transec ts .
Time-dependent vertical variations in bacterial
production along a north-south (station 32 to 24)
main channel transect. Values on contour lines
represent TdR in p mo 1 L h
Lateral distribution of bacterial production along
transect 2 for the period 13 Feb. - 21 Mar. 1985.
Lateral distribution of bacterial production along
transect 2 for the period 29 Apr. - 23 May 1985.
Lateral distribution of bacterial production along
transect 2 for the period 28 May - 19 June 1985.
Lateral distribution of bacterial production along
transect 2 for the period 11 July - 15 Aug. 1985.
Lateral distribution of bacterial production along
transect 2 for the period 11 Sep. - 10 Oct. 1985.
Mean bacterial production, expressed as TdR, within
() and below (0) the euphotic zone for transects 1-
3 during 1985.
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47
48
49
50
51
52
53
54
55
56
57
Mean bacterial production, expressed as TdR, along
transects 1 (A), 2 (0), and 3 (~) during 1 985.
Mean bacterial production, expressed as TdR, within
the euphotic zone along transects 1 (A), 2 (0), and
3 (D) during 1985.
Mean bacterial production, expressed as TdR, below the
euphotic zone along transects 1 (A), 2 (0), and 3
<~) dur ing 1985.
Mean bacterial production, expressed as TdR, along the
west flank (A), main channel (0), and east flank O)
of Chesapeake Bay during 1985. Values are means of
data from all three lateral transects.
Time-dependent vertical variations in amino acid
metabolism along a north-south (station 32 to 24)
main channel transect. Solid triangles at the top
indicate cruise dates. Values on contour lines
represent amino acid turnover rates expressed as %
of amino acid pool h .
Lateral distribution of amino acid metabolism,
expressed as amino acid turnover rates, along
transect 2 for the period 13 Feb. - 21 Mar. 1985.
Lateral distribution of amino acid metabolism, 101
expressed as amino acid turnover rates, along
transect 2 for the period 29 Apr. - 23 May 1985.
Lateral distribution of amino acid metabolism, 102
expressed as amino acid turnover rates, along
transect 2 for the period 28 May - 19 June 1985.
Lateral distribution of amino acid metabolism, 103
expressed as amino acid turnover rates, along
transect 2 for the-period 11 July to 15 Aug. 1985.
Lateral distribution of amino acid metabolism, 104
expressed as amino acid turnover rates, along
transect 2 for the period 11 Sep. - 10 Oct. 1985.
Mean amino acid metabolism, expressed as amino acid 106
turnover rates, within () and below (0) the
euphotic zone for transects 1-3 during 1985.
Pafte
93
94
95
97
98
ino
Mean amino acid metabolism, expressed as amino acid
turnover rates, within the euphotic zone along
transects 1 (A), 2 (0), and 3 O) during 1985.
107
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Number
Pafte
59 Mean amino acid metabolism, expressed as amino acid 108
turnover rates, below the euphotic zone along
transects 1 (A), 2 (0), and 3 (~) during 1 985 .
60 Mean amino acid metabolism, expressed as amino acid 110
turnover rates, along the west flank (A), main
channel (0), and east flank (Q) of Chesapeake Bay
during 1985. Values are means of data from all
three lateral transects.
61 Time-dependent vertical variations in glucose 112
metabolism along a north-south (station 32 to 24)
main channel transect. Solid triangles at the top
indicate cruise dates. Values on contour lines
represent glucose turnover rates expressed as % of
glucose pool h
62 Lateral distribution of glucose metabolism, expressed 113
as glucose turnover rates, along transect 2 for
16 April 1985.
63 Lateral distribution of glucose metabolism, expressed 114
as glucose turnover rates, along transect 2 for the
period 29 Apr. - 23 May 1985.
64 Lateral distribution of glucose metabolism, expressed 115
as glucose turnover rates, along transect 2 for the
period 28 May - 19 June 1985.
65 Lateral distribution of glucose metabolism, expressed 116
as glucose turnover rates, along transect 2 for the
period 11 Jul. - 19 Jul. 1986.
66 Lateral distribution of glucose metabolism, expressed 117
as glucose turnover rates, along transect 2 for the
period 11 Sep. - 10 Oct. 1985.
67 Mean glucose metabolism, expressed as glucose turnover 120
rates, within () and below (0) the euphotic zone
for transects 1-3 during 1985.
68 Mean glucose metabolism, expressed as glucose turnover 121
rates, within the euphotic zone along transects 1
(A), 2 (0), and 3 (~) during 1985.
69 Mean glucose metabolism, expressed as glucose turnover 122
rates, below the euphotic zone along transects 1
(A), 2 (0), and 3 (O) during 1985.
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Mean glucose metabolism, expressed as glucose turnover
rates, along the west flank. CA), main channel (0),
and east flank O) of Chesapeake Bay during 1985.
Values are means of data from all three lateral
transects.
Mean oxygen consumption within the euphotic zone (),
beneath the euphotic zone to the benthos (0), and
beneath the euphotic zone to 15m depth (A) during
1985. Values are means of data from all three
lateral transects.
Mean oxygen consumption beneath the euphotic zone
along transects 1 (A), 2 (0), and 3 (~) during 1985.
Linear regression of mean oxygen consumption on
bacterial abundance. Symbols: (A) euphotic zone
means for the time periods 20 Aug. - 11 Sep.,
14 Sep. - 3 Oct., and 5 Oct. - 2 Nov. 1984; ()
beneath euphotic zone means for 1984 data collected
in the time periods given above; (A) euphotic zone
means for the time periods 12 Feb. - 17 Apr.,
29 Apr. - 7 June, and 19 June - 12 Sep. 1985; (0)
beneath euphotic zone means for 1985 data collected
in the time periods given above. Means were
calculated using data from all transects.
Mean chlorophyll a. concentrations within () and below
(0) the euphotic zone during 1985. Values are means
of data from all three lateral transects.
Mean chlorophyll a. concentrations within (A) and below
(B) the euphotic zone across transects 1 (A), 2 (0),
and 3 O) during 1985.
Mean chlorophyll a. concentrations along the west flank
(A), main channel (0), and east flank O) of
Chesapeake Bay during 1985. Values are' means of
data from all three lateral transects.
Mean phaeopigment concentrations within () and below
(0) the euphotic zone during 1985. Values are means
of data from all three lateral transects.
Mean PON concentrations within () and below (0) the
euphotic zone during 1985. Values are means of data
from all three lateral transects.
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Mean PON concentrations along transects 1 (A), 2 (0),
and 3 CD during 1985. Means include data from
within and below the euphotic zone.
Mean PON concentrations along the west flank (A), main
channel (0), and east flank Q) of Chesapeake Bay
during 1985. Values are means of data from all
three lateral transects.
Mean POC concentrations within () and below (0) the
euphotic zone during 1985. Values are means of data
from all three lateral transects.
Mean POC concentrations along transects 1 (A), 2 (0),
and 3 Q) during 1985. Means include data from
within and below the euphotic zone.
Mean POC concentrations along the west flank (A), main
channel (0), and east flank G) of Chesapeake Bay
during 1985. Values are means of data from all
three lateral transects.
Mean total BOD-5 along transects 1 (A), 2 (0), and 3
"O during 1985. Means include data from within and
below the euphotic zone.
Mean total BOD-5 within the euphotic zone along
transects 1 (A), 2 (0), and 3 (~) during 1985.
Mean total BOD-5 below the euphotic zone along
transects 1 CA), 2 (0), and 3 (~) during 1 985.
Mean total BOD-5 along the west flank (A), main
channel (0), and east flank GD of Chesapeake Bay
during 1985. Means include data from within and
below the euphotic zone.
Mean total BOD-5 (closed symbols) and particulate BOD-
5 (open symbols) in the mesohaline portion of
Chesapeake Bay during 1985. Circles represent
values within the euphotic zone and triangles
represent values beneath the euphotic zone. Means
include data from all three lateral transects.
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Comparisons of mean particulate BOD-5 with mean
filterable BOD-5 (A) and % of total BOD-5 comprised
of particulate BOD-5 (B) in the mesohaline portion
of Chesapeake Bay during 1985. Values are means of
data from all three lateral transects. Symbols:
() particulate BOD-5 within the euphotic zone;
U) filterable BOD within the euphotic zone;
(0) particulate BOD-5 beneath the euphotic zone;
(A) filterable BOD-5 beneath the euphotic zone;
(¦) 2 particulate BOD-5 within the euphotic zone;
(~) 2 particulate BOD-5 beneath the euphotic zone.
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TABLES
Number Page
1 Station locations for 1985 Sea Grant/EPA Chesapeake 18
Bay study.
2 Cruise dates and descriptions for 1985 Sea Grant/EPA 19
Chesapeake Bay study.
3 Euphotic zone chlorophyll a. (mg m ) at stations along 41
the mainsten of the Bay from north (station 32) to
south (station 24).
? 1
4 Primary productivity (mg Cm d ) and chlorophyll 42
specific [mg C(mg Chi * d)~M at stations 1, 3, and
5 of the CHOP-PAX transect.
5 Exponential relationships between POC and Chi where 44
r = correlation coefficient and POC=a(e)'3
6 Coefficient of determination (r2) for power curve fits 46
of C/Chl on Chi where C/Chl = a(Chl)b.
_ 2
7 Euphotic zone chlorophyll a (mg m ) at stations along 51
CHOP-PAX transect from west (station 1) to east
(a tat ion 5).
8 Comparison of the means of key bacterial parameters 152
measured during summer conditions in 1984 (cruises
11-18, 8/20/84 to 9/11/84) and in 1985 (cruises 10-
13, 7/11/85 to 9/12/85).
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
AAt - Amino acid turnover race
AODC - Acridine orange direct count
BOD-5 - Biochemical oxygen demand
CBI - Chesapeake Bay Institute
Chi- Chlorophylla
Chop-Pax - Choptank-Patuxent transect (transect 2)
D.O. - Dissolved oxygen
F liOD - Filtered (dissolved) b ioc her: i c a 1 oxygen demand
Glut - Glucose turnover rate
N - Nitrogen
N/P - Nitrogen to phosphorus ratio
P- Phosphorus
PBOD - Particulate biochemical oxygen demand
Phaeop - Phaeopigments
POC - Particulate organic carbon
PON - Particulate organic nitrogen
PP - Primary production
TCA - Trichloroacetic acid
TdR - Thymidine incorporation rate
SYMBOLS
cci - centimeters
Km/d - Kilometers per day
L h ^ - Per liter per hour
-3-1
mgCm d - Milligrams of carbon per cubic meter per day
ml - Milliliters
pmo1 - Picomoles
r,r2 - Correlation coefficient
uCi - Microcuries
ug/1 - Micrograms per liter
um - Micrometers
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SECTION 1
INTRODUCTION
GENERAL CONSIDERATIONS
The Environmental Protection Agency's Chesapeake Bay Program (CBP) iden-
tified oxygen depletion as a major factor contributing to a general decline
in water quality and in the capacity of the Bay to support fishery resources.
Results of the CBP suggest that seasonal oxygen depletion during spring-
sucnner occurs over the mid-Chesapeake Bay as a consequence of vertical stra-
tification and high rates of heterotrophic metabolism in bottom water and the
bentho9. It is generally believed that increased nutrient loading from point
and nonpoint sources (D'EIia 1982; Taft 1982) has stimulated a set of
ecological interactions which have to an interannual increase in the area and
volume of water which is depleted in oxygen (Heinle et al. 1980; Taft et al.
1980; Officer et al. 1984). This set of ecological interactions can be
summarized as follows:
1) the photosynthetic production of organic matter by phytoplankton has
increased in response to higher nutrient inputs;
2) a significant fraction of this production ultimately accumulates
below the pycnocline and below the euphotic zone, especially in the main
channel of the Chesapeake Bay between the Chesapeake Bay Bridge and south of
the Potomac River;
3) this increase in the supply of organic matter, of phytoplankton
origin, stimulates the metabolism of heterotrophic organisms in bottom water
1
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and the benthos; and
4) the associated increase in oxygen demand reduces dissolved oxygen
levels, which depend on depth and the degree of vertical stratification of
the water column.
Annual phytoplankton production has apparently increased in response to
higher nutrient supply, a response that also appears to be related more to
nitrogen than to phosphorus supply (Boynton et al. 1982). However, several
important gaps exist in our understanding of how phytoplankton production is
related to seasonal oxygen depletion. These gaps, which are basic to the
development of water quality models, include:
1) a lack of understanding of the extent to which time- and space-
dependent variations in phytop lank ton production of the surface layer,
organic input to bottom water, and oxygen depletion are related on scales of
days-months and meters-kilometers,
2) how the importance of heterotrophic microorganisms as consumers of
organic matter and oxygen vary over similar time and space scales, and
3) the influence of these relationships and environmental factors on
the temporal and spatial extent of the oxygen depletion zone.
Relationships among phytoplankton production, accumulations of phyto-
plankton biomass in deep water,and bacterial production and respiration are
of particular importance since the key parameter of oxygen depletion is a
shift from phytoplankton-metazoan food webs (low oxygen demand per unit of
organic matter metabolized) to phytoplankton-bacteria1 food webs (high oxygen
demand per unit of organic matter metabolized). Naturally-occurring popula-
tions of marine bacteria were traditionally understood to be small (100's-
2
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1000's per ml) and the main role of the bacteria was thought to be as decom-
posers of particulate organic detritus (POC) in sediment (Steele 1974).
However, in the last ten years, as a result of several key technological
advances, our understanding of the biomass, functioning and roles of marine
bacterioplankton has undergone something of a revolution (Hobbie et al. 1977;
Fuhrman and Azam 1980; Williams 1981; Azam et al. 1983; Ducklow 1983). It is
now generally understood that bacteria constitute a sizable and dynamic pool
of living carbon in marine plankton systems. This is especially true in
estuarine systems, where primary production and inputs of a 1 lochthonous
organic matter and inorganic nutrients are high, and where shallow depths,
strong stratification and the two-layered estuarine circulation enhance
regenerative functions (Ducklow 1982; Wright and Coffin 1984). Bacterio-
plankton are efficient producers of organic carbon whose production rates
commonly range from 10 to 30% of daily photosynthetic production. Thus, in
considering the production and fate of POC in marine systems, it is now
important to include synoptic studies of the magnitudes and spatia 1/tempora1
variability of bacterioplankton biomass, production, and metabolic activity.
Because few such studies have been performed, we have only a sketchy
understanding of the interrelationships among physical forcings, nutrient
inputs, primary production, and bacterioplankton secondary production. A
system like the Chesapeake Bay, in which primary production is very high, and
in which nutrient inputs occur on several scales, presents a useful arena in
which to evaluate these interconnections. Furthermore, it is our contention
that the knowledge gained is of key significance in understanding the basic
physical and biological processes generating and maintaining anoxia in the
Bay.
3
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Given that the key biological parameter of oxygen depletion is the
consumption of organic material (e.g. phytoplankton carbon) by bacteria,
there are two major microbiological processes which contribute to anoxia in
Chesapeake Bay. One of these, sulfur cycling, is described by equations 1
and 2.
2[CH20] + So|" > S2" + 2C02 + 2H20 (eq. 1)
S2" + 202 > SoJ" (eq. 2)
Equation 1 depicts the mineralization of organic carbon, [CH20], by sulfate-
reducing bacteria, obligately anaerobic microorganisms which use sulfate as
an oxidant. The important product of this process is sulfide which is found
in the anoxic portion of the water column of the mid-Bay during summer
stratification. Oxygen consumption by sulfide (eq. 2) occurs abio logica1 ly,
but can be microbio logically mediated by sulfur-oxidizing bacteria. The
potential for carbon mineralization coupled to microbial sulfate reduction
and, thus, sulfide generation and subsequent oxygen consumption is very high
in marine and estuarine environments. In mid-Chesapeake Bay deep water and
sediments (ca. 15 mM sulfate and 0.3 mM oxygen at saturation), sulfate would
have about 100 times the oxidation capacity of oxygen at saturation.
Although sulfur cycling was not a topic of investigation in the Sea Grant/EPA
study reported herein, its relationship to our results will be discussed
briefly in a later section of this report.
The second key microbiological process contributing to anoxia is
depicted in equation 3.
[CH20] + 02 > C02 + H20 (eq. 3)
This process, in which oxygen is consumed directly during the mineralization
4
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of organic matter by microheterotrophs, has been the focus of the microbio-
logical portion of our 1984-85 studies.
THE 1984 STUDY
In 1984 we investigated phytoplankton-bacteria1 interactions and dis-
tributions in time and space in the mid-Bay region during mid summer-fall
when phytoplankton production and microheterotroph activity were at or near
their seasonal peaks and when vertical gradients of physical and chemical
properties were maximal. The spatial area covered by our 1984 study was
bounded by the east and west shores of the Bay and extended about 25 km in a
north-south direction centered on the mouth of the Choptank River. The
emphasis of our work in 1984 was on event scale variability and on the
significance of lateral variation on nutrient-phytop lankton-bacteria-oxygen
relationships in the Bay during summer under high flow conditions. Speci-
fically, we addressed the following problems:
1) Is phytop lankton production an important source of organic matter to
bottom water under summer conditions?
2) How important is phytoplankton production over the shallow flanks of
the main channel relative to local production in the channel per se as a
source of organic matter to bottom water under summer conditions?
3) Are heterotrophic microorganisms in the water column important
consumers of organic matter and dissolved oxygen and how does their impor-
tance vary in relationship to variations in phytoplankton production, organic
input to bottom water, and dissolved oxygen?
4) How do variations in vertical water column stratification (mixing)
influence these relationships?
5
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Objective 3 included obtaining preliminary answers to these questions:
1) What are the levels of bacterial abundance/biomass and how do these
vary vertically and horizontally over various time scales?
2) Does this variability correspond to variations in the distributions
of temperature, salinity, chlorophyll, and nutrients?
3) What is the magnitude of bacterial production relative to primary
production, and how does this vary?
A) Do bacterial abundance and production covary with other estimates of
microheterotrophic processes (amino acid uptake and respiration and oxygen
u t i1iza tion ) ?
5) From the answers to these questions, what can we conclude regarding
a) the mechanisms regulating variability in bacterial biomass and b) the
significance of bacterioplank ton metabolism in maintaining anoxia?
The major findings of our 1984 effort have been discussed in detail by
Tuttle et al. (1985). These findings were as follow!-: "
1) Variations in the dissolved oxygen content of mid-Chesapeake Bay
bottom water were much greater than expected on the basis of vertical stra-
tification arguments. Several east to west and west to east tilts of the
halocline were observed between 1 July and 2 November. The mean cross-Bay
tilt of the halocline was up to the west, resulting in the presence of
nutrient-rich, oxygen-depleted water at shallower depths along the western
shore than along the eastern shore. Halocline tilting events were associated
in the summer with the intrusion of anoxic water and hydrogen sulfide into
shallow waters of the western flank.
2) Low dissolved oxygen bottom water was not restricted to the well-
stratified summer months. Oxygen depletion was also observed during October
6
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due to restratification following vertical mixing events in September.
3) Variations in the particulate organic content of bottom water were
largely due to inputs of phytoplankton biomass and phytodetritus. The extent
and persistence of anoxia and low dissolved oxygen levels below the halocline
during summer probably depends upon this input.
4) The heterotrophic bacterial response to reoxygenation of bottom
water was rapid, with high rates of oxygen consumption even at low oxygen
tension. The rapidity of this response likely contributes to the maintenance
of low dissolved oxygen under summer conditions.
5) Phytoplankton productivity did not appear to be nutrient limited,
although transient phosphorus limitation might have occurred. N: P ratios
indicated that P would have been depleted before N, contrary to past obser-
vations which indicate that N is most likely to be limiting during summer.
6)LateraJ.. .^variations normal to the main axis of the Bay were large and
dominated the nutrient-phytoplankton-oxygen dynamics under stratified summer
conditions. Thus, seasonal patterns of variation in phytopLank ton
productivity based on measurements over the channel down the main axis of the
Bay are probably not representative of most of the Bay's area.
7) Microheterotrophs were present at the highest sustained cell densi-
ties so far observed in a marine environment throughout the study period,
especially during summer stratification and following restratification of the
Bay in October. Bacterial biomass appeared to be most closely coupled to
chlorophyll concentrations during stratification events.
8) Bacterial production and metabolic activity were indicative of the
large bacterial biomass. Thus, the bacterial population remained highly
7
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active throughout the study period.
9) Variations in bacterial biomass, production, and metabolic activity
were related to chlorophyll, particularly below the euphotic zone. This
supports our hypothesis that phytop lankton or phytodetritus provide the major
source of organic carbon for bacterial growth and respiration in the water
column during summer and fall.
10) For a portion of the study period, especially during partial fall
stratification, microheterotrophs were the dominant organisms causing oxygen
depletion in the mid-Bay. Based upon direct measurement of oxygen consump-
tion, estimates of oxygen consumption from heterotrophic bacterial produc-
tion, and net rates of oxygen consumption calculated during the restratifi-
cation events, water column oxygen consumption over the late summer and fall
at least equals the contribution of sediment oxygen demand.
THE 1985 STUDY
In 1984 we found few differences in physical, chemical, and biological
parameters within the north-south boundaries of the study area. Accordingly,
we extended spatial coverage by moving the northern-most of three lateral
transects to just south of the Chesapeake Bay Bridge at Annapolis and the
southern-most of the transects to just north of the mouth of the Patuxent
River. The locations of these transects are shown in Figure 1. In addition,
we added several deep channel stations between the transects. These changes
provided north to south coverage of about 80 km and afforded us the
opportunity to follow the development of low oxygen conditions in the north-
south direction. The cross-Bay transect off the mouth of the Choptank was
retained for the purpose of comparisons between 1984 and 1985. Assessment of
8
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T 2
¦v
CHESAPEAKE
BAY
Figure 1. Site map of Chesapeake Bay illustrating transect (T1, T2, T3)
locations ( ) and additional deep channel stations () occupied
during the 1985 study.
9
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cross-Bay variability was enhanced in that stations at all three transects
were occupied on every cruise.
Due to funding limitations, however, the frequency of cruises had to be
limited in 1985. Whereas our 1984 work, emphasized event scale variability
(many of the cruises were spaced 3-4 days apart), our 1985 study focused on
seasonal variations over the period February to October. We placed parti-
cular emphasis on the late spring (15 May-19 June) by making five of our 14
cruises during the time period at which we expected summer conditions to
become established.
The same parameters measured in 1984 were also determined in 1985.
However, on many of the cruises we added measurements of BOD-5 and bacterial
glucose metabolism in an effort to increase our understanding of the rela-
tionship of carbon flow from phytop lankton to microheterotrophs which results
in oxygen consumption. This report also contains a comparison of 1984-1985
during summer conditions. In this comparison we emphasize interactions among
freshwater flow, vertical stability, phytop lank ton production, bacterial
production and metabolism, oxygen consumption, and dissolved oxygen regime.
10
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SECTION 2
CONCLUSIONS
1. Phytoplankton production during February - May generates a quantity
of organic matter more than adequate to cause oxygen decline which occurs
annually during the spring in mid-Bay deep waters.
2. The development of the summer maximum in phytop lankton productivity
in the mesohaline region of Chesapeake Bay seems to depend upon the regenera-
tion of nutrients based upon phytop lankton carbon produced in the spring
which sinks into the deep water and is reminera1ized by bacteria. These
nutrients are recycled into the euphotic zone as a consequence of vertical
mixing and oscillations of the pycnocline. This is contrary to the hypo-
thesis that there is an interannual carry over of phytodetritus which fuels
oxygen depletion in the Bay (Taft et al. 1980).
3. Phytoplankton growth rates in 1984 and 1985 were not limited by
either N or P on a seasonal time scale. This is somewhat surprising since
1985 was a low flow year which should mean that inputs of new nutrients to
the Bay were lower than in high flow years such as 1984. If this is the
case, substantial reductions in nutrient inputs will be required to signifi-
cantly decrease phytoplankton production in the Bay.
4. Phytoplankton biomass during summer 1985 was greatly decreased
compared to 1984 even though phytoplankton production was similar in both
years. This suggests that phytoplankton production was more closely coupled
with zooplankton grazing in 1985 than in 1984.
11
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5. Bacterial biomass, production, and metabolism are tightly coupled to
phytop lankton production in the Bay, especially during summer conditions.
Phytodetritus is the source of carbon fueling bacterial oxygen consumption in
Bay deep waters.
6. Carbon utilizable by bacteria, measured as biochemical oxygen
demand, shifts abruptly from POC to DOC in the late spring, particularly
below the euphotic zone. This condition persists throughout the summer and
supports our contention that spring phytoplankton production can support
summer bacterial metabolism, nutrient regeneration, and oxygen consumption in
Baiy deep waters .
7. Carbohydrates rather than amino acids may be the preferred source of
carbon and energy for bacterial metabolism and production.
8. Despite a significant reduction in phytoplankton biomass in summer
1985 compared to summer 1984, bacterial biomass and amino acid metabolism
were similar both years under summer conditions and bacterial production in
1985 was higher. This and the fact the summer mean oxygen consumption rates
below the euphotic zone were nearly identical in 1984 and 1985 indicate that
increased reoxygenation of the deep water in 1985, not decreased bacterial
activity, was responsible for the lack of widespread anoxia.
9. Very high bacterial standing crops were found during the summer in
both years despite great differences in climatic conditions and water column
stability. This suggests that fundamental changes have occurred in the
ecosystem of mesohaline Chesapeake Bay over the years such that a significant
portion of primary production is channeled into bacterial production.
10. The potential for the establishment of anoxia tends to be greatest
12
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in the northern regions of the mesohaline Chesapeake Bay. This conclusion is
supported by the findings of greater bacterial biomass and production as well
as greater BOD near the Chesapeake Bay Bridge than near the mouth of the
Patuxent River.
11. Biological parameters exhibit greater variation in an east to west
than in a north to south direction. This implies that concentrating
monitoring or research efforts in the mesohaline portion of the Bay over wide
areas from north to south is unnecessary other than for the purpose of
delineating the extent of anoxia in any given year.
12. Water column oxygen consumption by microheterotrophs is a major
contributor to the development of anoxia, particularly during the spring and
the early summer. Later in the summer, sulfur cycling becomes an important
mechanism for maintaining anoxia.
13
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SECTION 3
RECOMMENDATIONS
FURTHER RESEARCH
Our 1984 investigations focused on event scale variability with cruises
spaced at 3 to 4-day intervals during the period of suracier conditions. In
contrast, the 1985 9tudy emphasized seasonal differences. While the greatly
different anoxic regimes which existed in 1984 and 19S5 wcru fortuitous for
comparing year to year variability, two sunner seasons of research (our 1984
summer work lasted less than one month) are insufficient to fully understand
the dynamics of oxygen depletion. We conclude that additional crucial
information could be obtained from a spring-summer study in which measure-
ments are made at or near the time intervals of the 1984 investigation.
Additional data are also needed to quantitatively assess the contribu-
tion of benthic and pelagic processes to oxygen depletion on both seasonal
and event scales. So far there has not been close enough coordination of
benthic and water column studies with relevance to the dynamics of the
establishment and maintenance of anoxia. It would be particularly useful to
be able to relate benthic and water column oxygen consumption and bacterial
activity with phytoplankton dynamics and carbon flow over short time
intervals.
Our research has demonstrated that a tight coupling exists between
phytoplankton and heterotrophic bacteria (and, thus, oxygen consumption) in
the mid-Chesapeake Bay. However, a major question which remains is whether
14
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the tributaries significantly influence raainstera water quality, whether the
mainstem affects the tributaries, or whether the two act independently. It
is also not known whether the trophic state of the tributaries is similar to
that of the mesohaline nainstem. The answer to this question is crucial to
the effective apportionment of limited resources between tributary and
mainstem restoration efforts. One approach to this question would be to
compare water quality characteristics, phytop lankton/bacteria 1 coupling, and
sulfur cycling in one or more of the tributaries emptying Into the mid-Bay
with the adjacent ma ins ten.
Tho results of our work suggest that there has been n major t-hift in the
food web structure of the Chesapeake Bay ecosystem. During spring and
summer, phytop 1 ankton production is consumed to a significant degree not by
grazers or filter feeders but rather by free-living bacteria in the water
column. Several factors, including nutrient enrichment, might be responsible
for such a shift. However, a key one could be the decline in populations of
filter feeding benthic animals such as oysters and clams caused by exposure
to hypoxic or anoxic waters, other detrimental water quality conditions,
disease, and over harvesting. For example, estimates suggest that the Bay's
oyster population may now be four million bushels or less compared to 80
million bushes in 1985 (Schneider 1987). Thus, a major removal mechanism for
phytoplankton biomass has been nearly eliminated from large ares of the Bay.
If it can be demonstrated through additional research that increased bivalve
populations could play an important role in removing phytoplankton and, by
removing this organic carbon source, decrease bacterial biomass and oxygen
consumption, the result could lead to major savings in the cost of restoring
15
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Bay water quality as well as revitalizing the shellfish industry.
MANAGEMENT
Our findings impact the design of monitoring regimes to evaluate the
influence of abatement procedures on the health of the Bay in general and on
anoxia in particular. During the summer, biological parameters are
surprisingly consistent over the 80 km north to south boundaries of the 1985
study area. We conclude that monitoring efforts, at least during the summer
period, could be restricted to very few sampling stations with emphasis
placed on event scale changes and east to west variability. The latter is
particularly important with regard to assessment of phytop lank ton production.
Secondly, we have demonstrated that a key feature of summer conditions
in the mesohaline portion of Chesappake Bay is the establishment of large
standing crops of bacteria. We have further shown that the magnitude of
bacterial abundance can be used to predict oxygen consumption. Therefore, we
recommend that measurements of bacterial abundance be included in the Bay
monitoring program. Data gained from these measurements will not only permit
a rapid and technically simple determination of oxygen consumption rates but
will also provide an indicator of Bay health, i.e. decreasing bacterial
abundances over the summer should be indicative of improved water quality in
the Bay and a desirable change in the Bay ecosystem.
16
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SECTION 4
MATERIALS, METHODS AND EXPERIMENTAL PROCEDURES
CRUISE SCHEDULE AfiD STATION DESCRIPTIONS
Sixteen stations were occupied during the course of the 1985 Maryland
Sea Grant/EPA study. Transect 1 consisted of three stations extending from
west to east just south of the Bay Bridge (Table 1, Fig. 1). Transects 2 and
3 consisted of five stations each extending west to east from Dares Brach to
Hills Point and just north of the mouth of the Pntuxunt RIvlt, respectively.
Three additional stations were occupied along the deep channel and formed
with stations SG3, SG24, and SG32 a six-station deep channel transect in a
north-south direction. The correspondence of 1985 stations with stations
occupied during our 1984 EPA study is indicated in Table I. A total of
fourteen two-day cruises were conducted between 02-12-85 and 10-11-85
(Table 2). Either the RV Orion, 52-ft. overall, or the RV Aquarius, 68-ft.
overall, wa6 used for each of the cruises.
SAMPLE COLLECTION
Water samples were collected with a submersible well pump connected to a
hose which was lowered to the desired sampling depth. No detectable
differences in physical or biological parameters could be detected when pump
supplied water was compared to in situ measurements or to water collected in
submersible sample bottles. This pump system is the standard water collec-
tion procedure about the UMCEES research fleet. Use of the pump system
17
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Table 1. Station locations for 1985 Sea Grant/EPA Chesapeake Bay study.
Corresponding 1984 station numbers in parentheses.
Stat ion
Transect v
Latitude
Long itude
Descr ipt ion
SGI (CP1)
2
38°
33.50'N
76°
29 .60'W
Dares Beach
SG2 (CP2)
2
38°
33.52'N
76°
27 .50'W
SG3 (CP3)
2
38°
33.57'N
76°
26 .34'W
SG4 (CP4)
2
38°
33 .54'N
76°
22 .50'W
SG5 (CP5)
2
38°
33 .52'N
76°
20.50'W
Hills Po int
SG21
3
38°
19.78'N
76°
24 .21 'W
Patuxent River
SG22
3
38°
19.94'N
76°
21 .64 'W
SG23
3
38°
20.20'N
76°
20.20'W
SG24
3
38°
20.60'N
76°
18.37'W
SG25
3
38°
20.92'N
76°
17 .86'W
Barren Island
SG26 (CP7)
-
38°
28.45'N
76°
23 . 25'W
SG27 (CP13)
-
38°
41 .00 'N
76°
25.50'W
SG28
-
38°
50.07'N
76°
24.20 'W
SG31
1
38°
59.25'N
76°
23.80'W
Bay Bridge
SG32
1
38°
58.85'N
76°
21.89'W
SG33
1
38°
58.69'N
76°
21,20'W
Bay Bridge
SG24B*
3
38°
20.67'N
76°
18.40'W
SG25B*
3
38°
20.90'N
76°
17 .85'W
*Stations replaced SG24 and SG25 on some cruises.
18
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Table 2. Cruise dates and descriptions for Sea Grant/EPA Chesapeake Bay
study.
Cruise Number
Funding Agency
Date
Stations Occupied
SGBl
NOAA
02-12&13-85
all
SGB2
NOAA
03-2I&22-85
a 11+SG3&24
repeated
SGB3
NOAA
04-16&17-85
all+SG3&24
repeated
SGB4
NOAA
04-29&30-85
a 11+SG3&24
repeat ed
SGB5
NOAA
05-16&17-85
al1+SG3&24
repeated
SGB6
NOAA
05-23&24-85
a 11+SG3&24
repeated
SGB7
NOAA
05-28&29-85
al1+SG3&24
repeated
SGB8
NOAA
06-06&07-85
al1+SG3&24
repeated
SGB9
NOAA
06-19&20-85
all+SG3&24
repeated
SGBl 0
NOAA
07-11412-85
a 11 + SG3&24
repeated
SGBl 1
EPA
07-196.20-85
a 11+SG3& 24
repeated
SGB12
EPA
08-15&16-85
a 11 + SG3&24
repeated
SGB13
EPA
091l&l285
all+SG3 repeated
SGB14
EPA
1010&1185
all+SG3&24
repeated
19
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greatly increased the speed of sampling compared to bottle sampling methods
so that more discrete samples could be taken. Thus, vertical water column
profiles could be described in greater detail than otherwise possible. In
addition, physical water quality parameters could be analysed in real time so
that samples for nutrients and biological measurements were taken from the
most appropriate depths. As the water from a sampled depth was pumped aboard
it was either collected directly into sample containers or in polypropylene
buckets from which samples were taken.
DETERMINATION OF PARAMETERS
Phys ic a 1 Wa t e r Qua I it y Parameters
Water depth was determined with an onboard sonar-type depth finder and
was confirmed by hydrographic wire depth. Secchi disk depth was determined
with a 12-inch diameter black and white, weighted disk attached to a line
calibrated at 10 cm intervals. Temperature and dissolved oxygen were deter-
mined using a YSI Model 57 (Yellow Springs Instrument Co.) digital dissolved
oxygen meter and salinity with a YSI Model 33 salinity meter. The dissolved
oxygen and salinity probes were usually submerged in containers through which
the pump Bay water circulated. At selected depths at certain stations,
dissolved oxygen concentrations were also determined by the CBI microwinkler
method (Carpenter 1965).
Part icu late Organic Carbon (POC) . Nitrogen (PON).
and Nutrient Determinations
Particulate organic carbon and nitrogen were measured by combustion.
Fifty to 100 ml of sample were filtered through pre-combusted CF/F glass
fiber filters and frozen. Upon returning to the laboratory, the filters were
thawed and analyzed with a Perkin-Elmer Model 240B elemental analyzer.
20
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Filtrates were collected and frozen for measurements of dissolved
inorganic nutrients (phosphate, nitrate, nitrite, ammonium). Nutrient con-
centrations were determined colormetrica11v using a Technicon autoanalvzer
and standard procedures .
Phytop lankton P iznen ts and Cell Dens i t ios
Chlorophyll a_ and phaeopig~ent concentrations were measured by
fluorometry. Fifty to 100 ml of sample were filtered through GF(F glass
fiber filters and frozen. Upon returning to the laboratory, the filters were
thawed, and pigments were extracted by grinding the filters in 90% acetone.
Fluorescence of the extracts was measured before and after acidification with
10?= HC1 usin^ a Turner Designs fluoroncter.
Phytoplankton cell densities were determined by the inverted microscope
technique. Samples were preserved with Lugol's solution and returned to the
laboratory for enumeration with an inverted microscope after concentrating
cells by settling for at least 48 hours.
Photosynthetic Produc t ion
The photosynthetic production of organic ©atter by phytoplankton was
measured by the technique at stations 1, 3, and 5 of transect 2. Primary
productivity was calculated from assimilation during 24-h incubations
under natural light conditions. Samples were inoculated with 5 uCi of
NaH^CO^ and incubated at surface water temperature under light levels of
100%, 50%, 22%, and 5% of incident radiation. Incubations were terminated by
filtration through a GF/F glass fiber filter. Filters were fumed over HC 1,
placed in scintillation vials with 10 ml of Ready-Solv and counted with a
Packard Tri-Carb Model 461C liquid scintillation spectrometer. Productivity
21
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3 I
was calculated for each light level (cig Cm d . Given the shallow depth
of the euphotic zone, PP-Irradiance curves (PP vs. % light) were determined
using a surface sanple only. Integrated euphotic zone production was
estimated from vertical Chi profiles, % light depths and PP/Chl (from the PP-
Irradiance curves).
Microb 1a 1 Me tabo 1 isn and Secondary Produc t ion
Dua1 Labe 1 Tec hninue
Natural rates of microbial amino acid and glucose metabolism were
determined using modifications of the methods suggested by Williams and Askew
(1 968) and Williams et al. ( 1976 ). A mixture of 15 carbon-14 rndio-labo1ed
amino acids (ICN Corp., product ;;10147), an algal protein surrogate, and
tritiated glucose (ICN Corp., product f?27020) were used. Bacterial
production was determined using modifications of the rnethod suggested by
Fuhrman and Azam (1980, 1982). Tritiated methy 1-thymd ine (ICN Corp.,
product #24060) was used in this case. Amino acid metabolism and bacterial
productions were determined simultaneously (as were amino acid metabolism and
glucose metabolism) using a dual label technique Ln which both substrates
were added to the individual samples.
The incubation containers for this determination were acid washed, all
polyethylene/polypropylene 10 ml syringes (Aldnch Chemical Co.) fitted with
gas-tight caps. The syringes were filled from a bucket which was continually
flushed with the pumped Bay water from the sampled depth. They were then
capped and transferred to a dark incubation box in which the in situ
temperature was maintained. Four syringes were filled with 10 ml of water
from each sampled depth. Metabolic activity in one of the four syringes was
stopped by addition of 100 ul of 1.2N l^SO^. This was the abiotic control.
22
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The remaining syringes were used for triplicate live incubations.
Each of the 10-ml samples was inoculated with 56.2 nanograms (nominally)
of the amino acid mixture, and 5 picomoles of "^H-glucose or 50 picomoles
(nominally) of H methy 1-thvmidine, and then incubated in the dark for
exactly 30 run. The samples were then filtered (0.2 um pore size membrane)
and rinsed with 4 ml of 5% ice cold TCA (trichloroacetic acid). Each
membrane filter was then placed in a Filmware bag (Nalge Co.); 5 ml of
Scintiverse (Fisher Chemical Co.) was added, and the bag heat sealed.
Liquid scintillation spectrometry was used to quantitatr the ^C and
simultaneously in each of the samples. Packard Model 4430 or 4530 Scintil-
lation counters set for ^H/^C dual label counting were used for this quan-
titation. The turnover rates for the .in, situ amino acid and glucose pools
were calculated from the assimilation rates for ^C or ^H. Bacteriop1ankton
production was calculated from the amount of TCA insoluble, partic le-asso-
3
ciated H thymidine.
Microb ia 1 Produc t ion - S in£1e Labe1 Tcc hn ique
3 ...
H-thymidine incorporation was also measured according to the method
described by Ducklow (1982). The concentration of added thymidine was 5 nM.
This method differed from the dual label technique in the following ways:
3 ¦
only H thymidine was added to duplicate samples and 30 ml samples were
incubated in plastic bottles containing an air phase. Thus, these experi-
ments were done at air saturation whereas the dual label experiments in which
the sample syringes were completely filled to exclude air were done at
ambient D.O. Comparison of results obtained by both methods was intended to
determine whether possible low measured incorporation rates at low ambient
D.O. were due to oxygen deprivation. Comparison of results at high ambient
23
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D.O. served as an interca 1 ibration control. Preliminary experiments indi-
cated that the use of dual labels had no appreciable effect upon H-thymidine
incorporation rates.
Bac ter ia 1 Biomass
Bacterial concentration was estimated by direct r. ic ru scop ic observation
of acridine orange-stained bacteria under ep i f luor escen t illumination (llobbie
et al. 1977). Ten milliliter, raw water samples were collected, placed in
particle-free screw-capped glass tubes, and preserved with 0.75 ml of
particle-free 372 formaldehyde. These samples were held at 4°C until
enumerated. The enumeration technique was essentially that of Hubble et al.
(1977). One milliliter Bay water samples were mixed with 1.0 nil uf particle-
free Bay water in particle-free vials. These samples were then- stained with
20 ul of 0.01%, particle-free acridine orange for about 4 minutes.
Particles, including the bacteria, were collected on 0.2 u porosity, Irgalan
Black (Ceiba Geigy Corp.) stained, Nuclepore polycarbonate filters. The
filters were washed twice with a total of about 3 ml of particle-free Bay
water, and were observed under epifluorescent illumination using a Leitz
Ortholux microscope at 1562x total magnification. Five randomly selected
fields were observed, and all bacterial cells within a 10x10 eyepiece grid
were counted. The mean of the five counts was used to calculate the total
bacterial concentration in the original water sample.
In many cases, AODC counts were also done according to the method of
Ducklow (1982). This technique differed from that described above chiefly in
that 10 rather than 5 fields were examined. Comparison of counts in the same
samples done by both methods afforded an intercalibration control.
24
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Microbia 1 Oxygen Coasumption
Microbial oxygen consumption races in the water column were determined
from short-tern changes in the oxygen concentration of enclosed water samples
incubated in the dark at .in. s itu temperatures. At each depth where these
samples were taken four, acid washed, 125 ml ground glass-stoppered bottles
were filled with water essentially as described in the Microbial Metabolism
section above. Care was taken to be sure that no air bubbles were trapped in
the bottles. Oxygen in one of these bottles was immediately fixed as
described in the CBI microwinkler method. The remaining three were incubated
at-_in situ temperature in light-tight containers for 6 hours after which the
oxygen in those bottles was also fixed. Oxygen concentrations were
determined by the microwinkler titration as described by Carpenter (1965).
Dark oxygen consumption was determined from the difference between the oxygen
concentration at the time of collection and that present after 6 hours of
incubat ion.
Biochemica1 Oxy&en Demand (BOD-5)
Biochemical oxygen demand was determined from changes in oxygen concen-
tration in samples incubated in the dark at 20°C for 5 days. The samples
were collected and oxygen concentration determined as described in the
Microbial Oxygen Consumption section. BOD-5 measurements were done in
duplicate and compared to immediately fixed samples when probe-determined
D.O. exceeded 4 mg/1. When D.O. was <4 mg/l, 1-liter samples were collected
and shaken to increase the initial oxygen concentration. In this case, the
initial D.O. of the shaken sample was determined for comparison to duplicate
shaken samples incubated for 5 days.
25
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At selected stations, filtered ECD-5 determinations were also made. In
this case, freshly collected water was filtered through Gelman type A/E glass
fiber filters at <7 in. iig. The filtrates were shaken, and the water
dispensed into ground, glass-stoppered bottles.
26
-------
SECTION 5
RESULTS
THE 1985 ANNUAL CYCLE-HYDROGRAPHY, NUTRIENTS, PHYTOPLANKTON
Hydrography
Seasonal variations in the vertical distributions of temperature and
salinity were generally typical of partially stratified estuaries located
in temperate latitudes. Surface temperature increased from less tiian 1°C
in February to 26°C in August (Fig. 2). The rate of increase was greatest
in May when surface temperature increased by 8°C from ca. 11°C at the
beginning of the month to ca. 19°C by the beginning of June. A thermo-
cline developed during April and persisted through August. Vertical
stratification in the salinity field was observed throughout the study
period but was strongest during April-August (Fig. 3). The effect of the
spring freshet was observed during April with a time lag of 1-2 weeks
between the northern most deep station (32) and the southern most deep
station (24) which gives a mean drift of ca. 6 km/d. Large intrusions of
high salinity bottom water occurred during April-May and again during
July-August.
These patterns were reflected in the density field (Fig. 4). Like
salinity, density stratification was observed throughout the study period.
Density gradients across the pycnocline ranged from 0.12 to 1.24 sigma-t/m
and generally decreased downstream from north to south. Peaks in the
density gradient across the pycnocline occurred during April and early May
27
-------
TEMPERATURE
26
20
24
22
22
20
2 0 -
22
20
E
20
24
z
a.
UJ
Q
26
2 0 -
22
24
10
20
22
SO
40
.. J
J
r
M
A
y
J
j
A
MONTH . 1983
Figure 2. Temporal variations in the vertical distribution of temperature*
28
-------
SALINITY
20-
0 -
E
2 0 -
X
a
UJ
o
2 0
*0 -
40 -
t
F
u
A
M
J
J
J
A
s
0
MQNT M , 196*
Figure 3. Temporal variations in the vertical distribution of salinity.
29
-------
SlOMA-t
20
E
Z
a
U1
O
SO
40
f
A
M
J
M
J
A
Figure 4. Temporal variations in the vertical distribution of sigma-t.
30
-------
as a consequence of the spring freshet and during July-August as a conse-
quence of surface warming and the intrusion of high salinity botton water.
Disso lved Inorganic Nutrients
To a first approximation, distributions of nutrients were related to
salinity and to vertical stratification (Figs. 5-7). Nitrate concentration
wa9 highest in the surface layer and decreased across the halocline. Max Ln:un
concentration occurred in the surface layer in association with the spring
freshet. Concentration minima occurred in the bottom layer in association
with intrusions of high salinity water. In contrast, ammonium and phosphate
tended to increase with depth and salinity with maximum concentrations
occurring in the bottom layer during late summer (Figs. 6 and 7).
N/P (nitrate + nitrite + ammoniun)/phosphate) was generally high
(Fig. 8), indicating that P would become limiting to phytop lankton growth
before N. However, the continued presence of these nutrients in the surface
layer suggest that phytop1ankton growth was not nutrient limited during 1985.
The shift in the frequency distribution from a peak in the 60-120 range
during March-May to a peak in the 15-30 range during May-October (Fig. 8)
probably reflects the combined effects of high nitrate supply during the
spring freshet, low nitrate supply during summer due to low fresh water
runoff, and nutrient regeneration during summer with phosphate being recycled
more rapidly than ammonium.
Dissolved Oxygen
The concentration of dissolved oxygen in bottom water decreased most
rapidly during winter-spring to less than 2 mg/1 in May (Fig. 9). Concen-
trations were lowest at the northern most station and increased toward the
31
-------
Figure 5. Temporal variations in the vertical distribution of nitrate (uM).
32
-------
& M WON I U M
MONTH, 1985
Figure 6. Temporal variations in the vertical distribution of ammonium
(uM).
33
-------
PHOS p H a t e
>0
E
X 20
1 0
SO
<05
s t a s:
<05
1 "I * U ~
0 1 0 5
"I »\
11
S T A 0 3
x
OS
\
S T A 2*
j r m
* M J j » s
MONTH . 1985
0 N D
Figure 7. Temporal variations in the vertical distribution of phosphate
(uM) .
34
-------
ao
40
o
a
60
40
20
12 MAR 16 MAY 1985
-B J
21 MAY -10 OCT 1985
j:i Jl.
5 10 I 5
I.
i0 60 I ? 0 ?40 400
N / P
Figure 8. Frequency distributions of the atomic ratio of dissolved
inorganic nitrogen (nitrate + nitrite + ammonium) to dissolved
inorganic phosphorus (orthophosphate); open bars - surface, solid
bars - near bottom.
35
-------
OXYGEN
2 0
10
X 20
20
SO
40
J
F
M
A
J
J
A
MONTH . 19 8 5
Figure 9. Temporal variations in the vertical distribution of dissolved
oxygen (mg/1); hatched areas indicate locations of mid-depth
oxygen minima.
36
-------
mouth of the Bay. The rate of decline during February-May was 0.1 mg/1/d at
all stations over the mid-Bay channel. This rate is equal to the rate of
decline reported by Taft et al. (1980) and to that calculated by Officer et
al. (1984). We emphasize that rate of oxygen decline determined here is the
net rate of decline over a period of several weeks. It includes both oxygen
consumption and re-aeration processes. Short-term oxygen consumption
(respiration) measurements discussed later in this report are estimates of
gross oxygen consumption and are not influenced by physical re-aeration
processes. Thus, the short-term measurements will give rates of oxygen
consumption higher than the net rotes considered here.
Hypoxia was maintained fror. May through September, but anoxic was
observed only during late summer at the northern most station. The oxygen
content of bottom water did not remain uniformly low during this period but
exhibited alternating periods of oxygen depletion and partial re-aeration, a
pattern which became more pronounced with distance to the south of station 32
(Fig. 9). Distributions of the dissolved oxygen indicate that oxygen
depletion occurs as bottom water moves up the Bay (e.g. Fig. 10) and that the
tendency for re-aeration increases south of station 32.
Oxygen minimum zones were occasionally observed in the pycnocline
(Fig. 9), a phenomenon that probably reflects the entrainment of oxygen
depleted bottom water into the surface layer and subsequent transport down-
stream (Fig. 10). High rates of heterotrophic metabolism in the pycnocline
may also contribute to the development of these oxygen minima (see section on
microbial activity).
37
-------
OXYGEN
Figure 10. Variations in the vertical distributions of salinity and
dissolved oxygen along the mainstem transect on cruise 10.
Station numbers are indicated by closed triangles at top.
38
-------
Phytop lank ton B ioraas s and Produc c iv i t v
Phytoplank ton bionass, as indicated by the concentration of chlorophyll-
a. (Chi), was high throughout the water column during February-May (Fig. 11)
when the Chi content of the euphotic zone (confined to the surface layer or
the upper 3-9m of the water column at all tines and places) was generally
greater than 100 uig/a" (Table 3). The Chi content of the water coluirn ranged
between 200 and 2200 mg/m^ and averaged 500 g/m^ through 16 May. Within 7
9 7
days Chi decreased dramatically to a mean of 161 mg/m" (88 to 264 ng/m on
23 May). Most Chi was confined to the surface layer during the remainder of
the study.
This rapid decline in phytoplankton bionass coincided with 1) a narked
decreased in the proportion of Chi accounted for by netplankton (>20 um) ,
2) decreases in both phytoplankton productivity (PP) and chlorophyll specific
productivity (PP/Chl), and 3) a decrease in the proportion of particulate
organic carbon (POC) accounted for by phytoplankton. Prior to 23 May,
netplankton accounted for 22 to 59X (nean=37"0 of Chi compared to less than
10% after the collapse. Dominant netplankton during this period of high Chi
included the diatoms Ceratau 1 ina pe lag ica . Leptocv1indr icus daqnicus .
Rhizosolen ia f r ag i 11 is s ina. Skeletonema costatus. Thalas s ionema
nitzschioides. and Thalass ios ira ap. and the dinof lage 1 lates Proroc entrum
spp. The nanoplankton (<20 urn) were dominated by cyanobacteria, micro-
flagellates, small centric diatoms and cryptophytes throughout the study
period.
Phytoplankton productivity (PP) at station 3 on the mainstem transect
averaged 1200 mg c/m /d during April and early May (Table 4). PP/Chl
averaged 6 mg C/(mg Chi x d) during this same period. The phytoplankton
39
-------
CHLOROPHYLL
0
32
0
40
32
2 0
0
32
0
46
0
e"M
0
2 0
32
4 0
month. I
Figure 11. Temporal variations in the vertical distribution of chlorophyll
(ug/1). ^
40
-------
TJt>1 f 3.
E upnoi1c
lont
Chloroph,l
i I"1?
. i
m ¦ )
at s: iUons
*lon? the mdinuen
32) to
south (jti
ton 24)
Otie
32
26
27
STATION
3 26
2«
We4n
C
12 Feb
11?
193
--
146
--
181
..
21 Hjr
176
201
167
164
167
249
1« 7
181
16 Apr
122
243
2 76
268
2S3
270
239
24
29 Apr
212
139
16S
1S4
99
63
139
37
16
187
?03
160
232
221
3S2
2?6
30
162
196
192
193
185
233
c
251
IV.
29*.
?e*.
36*.
521
2j f 6 J
5'.
7 2
71
76
49
70
66
16
?e «nj
t2
45
79
(Q
45
43
54
26
6 June
22
*0
63
92
133
74
71
55
1? June
58
74
70
71
SS
SU
63
16
11 Jul,
161
12
81
SO
60
40
77
57
19 Jul,
121
88
68
64
107
65
Ob
?e
Hem
80
65
72
67
71
57
C
64-.
29i
r.
241
«8*.
261
11 Au9
se
27
it
32
31
28
39
jB
11 Sept
48
33
34
S9
69
45
48
29
10 Oct
76
38
44
43
S9
S 2
S2
27
Hub
61
33
46
4S
S3
42
C
231
171
2 Si
301
381
291
41
-------
T.ble 4. Prwr, prodytH.H, (.; C e-J) inc cMoropn,M W,(H proflut. ,
(«s C (-9 Cnl-aJ-1) ,t il.noni 1. 3. .ne S o( :n, ChOP-Pai trisect.
0»lc
1
3
S
P
P/Chl
P
P/Chl
P
P/Chl
21 «ar
?ie
3
623
S
1 6 Apr
801
b
1?SS
8
104 1
<
29 Apr
741
7
110?
6
30 7
3
: t H«/
1 27<
t.
1 2 30
I
?6S
6
?j **.«j
t>
1
325
2
227
2
2E *¦*,
47S
16
263
7
21?
11
11 Jul/
133?
3!
ISSt
26
891
22
19 Jul/
1310
IS
1301
18
1216
20
1S Aug
1969
6?
1749
61
1230
SI
11 Sepl
789
38
1SS6
47
1320
44
10 Oct
1« 72
S2
162:,
«S
1210
S4
42
-------
2
collapse in late May coincided with a drop in PP to 329 mg C/m /d and a drop
in PP/Chl to 2 mg C/(mg Chi x d). Following these declines, PP and PP/Chl
increased to annual maxima in August, similar to the annual cycle described
by Taft et al. (1980). Assuming that PP/Chl at station 3 was representative
of rates at the other stations on the mainstem transect, PP averaged 1160 mg
C/m"/d during February-May prior to the collapse and 1750 mg C/m"/d during
June-August following the collapse. Thus, while Chi achieved its seasonal
maximum during spring in conjunction with the peak in fresh water flow, PP
achieved its seasonal maximum during late summer wher fresh water flow (and
associated nutrient inputs) was low.
The concentration of particulate organic carbon (POC) ranged from 130 to
3540 ug/1 and was significantly (P
-------
5- tipone"t1»l rf 1«11 cum pi bel-fr" "OZ »nd CM -i.ff r torrrlc 10"
tOfMltifn; jna POC " ( (e) e ch'.
Due-. n r « o
12 fft> - 23 n*y 222 0.6U 622 0.027
28 r.ty - 19 Jur.t 111 0.87 lib O.OEi
11 Julj - 19 Jul, 74 0.81 07 0.0«1
12 Aug - 10 Oct IIS 0.88 ««E 0.120
44
-------
TibU Cofffioml o' dtifitii n«;\on (r?) (or po-f cuf»f M , i of C/Chl on
Chi xirrf C/Chl « (CM)6.
CUtri « r' b
1? In 11 *«» J it 0.7) 501 -O.bT
?E >«, - 19 June '7 0.8S 33« -O.St
11 July - 19 July «E 0.86 29? -0.43
IS Aug ?S 0.83 SZ9 -O.O
11 Scpl M 0.7S S03 -0.57
10 Oct ?« 0.«6 290 -0.36
45
-------
phytoplankton accounted for 40-6 5" of total POC prior to the phytoplankton
collapse in late May and for less than 20% of POC thereafter. Frequency
distributions of C/Chl (Fig. 12) reflect this shift from a phytoplankton
dominated system during February-May to a detrital dominated system during
June-October, especially in the bottom layer where C/Chl ratios were much
higher than in the surface layer during June-October.
Ratios of POC to particulate organic nitrogen (C/N) varied between 4 and
20 with most ratios less than 10 (Fig. 13). Interestingly, C/N tended to be
higher during spring than during summer with little difference between the
surface and bottom layers during either season. These ratios are typical of
phytoplankton (Strickland 1965) and are much lower than ratios in organic
matter of terrestrial origin (Muller 1977) or of material from the upper
reaches of the Bay where C/N ratios exceed 20 (Flemer and Biggs 1971).
Apparently, most suspended organic matter was derived from local phyto-
plankton production, a conclusion reached by Biggs and Flemer (1972).
La tera 1 Var i a t ion
During the sunnier of 1984 we documented time-dependent variations in
density structure along the CHOP-PAX transect (Malone et al. 1986). These
variations influenced lateral distributions of dissolved inorganic nutrients,
oxygen, phytoplankton biomass and bacteria and appeared to be responsible for
high phytoplankton production over the flanks of the main channel relative to
production over the channel. Such lateral oscillations of the pycnocline
were observed during summer, 1985 but were much less pronounced (Fig. 14).
Likewise, while chlorophyll concentrations were higher over the flanks of the
main channel in 1985 (Fig. 15), we did not observe systematic variations in
the chlorophyll content of the euphotic zone (Table 7) nor in phytoplankton
46
-------
BO
<>0
«0
2 0
H 0
60
40
20
12 MAR - 16 MAY l?8S
n ¦ 7 5
a
?1 MAY - 10 OCT I0R5
n 155
9
! n
u . I U . J a
?' SO 100 ?00 «00 fDO ri.no
C / C h I
Figure 12. Frequency distributions of the ratio of particulate organic
carbon (ug/1) to chlorophyll (ug/1); open bars - surface, solid
bars - near bottom.
47
-------
100
60
80
60
40
a
12 MAR - 16 MAT 1905
n 77
, a , n_ ,
2 3 MAY- 10 OCT 1985
n' 156
j a
8 10
C / N
12 14 18
Figure 13. Frequency distributions of the ratio of particulate organic
carbon (ug/1) to particulate organic nitrogen (ug/1); open bars
- surface, solid bars - near bottom.
48
-------
0
0
20
0
0
20
30
0
0
20
0
0
20
30
0
2
4
6
e
10
I 4
OIST ANC E . km
Figure 14. Lateral distributions of salinity and oxygen along the CHOP-PAX
transect (from west on the left to east on the right). Solid
triangles at top indicate stations along the transect.
49
-------
3 0
SO
40
50
50
4C
£
x
a
w
o
20
20
30
i 0
0
2
e
» 2
A
) b
1 4
01 STANCE, km
Figure 15. Lateral distributions of chlorophyll (ug/1) along the CHOP-PAX
transect (from west to east). Solid triangles at top indicate
stations along the transect.
50
-------
Tibl* 7 Euphotu tor,, chlorophyll » (m, »-?) tt itJ'uoni ii0nS ChOP-PM
tr.niect from -eit (»Ht1on 1) 10 e«U (sutlon 5).
0«te
1
2
station
3
4
S
Hem
C
12 reb
124
131
146
93
101
119
181
21 H«r
70
8S
118
1S1
ISO
IIS
32
16 Apr
207
213
228
2S3
412
263
32
29 Apr
1S2
110
311
88
97
156
S8
16 Hi;
1SS
164
275
114
1S2
172
35
23 «»/
62
118
168
74
116
108
39
Hun
128
140
208
129
171
C
OX
311
371
Sll
701
28 Hi/
33
31
30
21
2U
27
22
3 June
29
3S
47
41
31
37
20
6 June
28
S8
78
60
80
61
34
14 June
37
S6
46
47
38
«S
17
17 Jul;
1S6
*3
59
<6
SI
51
13
19 June
177
103
93
63
62
80
23
27 June
S
48
41
23
36
31
SS
J Jul J
S2
32
32
42
38
39
21
1) Jul/
62
62
62
S9
45
58
13
18 Jul/
20
19
14
11
10
16
41
19 Jul/
114
124
102
89
83
102
17
Hein
47
54
Si
46
*S
C
631
561
SOI
221
Sll
22 Jul/
18
1-
13
IS
14
17
8 Aug
U
8
37
36
20
22
62
12 Aug
21
22
22
14
17
19
19
IS Aug
36
26
29
26
32
30
14
29 Aug
30
32
3<
41
24
32
19
11 Sept
27
32
38
36
49
36
23
10 Oct
32
2B
46
32
29
33
22
Hein
2S
23
31
29
26
C
3s:
411
361
371
A7l
51
-------
productivity (Table 4). As discussed below, this contrast between 1984 and
1985 may be related to differences in vertical stratification and fresh water
flow (and probably wind forcing).
1984 Ai.'D 1 985 INTERATRIAL CONTRASTS - HYDROGRAPHY, NUTRI F.NTS, PHYTOPU^TPN
A najor difference between 1934 and 19S5 the exceptionally hi^h
fresh water flow during July-August 1984 (Seliger et al. 1985). 1985 was a
drought year and freshwater flow was exceptionally low. As a consequence,
surface salinity was much higher and vertical stability lower during 1985
than during 1984 Cr , 16). Villi's this had little influence on the thickness
of the hypoxic layer or on the length of tire h\po.
-------
1
198 5
19 0 4
20
MONTH
Figure 16. Temporal variations in surface salinity, vertical stability
across the pycnocline, and the thickness of the hypoxic layer
having oxygen concentrations less than 1 mg/1 during 1984 (solid
line) and 1985 (broken line); circled point is from June 1984.
53
-------
S A L I N I T Y
9C < N 8 3 3 843 833 82 T 818
20
40
Z
~
~
~
T
~
0
0
0
0
40
50
I 0
0
Z 0
3 0
40
50
60
7 0
00
N DISTANCE, km $
Figure 17. Variations in the vertical distribution of salinity along the
mainstem transect in June of 1984 and 1985. Numbers above solid
triangles indicate CBI stations (above) and stations from this
study (below).
54
-------
OXYGEN
904
649
40
I 30
32
28
2 7
0
0
0
2 0
30
40
50
0
l 0
20
40
50
6 0
TO
90
OISTANCC. km
Figure 18. Variations in the vertical distribution of dissolved oxygen
along the mainstem transect in June of 1984 and 1985- Stations
are the same as in Figure 16.
55
-------
E UPhOTi C ZONE Chl
PROD/CHL
Figure 19. Temporal variations in the chlorophyll content of the euphotic
zone and chlorophyll specific productivity of the euphotic zone
during 1984 and 1985.
56
-------
phvtop lankton origin (due both to sinking of particulate organic matter and
to the release of dissolved organic natter by phytop lankton in the bottom
layer) into the bottom layer was probably lower during 1985. Poor coupling
between zooplankton grazing and phytop lankton production during 1984 may be
caused by oscillations of the pvcnocline and associated variations in
vertical mixing (Malone et al. 1986).
In this context, it is noteworthy that phytop lank ton production showed
little variation between 1984 and 1985 despite those differences in fresh
water flow (and associated nutrient inputs), vertical stability, and lateral
osc i 1 1 at ions of the pvcnocline. Phytop 1 .ink tun production during Ju 1 v-Oc tobo
along the CHOP-PAX transect averaged 1.20 g C/tn /d in 1984 compared lo 1.37
C/ir*"/d in 1985. *'e would also like to point out that these estimates of
summer production are substantially lower (and more reasonable when compared
to other estuarine and coastal ecosystems) than reported by Officer et al.
(1984) .
Disso lved Oxygen and Vert ica1 Strat if icat ion
While vertical stability clearly influences mixing between oxygenated
surface water and oxygen depleted bottom water, variations in the oxygen
content of bottom water were poorly related to variations in vertical stabi-
lity during 1985 (Fig. 20). Likewise, during spring, vertical stability
decreased downstream from north to south yet the rate of decrease in the
oxygen content of bottom water did not vary along the mainstem transect
(Fig. 21). However, bottom water oxygen concentration was strongly corre-
lated with temperature which accounted for 88% of the variation in bottom
water oxygen during the spring decline (Fig. 20). This suggests that
57
-------
I 2
e
4
0 « 0 6
0
0
0.2
0. 8
I 0
a b, / & 2
O
£¦
0 88
3 n
oo
0 4 8 12 16 20 24 28
T
Figure 20. Mean oxygen concentration of the bottom layer in relationship to
vertical stability across the pycnocline and to bottom water
temperature (circle, station 32; triangle, station 3; diamond,
station 24); regressions run for 12 Feb. - 16 May.
58
-------
2
a
4
0
2
a
4
0
2
T
T
T
MONTH.1985
Figure 21. Temporal variations in
oxygen in bottom layer
across the pycnocline
the mean concentration of dissolved
(solid line) and vertical stability
(dashed line).
59
-------
heterotrophic metabolism governs the rate of decline in oxygen over the
observed ranged of vertical stability in the Bay. The predominant effect of
vertical stability appears to be on the spatial extent of anoxia during the
summer .
Produc t ion and Fate o f Phvtop lank ton Biomass
Seasonal variations in phytop 1ankton biomass and productivity were out
of phase in the Bay with biomass peaking during spring and productivity
during summer. The large accumulation of biomass during February-May
suggests the possibility that the decline in dissolved oxygen during this
period was a consequence of the heterotrophic metabolism of organic matter
produced by p hy t.o p 1 ank ton over the sane period. The rapid decline in biomass
during late May may have been due to a mass mortality of phytop1ankton below
the euphotic zone caused by low oxygen concentrations and/or a rapid increase
in grazing pressure. In either event, sinking of this organic matter into
the benthos could provide a source of organic substrates for subsequent
oxygen removal during summer. The development of the summer ruxiiiium in
productivity would then depend on the regeneration of nutrients based on this
organic matter and the recycling of these nutrients into the euphotic zone as
a consequence of vertical mixing and oscillations of the pycnocline. This is
contrary to the hypothesis that there is an interannual carry over of
phytodetritus which fuels oxygen depletion in the Bay. However, our results
do not omit the possibility that all the organic matter produced in any given
year is not consumed during that year.
Nutrients and Phvtoplankton Produc t ion
Our results indicate that phytoplankton growth rates (biomass specific
productivity) were not limited by either P or N on a seasonal time scale.
60
-------
The nutrient concentrations we observed in 1985 were simply too high to limit
the rate of phytop lankton growth. This is sonewhat surprising since 1S85 was
a low flow year which should tiean that inputs of new nutrients to the Bay
were lower than they would have been had fresh water runcff been greater. If
this is the case, substantial reductions in nutrient inputs will be required
to significantly decrease phytoplarVton production in the Bay. We emphasize,
however, that our results do not preclude the possibility that phytop lankton
biotnass yield might be nutrient limited.
In this regard, it would be a mistake to conclude that the high N/P
ratios reported here and elsewhere indicate thnt N ronoval is not necessary.
Differences in the cycles of N and P and in the physical chemistry of the
compounds involved make interpretation of N/P ratios in terms of nutrient-
limited phytoplankton growth difficult.
THE 1985 ANNUAL CYCLE - MICROBIOLOGY
Eac teria1 Abundance
Temporal Changes along the Main Channel
Bacterial abundance was low in late winter, averaging less than 10^
cells/ml (Fig. 22). Bacterial numbers increased rapidly as water
temperatures increased, approaching summer abundances by late April. During
the period mid-February to late April, bacterial abundances exhibited little
change from surface to bottom, but numbers increased earlier up Bay at
stations 32 and 3 than at station 24 farther to the south.
Following a peak in late April, bacterial numbers decreased rapidly at
all three stations (Fig. 22), preceding the phytoplankton decline by 1 week
(Fig. 11, Tables 3 and 4). Bacterial abundance remained low, but began to
61
-------
13 10
Sta 32
a
Sta 03
' 1
0) 30
Sta 24
(O
ID
UJ
O
z
<
a
z
ID
CD
<
_l
<
cc
LU
h-
O
<
m
20 60 100 140 180 220 260 300
Time (days after Jan 1, 1985)
Figure 22. Time-dependent vertical variations in bacterial abundance along
a north-south (station 32 to 24) main channel transect.
62
-------
increase about one to two weeks after the phytoplankton decline. The
increase was especially noticeable at station 3. By mid-June, sunmer
conditions had become established, marked by higher bacterial numbers above
the pycnocline and lower numbers below it. During this period, deep waters
at stations 32 and 3 generally harbored larger bacterial populations than at
station 24.
Temporal Changes along Transect 2
Figures 23-27 provide a synopsis of temporal changes in bacterial
abundance along lateral transect 2 which was studied intensively during our
1984 investigation (Tuttle et al. 1985). Salinity contours for this transect
are provided to indicate the pycnocline depth and degree of stratification
(Figs. 28-32). Early cruises (Figs. 23 and 24) were often characterized by
similar numbers of bacteria in surface waters, in deep waters, and from flank
to flank. The presence of mid- and deep water maxima of bacterial abundance
were common during the late winter and spring. Examplts of this occurrence
include cruises 2 and 3 (Fig. 23), cruises 4-6 (Fig. 24), and cruises 7-9
(Fig. 25). Bacterial abundance contours which were typical of summer 1984
(Tuttle et al. 1985) did not occur along transect 2 until mid-July (Fig. 16).
These "typical" abundance contours, characterized by high bacterial numbers
on the flanks and in surface waters and by decreasing bacterial abundances
with depth, were common along transect 2 from mid-July to the end of the
study period (Figs. 26 and 27).
The transect 2 series also illustrates recovery of the bacterial
community following the mid-May bacterial decline (cruise 5, Fig. 24). Not
until 6 June (cruise B, Fig. 25), two weeks following the phytoplankton
63
-------
I J 03)
; 4
* n
1
v
-
*
/ >
-
2 M
JO
0
O
J
. \
\
*
- \
-
n
-
1 u
1 'J
V
2.
1 *
1 e
* 1
i «
20
2 y
-
7 -A
-¦
^ fc
-
20
1
JO
(^~E6/n \ '.)-
C R 2
* ^ ! : ,
'J
Z
<
z
D
CD
<
_J
<
rv
(J
<
CD
* 6 a 10 12
~ it>TANC£ K WO m wtSUHN S^owt (km)
Figure 23. Lateral distributions of bacterial abundance along transect 2
for the period 13 Feb. - 16 Apr. 1 985.
64
-------
a
x
CR5
4 a a iu 12
distance from wsstimn smohe
-------
i o -
t J -
a
*'
1 A -
X
\ A -
i
1 O -
au -
2 i -
V ^ -
V tt
2 0 -j
30 -i
? r~
n
6 n iu 1 2
OibfA^CE KRQm wtSTEHN S^OHE
Figure 25- Lateral distributions of bacterial abundance along transect 2
for the period 28 May - 19 June 1985.
66
-------
3
X
3
X
J
-
4
-
6
-
rt
-
1 o
-
l 2
-
1 4
-
i e
-
» «
-
2U
-
7 2
-
2 «
-
2
2 M
-
JO
O
o
2
-
6
-
n
-
1 u
i 2
' 4
-
' *
1 *1
2 U
2 2
-
2 A
2 ft
2rt
-
JU
CJ
o
A
V
1
*
-
M
-
1 u
-
» 2
1 4
i
1 e
i rt
-
20
..
CR10
(¦f*E6 / ri'i1)
11>
Z
<
Q
2
D
£
<
Distance FROM w£STtHN Si-iOHE (Um)
<
(j
<
m
Figure 26. Lateral distributions of bacterial abundance along transect 2
for the period 11 July - 15 Aug. 1985.
67
-------
Figure 27. Lateral distributions of bacterial abundance along transect 2
for the period 11 Sep. - 10 Oct. 1985.
68
-------
0>STAf*C£ ~NOm wtbltHN br»OHE {Urn)
Figure 28. Variations in salinity with depth along transect 2 for the
period 13 Feb. - 16 Apr. 1985.
69
-------
M
i V
V
l .*
X
1
ft
-J
1 «
^::
j j
j *
-¦ t>
'S M
J ^
O
J
«
ft
n
i ^
a
"*"
r
i ft
j
r
J
V J
V -*
^ ft
2tt
JU
(J
J
-*
ft
rt
l J
V
* -
\ 4
X
r/
t ft
-j
i *
1 o
2 U
2 'J
2 *
2 ft
2t)
JU
¦J4 - 29 - rt 3 )
(op1)
<
'J)
* 6 a lO 12
UiSTamCE KWQm Wt SItWN bnUHE (w»r»)
I 1
1 O
Figure 29. Variations in salinity with depth along transect 2 for the
period 29 Apr. - 23 May 1985.
70
-------
1 ( j
w -
* '
i -
r
-/
1 o
'
1 tt
2U - ¦
J
? *
V *>
2 M i
JO
CR9
(LJ6 - 1 -J-B3)
<
IJ)
* a fl 10 i 2
OlSTANCC »HOM WESIENn SciOWE (urn)
Figure 30. Variations in salinity with depth along transect 2 for the
period 28 May - 19 June 1985.
71
-------
CR1 2
¦J)
CUM-15-03)
a 6 a IO 12
DISTANCE FNUm wtSTLH/M inOHE (Urn)
Figure 31. Variations in salinity with depth along transect 2 for the
period 11 July - 15 Aug. 1985.
72
-------
3
X
C p p1)
Distance ~MCJm Wtb'tWN (*'»0
<
'/)
Figure 32. Variations in salinity with depth along transect 2 for the
period 11 Sep. - 10 Oct. 1985.
73
-------
decline, did bacterial numbers reach or exceed abundances which had been
present on 29 April (cruise 4, Fig. 24). This recovery was probably fueled
by dead or dying phytoplankton.
Differences with Depth
Figure 33 compares nenn bacterial abundances within and below the
eup'notic zone. Not until the end of May, one week after the phytop lankton
decline, did mean surface water abundance exceed bacterial numbers below the
euphotic zone. Over the next three weeks, bacterial abundance increased in
parallel within and below the euphotic zone, probably at the expense of
decaying phytoplankton. Summer conditions, i.e. bacterial abundance signifi-
cantly lower below the euphotic zone, were reached across the mid-Bay study
area by late June and persisted throughout the remainder of the study.
North to South Differences
Mean bacterial numbers within the euphotic zone were often greater at
the two more northern transects, but this was especially true from late June
to mid-October (Fig. 34). Mean sunnier abundances often exceeded 10^/mI at
either transect 1 or 2 on a given cruise date, but never exceeded 8xl0^/ml at
transect 3 over the time span of the last five cruises. The differences were
less pronounced below the euphotic zone (Fig. 35), but mean bacterial numbers
at transect 1 or 2 always exceeded mean bacterial numbers at transect 3 from
early June to mid-October.
East to West Differences
In accordance with depth and with north-south transect location, mean
bacterial abundance was nearly uniform laterally until late May (Fig. 36).
Following the phytoplankton decline over the study area, however, mean
bacterial numbers were greater over the flanks than over the deep channel.
74
-------
I4<-
r-
fcr-
MCNTh
Figure 33. Mean bacterial abundances within () and below (0) the euphotic
zone for transects 1-3 during 1985. Bacterial abundances are
expressed in millions of bacterial ml~*.
75
-------
IZr
o
X
8r
i
£
/ O
I 6<-
ui ;
U
2
MONTH
Figure 34. Mean bacterial abundances within the euphotic zone along
transects 1 (A), 2 (0), and 3 O during 1 985. Bacterial
abundances are expressed in millions of bacterial ml~*.
76
-------
MONTH
Figure 35. Mean bacterial abundances below Che euphotic zone along
traosecC s 1 (A), 2 (0), and 3 O) during 1985. Bacterial
abundances are expressed in millions of bacterial ml
77
-------
to
O
E
.2
w
0)
o
OJ
cc
1985
Figure 36. Mean bacterial abundances along the west flank (A), main channel
(0), and east flank
-------
From mid-June until the end of the study, abundances tended to be highest
over the eastern flank.
Bacterial Size
As we counted bacteria to estimate abundance, we divided the microor-
ganisms observed into the following size classes: <1 um, 1 to 2 urn, and
>2 urn. For our purposes here, we consider only two size classes, namely
bacteria less than or greater than 1 um.
Early in the year significant portions of the bacterial community were
>1 um (Figs. 37 and 38). The proportion of these larger bacteria decreased
until by mid-May, most of the bacteria observed were smaller than 1 un. This
pattern of size distribution was consistent for northerly and southerly
transects and within and below the euphotic zone (Figs. 37 and 38). The
proportion of larger bacteria was greatest at transect 2. The proportion of
larger to smaller bacteria at transect 2 increased as bacterial abundances
increased following the phytop 1ankton decline in late May. This change in
size proportions was independent of depth (Figs. 37 and 38). A similar but
smaller change occurred at transect 3.
During summer conditions, bacteria <1 um consistently comprised 85 to
95£ of the bacterial community. Except in early June when the proportion of
larger bacteria increased from east to west, little difference in bacterial
size was observed from flank to flank (Fig. 39). The reason for these
changes in bacterial size are unclear. One possible explanation is that
increasing bacterial growth rates throughout the spring as water temperature
increases select for smaller sized bacteria. On the other hand, we may
simply have observed shifts in the bacterial species composition.
79
-------
VOOr
/
W
MONTH
Figure 37. The percent of total bacterial abundance within the euphotic
zone attributable to bacteria smaller than 1 um along transects
1 (A), 2 (0), and 3 (~) during 1 985.
80
-------
iOOr
80-
70-
Q
6C-
c
1-50-
40-
30-
20-
10-
MONTH
Figure 38. The percent of total bacterial aoundance below the euphotic zone
attributable to bacteria miliar than 1 um along transects 1
(A), 2 (0), and 3 O) durinc 1985.
81
-------
90-
8C-
70-
60-
40-
A
30-
20-
10
HONTh
Figure 39. The percent of total bacterial abundance attributable to
bacteria smaller than 1 um along the west flank (A), main
channel (0), and east flank O) of Chesapeake Bay during 1985.
Percentages are calculated from means of data from all three
lateral transects.
82
-------
Bacterial Produc t ion
Temporal.Changes along the Main Channel
In agreement with bacterial abundance, bacterial production was low in
late winter with TdR averaging about 10 pmol/l/h (Fig. 40). Production
increased rapidly in the spring with increasing water temperatures , almost
reaching summer levels by late April. At this time, mid and bottom water TdR
often equaled or exceeded summer values. Bacterial production decreased
abruptly in mid-May at all three deep stations, coincident with decreases in
bacterial abundance and one week before the rapid reduction in phytop lank ton
biomass and production over the entire study area. The decreases in
bacterial production vure greater at station 24 than at either of the niore-
no r t hern stations.
Rapid recovery of bacterial production followed the phytop lankton
decline, particularly in near surface waters at station 32 (Fig. 40). High
surface water production (TdR >200 ptr.ol/l/h) was characteristic of station 32
from late May to mid-July and from late August to late September. Farther
south at station 3, high surface water production extended from early June to
late August. High production in surface waters was less frequent at station
24. Episodes of very high, near surface water production occurred at all
three deep stations, but were not necessarily coincident along the main axis
of the Bay.
Bacterial production in deep water was usually highest to the north at
station 32 where TdR often equaled or exceeded 100 pmol/l/h from mid-April to
mid-September (Fig. 40). A notable exception to this pattern occurred during
July and early August just before the onset of anoxia (Fig. 9). Deep water
bacterial production at station 3 was similar to station 32 except that
83
-------
a
00 100
Sta 32
10 20
50 20
LJ
Sta 03
Sta 24
o
£
a
z
O
H
o
D
Q
O
GC
0.
I
<
cc
LU
H
O
<
CD
20 60 100 140 180 220 260 300
Time (days after Jan 1, 1985)
Figure 40. Time-dependent vertical variations in bacterial production along
a north-south (station 32 to 24) main channel transect. Values
on contour lines represent TdR in p mol L h
84
-------
production in near bottom water was usually lower farther north (i.e. at
station 32) and TdR isoclines closely followed salinity contours (Fig. 3)
during the summer. At station 24, bacterial production below the pycnocline
(10-12 m depth) never exceeded TdR values of 50 pmol/l/h.
A synopsis of bacterial production profiles across lateral transect 2 is
shown in Figures 41-45. Early in the year bacterial production exhibited
little in the way of a pattern with TdR in deep channel water likely to be as
high as in surface water on the flanks (Fig. 41). Later in the spring the
response of bacterial production to the spring phy top 1 .ink t on bloom and the
subsequent reduction in bacterial production are clearly evident (Fig. 42).
Within 12 days after the bacterial decline and within 7 days after the
phytoplankton decline (cruise 7, Fig. 43), bacterial production had recovered
to spring bloom levels and thereafter continued to increase into the summer.
By early July (Fig. 44) summer conditions had become established, marked by
high bacterial production in surface waters and on the flanks and much lower
production in the deep waters, decreasing with depth at deep channel
stations. From late April (Fig. 42) on, bacterial production isoclines
followed salinity contours (Figs. 29-32, 42-45). A noticeable decrease of
bacterial production was observed from mid-September to mid-October
(Fig. 45), signaling the end of summer conditions.
Differences with Depth
As we observed with bacterial abundances (Fig. 33), mean bacterial
production early in the year was similar within and beneath the euphotic zone
(Fig. 46). However, mean bacterial production within the euphotic zone began
to exceed production below the euphotic zone nearly two weeks before
85
-------
Dlh'AM-K ~HUM Situ** (Ur»>)
Figure 41. Lateral distribution of bacterial production along transect 2
for the period 13 Feb. - 21 Mar. 1985.
86
-------
( p m ol/'*hr )
3
X
<120
DO-r-
Figure 42. Lateral distribution of bacterial production along transect 2
for the period 29 Apr. - 23 May 1985.
87
-------
-
4
-
6
-
M
-
1 CJ
-
1 ^
_
1 4
-
i ft
t rt
-
-
2 2
-
2 ¦*
2 ft
-
^ *1
-
--
u
o
2
~1
4
-
*
-1
n
-
i o
-
> 2
" '
\ +
X
i ft
-
' M
-
2 c;
j j
-¦
2 +
-¦
2 ft
-1
2M
-t
I
JU
o
o
-r
2
"
4*
ft
rt
-
) C)
-¦
» 2
-1
3
r
1 *
1 0
I
i
*
1 tt
i
20
2 2
- \
2 -4
2 6
-
2 a
A
JO
l
-r-
<175
<250
CR8
6 - : o n.lj
0
0
D
a
o
<
or
Li
r~
u
<
CD
a a a 1 2
DISTANCE FMOM WtiTtHN Si-iO HE (hfr>)
Figure 43. Lateral distribution of bacterial production along transect 2
for the period 28 May - 19 June 1985.
88
-------
¦ 2
-
i 4
-
tt
-
; m
-¦
-
-¦
v *
-
-
M
-
l;
u
J
v
4
\
- \
fl
\
1 ^
-
1 4
--
' f\
-
< n
-¦
VCJ
-
J 2
7 *
-
^ ©
-
^ n
-
JO
CR 1 O
(pmol/l + lir )
1 u
1
V
1 J
1 4
"1
z
n
1 6
1 O
!
1
H
2CJ
-{
V 2
n
2 ¦*
t
2C
-j
2 £3
'1
30
t
-r
0
u
J
G
0
V
Q
<
'1
' , I
(J
<
CD
i 1 1 i r 1 1 rr
* e o io 12
GlS TanCC FRQm wk'STEHN SmOWt (km)
-I 1
Figure 44. Lateral distribution of bacterial production along transect 2
for the period 11 July - 15 Aug. 1985.
89
-------
J
*
-
6
-
a
-
1 u
-
V
1 V
-
T
«
1 rt
"1
2 O
-1
23
-
2 *
-¦
2 b
-
V PI
JU
- r-
o
T
2
1
-i
6
-i
B
-
y u
-
i V
-
X
-i
1 ft
i «
-
2 a
~i
j j
-
2 *
-
2 e
2 CI
i
3 O
o
prr <
f p rn (jl/l * h r
1 0 12
Z
0
u.
0
D
C)
0
<
(V
u
.<
lTj
t r ¦ i » i 1
1 O 12 1 *
^ 6 n
OI S T A M C t KHOm wkiTtHN SnOMC
-------
uo-
Figure 46. Mean bacterial production, expressed as TdR, within () and
below (0) the euphotic zone for transects 1-3 during 1985.
91
-------
bacterial abundances exhibited the same pattern. The time difference between
the changes in within and beneath euphotic zone ratios of bacterial biomass
and production nay be indicative of an increase in the biomass specific
growth rate of euphotic zone bacteria. Alternatively, bacteria below the
euphotic zono may have been subjected to increased removal pressure (e.g.
predat ion) .
Following the mid-May decline of bacterial production, euphotic zone
production oscillated throughout the remainder of the study (Fig. 46). Peaks
of production occurred in late June, late July and in id-September. Four peaks
w£re found below the euphotic zone, three of which were co inc idi.-nt with
euphotic zone maxima and a fourth which occurred in late May during the
phytoplankton decline (Tables 3 and 4). The frequency of oscillation and/or
timing of actual peaks tnay have been different than the data indicate. More
frequent sampling intervals (such as the twice weekly "sTmplirig intervals of
1984) would have been necessary to resolve this problem.
North to South Differences
Bacterial production was generally highest farthest to the north from
February to mid-July (Fig. 47). This was true both within (Fig. 48) and
below (Fig. 49) the euphotic zone. Mean bacterial production below the
euphotic zone was greater at either transect 1 or 2 than at the southern-most
transect 3. When expressed as transect means (Figs. 48 and 49), the produc-
tion data from mid-May to mid-October indicate three coincident peaks within
and below the euphotic zone at transect 1, two of which (mid-June and mid-
September) also occurred along transects 2 and 3. The third peak in late May
occurred only below the euphotic zone at the two southern-most transects
(Fig. 49).
92
-------
2*0
200
too
40
Figure 47. Mean bacterial production, expressed as TdR, along transects 1
(A), 2 (0), and 3 (D) during 1985.
93
-------
280
80-
40
Figure 48. Mean bacterial production, expressed as TdR, within the euphotic
zone along transects 1 (A), 2 (0), and 3 (D) during 1985.
94
-------
22a
MONTH
Figure 49. Mean bacterial production, expressed as TdR, below the euphotic
zone along transects 1 (A), 2 (0), and 3 (~) during 1985.
95
-------
East to West Differences
Mean bacterial production was relatively constant across the Bay until
the spring phytop lank ton bloom in late April to mid-May when production was
greatest along the eastern flank (Fig. 50). Following the mid-May bacterial
decline, however, bacterial production tended to be much greater over both
flanks than over the deep channel (with the exception of the east flank along
transect 2 in late July). Unlike bacterial abundance (Fig. 36), bacterial
production in the late spring to nid-sunr.er was often greater to the west
than to the east (Fig. 50). During mid-August to mid-September, however,
production to the east exceeded production to the west, consistent with the
same time period in 1984 (Tuttle et al. 1985). In contrast to 1984,
bacterial abundance was also higher to the east than to the west in the late
summer (Fig. 36).
Am ino Ac id Metabo 1 ism
Temporal Changes along the Main Channel
Amino acid metabolism (expressed as turnover rate) was low in late
winter but began to increase rapidly in the spring (Fig. 51) in a pattern
similar to bacterial abundance (Fig. 22) and production (Fig. 40). This
increase was most rapid at station 32 until mid-April when contours were
similar for-station 24. High amino acid turnover rates accompanied the
spring phytoplankton maximum with the greatest responses in mid and deep
waters at stations 3 and 24. Turnover rates observed at these stations were
the maximum values measured during the study. While amino acid metabolism at
station 32 also increased, the response was less than at the more southerly
mainstem stations except in surface waters. The bacterial decline in mid-May
96
-------
320
280
240
it 200
o
| 160
cc
T3
H
120
80
40
1985
Figure 50. Mean bacterial production, expressed as TdR, along the west
flank (A), main channel (0), and east flank O) of Chesapeake
Bay during 1985. Values are means of data from all three
lateral transects.
97
-------
11 I I
STATION 2^
MAR ! APR MAY
auc.
ccr
2
ifi
i
o
CD
<
H
U u.
^ -
\
o *
. w
u
<
0
z
1
<
100 1 -»0 1 BO 220
TIME (day* aft«r Jan 1. 19H5)
2 60
300
Figure 51. Time-dependent vertical variations in amino acid metabolism
along a north-south (station 32 to 24) main channel transect.
Solid triangles at the top indicate cruise dates. Values on
contour lines represent amino acid turnover rates expressed as %
of amino acid pool h .
98
-------
was accompanied by greatly decreased amino acid metabolism ail along the
north-south transect. Unlike bacterial production (Fig. 40), amino acid
metabolism failed to recover to levels measured during the spring
phytoplankton bloom except at station 32, and only there in surface waters or
near the pycnocline. During summer conditions, subpycnoc1ine amino acid
metabolism did not exceed 1 to 2S/h except in mid-September at stations 32
and 3. The mid-water maxima found then at these stations were not observed
at station 24.
Amino acid metabolism across transect 2 increased from February to
March, decreased in mid-April and then increased to maxicium levels in late
April during the spring phytop lankton maximum (Figs. 52 and 53). The highest
turnover rates were measured in mid and deep water, but rapid metabolism also
occurred near the western shore and across the eastern flank (Fig. 53). The
yearly maximum in metabolic rates measured on 29 April were followed by a
sharp decline of amino acid metabolism on 16 May during the decline in
bacterial abundance (Fig. 24) and production (Fig. 42). Within one week
(cruise 6, Fig. 53), amino acid metabolism increased in surface and mid-
waters coincident with the dramatic phytoplank ton decline observed on the
same day. However, turnover rates on this and on subsequent cruises through-
out the remainder of spring and summer (Figs. 54-56) never reached values
measured on 29 April.
The relationship of amino acid metabolism (Figs. 52-56) to salinity
(Figs. 28-32) was irregular. On cruises 4 (Figs. 29 and 53), 9 (Figs. 30 and
54), 10 (Figs. 31 and 55), 13 and 14 (Figs. 32 and 56), amino acid metabolism
tended to follow salinity contours. On cruises 8 (Figs. 30 and 54) and 11
(Figs. 31 and 55) amino acid metabolism isoclines appeared to be in opposi-
99
-------
\ ,C
1 n
1 2
1 ¦*
i to
1 «
2U
2 2
2 *
2 b
2 «
JO
O
2
M
i CJ
1 2
1 *
' h
' M
2 C>
2 2
2 -*
2 «
2 n
M)
u
2
1 C)
y 2
1 -4
i r»
1 m
20
7*?
2 -»
2 6
2 0
JO
t. u -.' - i nr.)
(S«/ M r )
I r~
1 O
as
CR3
>L
CO
_l
0
m
<
u.
L!
5
o
6
<
0
7
>
<
* e a i o '2 i
UibltNCE FROu WIS1E»N SmOW£ (urn)
Figure 52. Lateral distribution of amino acid metabolism, expressed as
amino acid turnover rates, along transect 2 for the period 15
Feb. - 21 Mar. 1985.
100
-------
" l'
-
b
-
M
-
i a
-
i 2
-
1 «
-
1 t>
-
i a
-1
20
-
2 2
-
2 *
-
2b
-¦
2fl
-
JO
u
3
~K
2
J\
a
-¦
f!
-
1 ^
-
1 *
-
1 t>
-
fi
-
y J
-
2 «*
-
26
~!
2rt
i
JU
!
t
o
o
-H
2
-
a
-
c>
-
M
-
y u
-
1 2
-
\ A
-
1 6
1 M
2 (J
-
2 2
-
2 <*
-
2b
-
2«
JU
-r-
O
('>') /* h r }
C R 5
/
(0:)- 1 to tl D )
>. _ &
5
VI
_l
0
3
<
r~
Ll
\
4,
o
6
<
0
y
]>
<
I T 1 J T " »'
2 ' * «
Oi S r AnC£ KWOm vstbTt^'s S«ONt ( u m )
1 1 r--
a i o
1 r
12 1 ¦
Figure 53. Lateral distribution of amino acid metabolism, expressed as
amino acid turnover rates, along transect 2 for the period 29
Apr. - 23 May 1985.
101
-------
Figure 54. Lateral distribution of amino acid metabolism, expressed as
amino acid turnover rates, along transect 2 for the period 28
May - 19 June 1985.
102
-------
('AS/hr ^
n
i o
y J
:»
*¦ "
\ *
X
i b
J
'
1 M
V
J J
J *
-
y b
-
-
JO
-t
C R 1 1
2
¦71
I
0
u)
<
(-
Ll
<
0
7
2
4
A 6 n 10 12
OiSTamCC from wtSTtMN b>-iO«e (Witi)
? igure 55. Lateral distribution of amino acid metabolism, expressed as
amino acid turnover rates, along transect 2 for the period 11
July to 15 Aug. 1985.
103
-------
J *1
\:)
'J U
~ ~
/ -*
V t>
J n
) CJ
Cr.\) - i - tsa >
('>;/' Mr)
i i; w
1 «
"7
/
/
>
S
j
o
3
<
'J
5
<
0
2
<
Figure 56. Lateral distribution of amino acid metabolism, expressed as
amino acid turnover rates, along transect 2 for the period 11
Sep. - 10 Oct. 1985.
104
-------
tion to salinity contours.
Differences with Depth
Mean amino acid turnover rates for the entiie study area are shown in
Fig. 57. In agreement with bacterial production estimates (Fig. 46), amino
acid metabolism was most rapid beiow the euphotic zone until the mid-Ilay
bacterial decline when it became higher in the euphotic zone and remained so
throughout the study period. The amino acid metabolism data were charac-
terized by a series of oscillations with 5 peaks, the largest of which
coincided with the spring phytop lank ton bloom. Ail these peaks occurred
s imu 11 jr.L'ous 1 y above and below the euphotic zone and coincided with peaks of
bacterial production (rig. 46).
North to South Differences
From February to mid-April, mean amino acid turnover rates were highest
along transect 1 within and below the euphotic zone (Figs. 58 and 39). This
was also true of the late June to mid-August period, but the differences
between rates at transects 1 and 3 were small, particularly within the
euphotic zone. All five amino acid metabolism peaks were coincident at
transects 2 and 3 within and below the euphotic zone.
Mean amino acid metabolism at transect 1 did not always coincide with
the two transects to the south. For example, the mid-September peak across
transects 2 and 3 did not occur at all in the euphotic zone at transect 1
(Fig. 58) and was only a minor peak below the euphotic zone (Fig. 59). The
late May euphotic zone peak at transect 1 occurred five days later than at
transects 2 and 3 (Fig. 58).
105
-------
u
10-
9
8
7
<
4
3
2
1
l
l
i
MONTH
Figure 57. Mean amino acid metabolism, expressed as amino acid turnover
rates, within () and below (0) the euphotic zone for transects
1-3 during 1985.
106
-------
MONTH
Figure 58. Mean amino acid metabolism, expressed as amino acid turnover
rates, within the euphotic zone along transects 1 (A), 2 (0),
and 3 (~) during 1985.
107
-------
MONTH
Figure 59. Mean amino acid metabolism, expressed as amino acid turnover
rates, below the euphotic zone along transects 1 (A), 2 (0), and
3 (D) during 1985.
108
-------
The fact that the 29 April peak was substantially greater at the
southerly transects than at transect 1 (Figs. 58 and 59) may be significant
with respect to bacterial production means measured on the same day (Figs. 48
and 49). Comparison of bacterial production with amino acid metabolism at
each of the transects suggests that amino acid metabolism was uncoupled from
bacterial production, especially at transects 2 and 3; that amino acid pools
were lower in late April than during the summer when bacterial production was
higher but amino acid turnover was lower; or that the bacterial community at
transect 1 was also using carbon and energy sources other than amino acids to
support growth. We have insufficient information to decide among these
possibilities.
We did not measure amino acid respiration during 1985. However, respi-
ration measurements made during summer and fall cruises in 1984 indicated the
respiration accounted for 40% of total amino acid metabolism (Tuttle et al.
1985). Summer 1985 amino acid turnover rates corrected for respiration are
similar to the rates found in summer 1984.
East to West Differences
From late April throughout the remainder of the study, mean amino acid
metabolism along the flanks exceeded metabolism over the main channel
(Fig. 60). After the mid-May bacterial decline the differences along the
eastern flank compared to the western flank were small, but mean amino acid
turnover to the east exceeded that to the west at each of the peak events.
Glucose Metabolism
Our 1984 amino acid metabolism data correlated well with chlorophyll a.
and bacterial production estimates (Tuttle et al. 1985). This suggested that
amino acids were a reasonable surrogate for phytoplankton carbon capable of
109
-------
34f
32-
A
14-
8
4
2
M
M
A
J
A
J
5
O
MONTH
Figure 60. Mean amino acid metabolism, expressed as amino acid turnover
rates, along the west flank (A), main channel (0), and east
flank O of Chesapeake Bay during 1985. Values are means of
data from all three lateral transects.
110
-------
being utilized for microheterotroph production during late August to early
November 1984. Nevertheless, our 1985 study was conducted over a longer time
period, thereby increasing the likelihood that phytoplank ton community struc-
ture and thus carbon and energy sources for bacterial production could
change. We added glucose metabolism determinations to our 1985 measurements
to assess the possibility that bacterial growth and metabolism in Chesapeake
Bay waters might be fueled by organic carbon and energy sources other than
amino acids.
Temporal Changes along the Main Channel
We began measurements of glucose metabolism in mid-April (Fig. 61).
Glucose turnover rates were already substantial at this time with particu-
larly high values at station 32 at all depths and in mid-water at station 24.
The spring phytoplankton maximum (Fig. 11) coincided with high glucose turn-
over ratesin deep' water, raost~notably up Bay at stations 32 and 3. Deep
water rates to the south at station 24 were lower, but very high rates of
glucose turnover were found in surface and mid-water at this location.
Glucose metabolism decreased in concert with the bacterial decline in
mid-May. Its recovery, coinciding with the phytoplankton decline, was rapid
but turnover rates in deep water never again reached the values found in late
April (Fig. 61). Surface and mid-water rates, however, remained high until
late July (when measurements had to be discontinued at stations 32 and 3) or
mid-August (station 3). Summer deep water glucose metabolism tended to be
greater at stations 32 and 3 than at station 24 to the south.
A cross-Bay synopsis of glucose metabolism at transect 2 is depicted in
Figures 62-66. In mid-April (Fig. 62) glucose turnover rates were highest
111
-------
\o Data
No Data
T] ri I I n I nr
' ' ^2°-oo 1 20 ,15
No Data
Sl'A'l I ON 3
3 O
1 o
1 0 -
20
JO -
40 -
No Data
No Data
2
-------
At,
/'
< j i=; 3
(- i f> (id)
C *<*'» \ \ r j
+ ti ri \ O i 1
Ui^ t ANCr. ~ h (J »,» w r !si> »-»«s. <*.»»)
Figure 62. Lateral distribution of glucose metabolism, expressed as glucose
turnover rates, along transect 2 for 16 April 1985.
113
-------
Figure 63. Lateral distribution of glucose metabolism, expressed as glucose
turnover rates, along transect 2 for the period 29 Apr. - 23 May
1985.
114
-------
t-t \
'J (;
'J v
x I i r ^
<
.it;
cj
i ^
I «5
*. n
/ /
j *
j «>
^fi
JO
CR9
l *
r
-» r
^ ti rt 1 O 1 V
Ulb I ANwt ~*WOM WtSItMfS. SwOMt (NffU
Figure 64. Lateral distribution of glucose metabolism, expressed as glucose
turnover rates, along transect 2 for the period 28 May - 19 June
1985.
115
-------
Figure 65. Lateral distribution of glucose metabolism, expressed as glucose
turnover rates, along transect 2 for the period 11 Jul. - 19
Jul. 1986.
116
-------
C - x
7
h
M -
1 O -
t 'J -
\ * -
1 f* -
1 M
i»U -
J V - ¦
V * -
J fi
n -¦
3 U -i -
O
(J k
-V - \
1 c
i v ¦
» A
1 to -
t « -
v'o -
J J -I
a * -
^ « ¦
V M -
JC) <
(%/- h r ;
CRU
2
J)
j
0
D
<
h
U
'J
0
0
0
D
J
0
* n a i a w
Olb fANCk ~'MOM AKbltMN < I* r r» >
Figure 66. Lateral distribution of glucose metabolism, expressed as glucose
turnover rates, along transect 2 for the period 11 Sep. - 10
Oct. 1985.
117
-------
near the main channel of the Bay with maximum rates of 6-7%/h. Glucose
turnover increased greatly by late April (Fig. 63), in concert with the
spring phytop lank ton maximum (Tables 2 and 3) and peaks of bacterial abun-
dance (Fig. 24), production (Fig. 42), and amino acid metabolism (Fig. 53).
Highest rates occurred in mid and deep water and along the eastern flank
(Fig. 63). Glucose metabolism decreased dramatically (5 to 10-fold in mid
and deep water) in mid-May (cruise 5, Fig. 63) with the bacterial decline,
but nearly doubled one week later in the surface water as phytoplankton
biomass and production decreased (Tables 2 and 3). Following a second
decline at the end of May, glucose turnover increased again into mid-June.
Bottom water glucose metabolism on 19 June (cruise 9, Fig. 64) was exceeded
only by rates found during the spring phytop 1ankton bloom.
Summer conditions predominated in July throughout the remainder of the
study, i.e. high rates of glucose metabolism were found in near surface
waters and lower and decreasing rates occurred beneath the pycnoc1ine
(Fig. 65 and 66). Unlike all the other bacterial parameters measured, how-
ever, this "summer" pattern was observed as early as 23 May and persisted
through May and June (Figs. 63 and 64). Glucose metabolism decreased in
September and October as water temperatures began to decline (Fig. 66).
Except for cruise 3 (Fig. 62), cruise 7 (Fig. 64), and possibly cruise 5
(Fig. 63), glucose turnover isoclines were similar to salinity profiles
across the transect (Figs. 29-32). In this respect, glucose metabolism
tended to follow bacterial production (Figs. 42-45) more closely than did
amino acid metabolism (Figs. 53-56).
Differences with Depth
Mean glucose turnover rates below the euphotic zone exceeded rates
118
-------
within the euphotic zone until the mid-May bacterial decline (Fig. 67). The
timing of this change was consistent with bacterial production (Fig. 46) and
amino acid turnover (Fig. 57), but occurred two weeks earlier than the shift
in bacterial abundance (Fig. 34). However, the mid-May change occurred only
at transects 1 and 2 (Figs. 68 and 69). Across transect 3, mean glucose
turnover within the euphotic zone was always greater than below it.
North to South Differences
From early June to late July when measurements at transects 1 and 3 had
to be discontinued, mean glucose metabolism at the nortliern-nost transects
was usually higher than at transect 3 (Figs. 68 and 69). This was not true
in late April when glucose metabolism at transect 3 within the euphotic zone,
but not below it, exceeded glucose metabolism at the more northern transects.
Glucose metabolism nearly always exceeded amino acid netabolism in both
surface and bottom waters, often by a factor of 2 or more. Assuming that
natural glucose and amino acid concentrations in Bay water were similar, we
suggest that carbohydrates were preferred over amino acids as carbon and
energy sources for microheterotrophic growth and metabolism. However, this
cannot be proven in the absence of measurements of carbohydrate and amino
acid concentrations.
Beginning with the bacterial decline in mid-May, mean glucose metabolism
on the flanks exceeded glucose metabolism over the deep channel (Fig. 70).
Ho discernible pattern was observed in east and west flank glucose turnover
rates.
Oxygen Consumpt ion
Mean oxygen consumption data for the mesohaline portion of the Bay are
119
-------
50r
40-
-C
10-
FMAMJJA50
MONTH
Figure 67. Mean glucose metabolism, expressed as glucose turnover rates,
within () and below (0) the euphotic zone for transects 1-3
during 1985.
120
-------
40-
1
FMAMJJA50
MONTH
Figure 68. Mean glucose metabolism, expressed as glucose turnover rates,
within the euphotic zone along transects 1 (A), 2 (0), and 3 Q)
during 1985.
121
-------
5C-
40-
10-
MONTH
Figure 69. Mean glucose metabolism, expressed as glucose turnover rates,
below the euphotic zone along transects 1 (A), 2 (0), and 3 (~)
during 1985.
122
-------
50-
40-
10-
MONTH
Figure 70. Mean glucose metabolism, expressed as glucose turnover rates,
along the vest flank (A), main channel (0), and east flank O
of Chesapeake Bay during 1985. Values are means of data from
all three lateral transects.
123
-------
plotted in Fig. 71. Apart from the decrease in oxygen consumption rates
within the eup'notic zone from mid-February to mid-April, the data indicate an
increasing trend in oxygen consumption rates culminating in a late July to
mid-August euphotic zone peak and peaks below the euphotic zone in early June
and late July. Higher mean oxygen consumption rates below the euphotic zone,
calculated by averaging to 15m depth compared to averaging to the bottom,
reflect decreases in oxygen consumption rates with depth as well as elevated
oxygen consumption rates near the pycnocline.
Elevated euphotic zone oxygen consumption rates from late April to late
Hay coincide with the spring phytoplankton bloom (Table 3) while the summer
euphotic zone oxygen consumption maximum corresponds with high primary
production during the same time period (Table 4). Despite this, there are no
clear-cut relationships among the deep water oxygen consumption maxima and
bacterial or phytoplankton parameters on a cruise-by-cruise basis. However,
bacterial abundance and activity tended to be uniformly high during the
summer period. We cannot rule out the possibility that organisms other than
bacteria might have been responsible for a portion of the observed oxygen
consumpt ion.
Oxygen consumption below the euphotic zone was usually greatest at the
northern-most and least at the southern-most lateral transect (Fig. 72).
Peak mean values along transect 1 in early June and transect 2 in early July
do not correspond with peaks in bacterial or phytoplankton parameters. How-
ever, the generally high deep water oxygen consumption rates which occurred
along transects 1 and 2 are consistent with high phytoplankton production and
high bacterial abundances and activity characteristic of summer conditions.
Higher oxygen consumption rates along the northern-most transects are also
124
-------
zo
2
0i-
MONTH
Figure 71. Mean oxygen consumption within the euphotic zone (), beneath
the euphotic zone to the benthos (0), and beneath the euphotic
zone to 15m depth (A) during 1985. Values are means of data
from all three lateral transects.
125
-------
2.4r
2.0
1.6
O)
E
I 1.2
a
£
3
(A
C
o
O
(M
o
0.8
0.4
1 1 1 i i i i i i
FMAMJJASO
1985
Figure 72. Mean oxygen consumption beneath the euphotic zone along
transects 1 CA), 2 (0), and 3 (~) during 1985.
126
-------
consistent with oxygen depletion observed at the deep stations throughout the
study period (Figs. 9 and 21).
Despite improvements in our experimental procedures for measuring oxygen
consumption rates (see SECTION 4) we still observed, as in our 1984 data
(Tuttle et al. 1985), considerable scatter in the oxygen consumption determi-
nations. However, when we aggregated oxygen consumption data according to
time periods based upon season and degree of stratification (1984 data,
Tuttle et al. 1985) or by seasonal periods only (1985 data) and further
separated the resulting aggregates into euphotic zone and below euphotic zone
mearts, we found surprisingly good agreement between mean oxygen consumption
rates and bacterial abundances (Fig. 73). Attempts to repeat this exercise
with bacterial production estimates and bacterial metabolism measurements
were unsuccessful. The apparent relationship between oxygen consumption rate
and bacterial abundance required further statistical evaluation. Neverthe-
less, the ability to estimate oxygen consumption by measuring bacterial
abundance, a much easier and more precise determination than directly
measuring oxygen consumption, could prove to be a useful addition to Bay
monitoring efforts.
Carbon Sources for Bacteria and Oxygen Consumption
We have previously discussed variations in phytoplankton biomass and
productivity in terms of phytoplankton dynamics. In this section, we focus
on phytoplankton as a source of carbon fueling bacterial metabolism and
oxygen consumption.
Chlorophyll
Mean chlorophyll concentrations within and below the euphotic zone for
127
-------
0.07 r
0.06
0.05
0.04
0.03
0.02
C.01
y - 0.0045x + 0.0066
r « 0.95
0
''''
jii i i i i i i
0
8 10 12 14
Bacterial Abundance, cells ml-1 (x106)
Figure 73. Linear regression of mean oxygen consumption on bacterial
abundance. Symbols: (A) euphotic zone means for the time
periods 20 Aug. - 11 Sep., 14 Sep. - 3 Oct., and 5 Oct. - 2 Nov.
1984; () beneath euphotic zone means for 1984 data collected in
the time periods given above; (A) euphotic zone means for the
time periods 12 Feb. - 17 Apr., 29 Apr. - 7 June, and 19 June -
12 Sep. 1985; (0) beneath euphotic zone means for 1985 data
collected in the time periods given above. Means were
calculated using data from all transects.
128
-------
each cruise are plotted in Figure 7&. Early in the year, mean chlorophyll
below the euphotic zone exceeded euphotic zone chlorophyll. This pattern
reversed in late April and mean euphotic zone chlorophyll thereafter
continued to exceed mean chlorophyll concentrations found below the euphotic
zone .
Two distinct chlorophyll maxima were observed within the euphotic zone
(Fig. 74). The first of these, extending from mid-April to mid-May, was
indicative of the spring phytoplankton bloom (Tables 3 and 4) while the
second, occurring in late July, represented a summer production maximum.
Below the euphotic zone, however, chlorophyll a_ concentrations decreased
consistently from mid-April to late May and then remained relatively low
through mid-October. This pattern suggests that removal of intact phyto-
plankton below the euphotic zone by zooplankton grazing and/or by bacterial
degradation increased after mid-April.
The variations of mean chlorophyll content of the euphotic zone were
qualitatively similar to each of the east-west transects (Fig. 75A). Below
the euphotic zone, however, significant inputs of chlorophyll occurred during
the summer at transect 1 (Fig. 75B). During both the spring and summer
maxima of phytoplankton bioraass, higher concentrations of chlorophyll were
found at transect 1 than at transect 3 (Figs. 74A and 74B). This relation-
ship was true both within and below the euphotic zone.
From late April to mid-August, mean chlorophyll concentrations over the
Bay flanks were greater than over the main channel (Fig. 76). This pattern
is consistent with measurements of bacterial abundance (Fig. 36), bacterial
production (Fig. 50), and bacterial metabolism (Figs. 60 and 70). However,
the same was not necessarily true of primary productivity (Table 4) or of
129
-------
40
d 30
W
F
A
M
M
MONTH
Figure 74. Mean chlorophyll ^concentrations within () and below (0) the
euphotic zone during 1985. Values are means of data from all
three lateral transects.
130
-------
40
30-
40r
30-
10-
1
F
M
A
M
J
J
S
0
A
MONTH
Figure 75. Mean chlorophyll a. concentrations within (A) and below (B) the
euphotic zone across transects 1 (A), 2 (0), and 3 (~) during
1985.
131
-------
50
40
o>
3.
30
>»
20
10
1985
Figure 76. Mean chlorophyll a. concentrations along the west flank (A), main
channel (0), and east flank O) of Chesapeake Bay during 1985.
Value6 are means of data from all three lateral transects.
-------
euphotic zone chlorophyll (Table 7) measured across transect 2. The apparent
discrepancy is likely due to the fact that the means plotted in Figure 76
included measurements, made within and below the euphotic zone and at all
three transects, whereas the data compiled in Tables 4a and 7 represent
transect 2 measurements made only within the euphotic zone.
Phaeop igments
Mean phaeopigment concentrations (Fig. 77) tended to follow mean chloro-
phyll concentrations through time (Fig. 74). 5oth spring and summer peaks
were observed within the euphotic zone, but the magnitude of the mid-July
pHaeopignent peak relative to the spring phaeopigment maximum was greater
than expected from the ratio of mean summer maximum to mean spring maximum
chlorophyll (Fig. 74). We attribute this difference to increased metabolism
of phytoplankton biomass within the euphotic zone during the summer on the
basis that high phaeopigment to chlorophyll ratios were indicative of phyto-
plankton decay. Our interpretation is consistent with the notion that the
system shifted from phytoplankton to detrital dominance following tlu; phyto-
plankton decline in mid-May (Fig. 12).
Particulate Organic Nitrogen
Mean particulate organic nitrogen (PON) concentrations (Fig. 78) within
the euphotic zone exhibited a pattern consistent with phaeopigments
(Fig. 77), i.e. the summer maximum exceeded the spring maximum. Below the
euphotic zone, PON peaks in late June, late July, and mid-September coincided
with peaks of bacterial production (Fig. 46) and amino acid metabolism
(Fig. 57), but the trend over the summer indicated relatively low PON in
deeper waters during the summer compared with the spring. Given that
133
-------
a
7
6
4
UJ
3
2
i
MONTH
Figure 77. Mean phaeopigment concentrations within (#) and below (0) the
euphotic zone during 1985. Values are means of data from all
three lateral transects.
134
-------
300
ZOO
Q- m-
JD
MONTH
Figure 78. Mean PON concentrations within () and below (0) the euphotic
zone during 1985. Values are means of data from all three
lateral transects.
135
-------
bacterial biomass and production remained at high levels during summer condi-
tions, the PON data suggest a decreased bacterial dependence upon particulate
organic matter below the euphotic zone during the summer compared to other
seasons .
Mean PON concentrations for each of the three lateral transects are
shown in Figure 79. Except for the tendency of POH concentrations to be
highest at transect 1 during the period mid-June to mid-August, there was no
clear-cut trend among the transects. In contrast, mean POM concentrations
were consistently greater over the Bay flanks than over the main channel from
late April to mid-August (Fig. 80).
Particulate Organic Carbon
Variations in mean particulate organic carbon (POC) concentrations
(Fig. 81) were similar to changes in PON (Fig. 78) except that euphotic zone
POC began to exceed POC below the euphotic zone two weeks earlier than this
shift was observed in PON and the summer euphotic zone POC maximum occurred
in mid-August rather than in late July. POC peaks coincident with bacterial
production (Fig. 46) and amino acid metabolism (Fig. 57) occurred during the
summer below the euphotic zone, but the trend of below euphotic zone POC
indicated increasing accumulation of POC in deep water following the phyto-
plankton decline in late May which continued until the end of the study.
This trend suggests that from late spring through the summer, POC may have
been less susceptible to bacterial attack below the euphotic zone than it had
been earlier in the year.
Where POC data are available for each of the three lateral transects
(Fig. 82), we found no evidence for consistently higher POC at one transect
compared to the others. However, in agreement with PON (Fig. 79), POC at
136
-------
400 n
350
300
250
150-
100
50
MONTH
Figure 79. Mean PON concentrations along transects 1(A), 2 (0)r and 3 Q)
during 1985. Means include data from within and below the
euphotic zone.
137
-------
300
250-
~ ~
200
Q- 200-
MONTH
Figure 80. Mean PON concentrations along the west flank (A), main channel
(0), and east flank O of Chesapeake Bay during 1985. Values
are means of data from all three lateral transects.
138
-------
2.0
LS
14
L2
X 10
0.6
0.4
MONTH
Figure 81. Mean POC concentrations within () and below (0) the euphotic
zone during 1985. Values are means of data from all three
lateral transects.
139
-------
MONTH
Figure 82. Mean POC concentrations along transects 1 (A), 2 (0), and 3 O
during 1985. Means include data from within and below the
euphotic zone.
140
-------
transect 1 exceeded POC at transects 2 and 3 during mid-summer. Insufficient
data are available to calculate within and below euphotic zone means for each
of the transects. Mean POC concentrations were greater over the flanks than
over the main channel from late spring to at least mid-August (Fig. 83).
This difference is consistent with chlorophyll (Fig. 76) and PON (Fig. 80).
Biochemical Oxygen Demand
We emphasize that only degradable organic carbon (defined here as that
organic matter subject to bacterial metabolism) can fuel oxygen consumption.
While amino acid and carbohydrate analyses would have been usoful for compar-
ison with amino acid and glucose metabolism measurements, budgetary
restraints necessitated our use of biochemical oxygen demand (BOD) determi-
nations to estimate labile carbon. Experimentally determined oxygen consump-
tion was converted to carbon equivalence according to equation 4.
C^Hj^Og + 6O2 Jl. 6CO2 + &H2O (eq. 4)
The use of carbohydrates to calculate carbon equivalence is supported by our
finding that glucose turnover exceeded amino acid turnover.
Mean total BOD throughout the water column was variable during the
spring when no single transect exhibited consistently high or low BOD in
relation to the other two (Fig. 84). During the summer, however, mean total
BOD remained significantly higher to the north at transect 1 than to the
south at transect 3. This difference was due mainly to BOD within the
euphotic zone (Fig. 85) rather than below it (Fig. 86), even though BOD below
the euphotic zone also tended to be greater to the north than to the south
(Fig. 86). During rapid oxygen depletion of bottom waters in the spring
(Fig. 21), total BOD beneath the euphotic zone was greater to the north than
.141
-------
MONTH
Figure 83. Mean POC concentrations along the west flank (A), main channel
(0), and east flank O of Chesapeake Bay during 1985. Values
are means of data from all three lateral transects.
142
-------
1000
800
E
O
O)
E
in"
i
Q
o
CD
600
400
0
- \Jo \°
T \
^ - n- <^T^~ ^
K"~
b o
¦ i j 1 1
° /§-\
P~ ~o / \ \
' y \ 0 \
\ /"
\ / O
\ /
X
? " A
n
M I A I M I J I J I A I S I O
1985
Figure 84. Mean total B0D-5 along transects 1 (A), 2 (0), and 3 (D) during
1985. Means include data from within and below the euphotic
zone.
143
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1400
/
1000
/
cn
E BOO
LD
I
o 600
S'
u
400.
200
MONTH
Figure 85. Mean total BOD-5 within the euphotic zone along transects 1 (A),
2 (0), and 3 (D) during 1985.
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coo
£600-
600-
400-
200
a
MONTH
Figure 86. Mean total BOD-5 below the euphotic zone along transects 1 CA),
2 (0), and 3 (O) during 1985.
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to Che south (Fig. 86) whereas euphotic zone BOD showed no coherent pattern
among the three transects (Fig. 85). The May declines of bacteria and phyto-
plankton coincided with BOD decreases at all three transects. These
decreases were more severe and times slightly later at transect 1 than
farther south (Figs. 84-86).
Within the euphotic zone, summer BOD concentrations equaled or exceeded
spring levels (Fig. 85) whereas below the euphotic zone, summer concentra-
tions tended to average about half the spring means (Fig. 86). These dif-
ferences are consistent with mean PON (Fig. 78) and POC (Fig. 81) concentra-
tions and suggest that increased bacterial production and metabolism below
the euphotic zone during the summer tends to maintain BOD at relatively low
levels and/or the flux of particulate BOD to deep water decreases during
summer conditions.
Total mean BOD concentrations over the flanks exceeded those over the
main channel from late April through mid-October (Fig. 87). Apart from the
period of mid-May to early June when east flank BOD concentrations were
higher than to the west, there were no consistent differences in east and
west flank ROD means.
Mean total BOD within the euphotic zone reflected the spring phytoplank-
ton biomass maximum and high summer phytop lankton production (Fig. 88).
Following the May phytoplankton decline, euphotic zone BOD increased into the
summer, reaching concentrations higher than during the spring peaks. Parti-
culate BOD within the euphotic zone changed in concert with total BOD, but it
usually comprised a smaller portion of total BOD in the summer than it did in
the spring (Figs. 88 and 89) comprising as little as 30 to 40% of total
euphotic zone BOD in July and September (Fig. 89B).
146
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1200
1000
800
600
200
MAMJJASO
1985
Figure 87. Mean total B0D-5 along the west flank CA), main channel (0), and
east flank Q) of Chesapeake Bay during 1985. Means include
data from within and below the euphotic zone.
147
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1000
ft)0
U>0
500
4oo
v. /
03
200
Figure 88. Mean total B0D-5 (closed symbols) and particulate B0D-5 (open
symbols) in the mesohaline portion of Chesapeake Bay during
1985. Circles represent values within the euphotic zone and
triangles represent values beneath the euphotic zone. Means
include data from all three lateral transects.
148
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100C
m
600
r too
5 400
o
CD
200
80r
CD
LU
feO
o
20-
MONTH
Figure 89. Comparisons of mean particulate BOD-5 with mean filterable B0D-5
(A) and Z of total B0D-5 comprised of particulate B0D-5 (B) in
the mesohaline portion of Chesapeake Bay during 1985. Values
are means of data from all three lateral transects. Symbols:
() particulate B0D-5 within the euphotic zone; (A) filterable
BOD within the euphotic zone; (0) particulate B0D-5 beneath the
euphotic zone; (A) filterable B0D-5 beneath the euphotic zone;
tt) Z particulate B0D-5 within the euphotic zone; O %
particulate B0D-5 beneath the euphotic zone.
149
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Below the euphotic zone, the shift in the proportion of particulate BOD
to total BOD was even more striking (Figs. 89A and 89B). From mid-June into
October, dissolved BOD was about double the particulate BOD (Fig. S9A).
Particulate BOD below the euphotic zone during this time period accounted for
no more than 20 to 40" of the total BOD (Fig. 89B).
Mean particulate BOD (Fig. 89A) below the euphotic zone remained
reasonably constant through the summer while POC exhibited an increasing
trend (Fig. 81). For example, particulate BOD accounted for about 35% of POC
in late June but only about 15% of POC in mid-September.
During the suciner, mean / PBOD/POC averaged 51" within the euphotic zone
(n=34), 26% within the region of the pycnocline (n=23), and 18% beneath the
pycnocline (n=18). Ke suspect that most of the particulate organic material
which reached the deep water below the pycnocline during the 6ummer is either
reasonably refractory or is composed of relatively large material which
rapidly sinks to the bottom and is metabolized in the sediments. Therefore,
bacteria in the deep water are likely supported chiefly by dissolved organic
matter. This contention is supported by the fact that FBOD below the
euphotic zone comprises about 2/3 or more of total BOD there and that the
mean ratios of euphotic zone FBOD : below euphotic zone FBOD does not differ
by more than 10% from the ratios of euphotic zone bacterial abundance : below
euphotic zone bacterial abundance over the summer. Thus, the summer pattern
of bacterial abundance (see e.g. Figs. 26, 27, and 33) can be explained to a
first approximation in terms of FBOD.
1984 AND 1985 INTERANNUAL CONTRASTS - MICROBIOLOGY
As we have discussed earlier in this report, 1984 and 1985 were sharply
150
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contrasting years in terms of rainfall, water column stability (Figs. 16 and
21), surface salinity (Fig. 16), and oxygen regime (Fig. 9). We have also
documented differences in phytoplankton biomass during the summer seasons of
the two years (Fig. 19), marked by substantially higher standing stocks in
1984 than in 1985. However, average summer phytoplankton production was
comparable in both years. In this section, we compare bacterial parameters.
Microheterotrophs
Comparisons of bacterial parameters between the summers of 1984 and 1985
are difficult to make due to the differing boundaries of the study areas
investigated, frequency of cruises, and time spans covered. For example, we
have found and reported herein differences in a north to south direction in
1985 while no apparent differences existed over the more limited north to
south range of the 1984 study. Furthermore, bacterial parameters were
measured beginning only in late summer (20 August) of 1984. Were we able to
match time periods for the two years, we would have at best 1985 data from
only the September cruise. Fortunately, we observed similar patterns in the
bacterial parameters during summer conditions in both years, namely high
values near the surface and decreasing with depth (e.g. Figs. 26, 27,
44, 55, and 56). Therefore, we compare here data for transect 2 (Choptank
transect) for the period 8 August to 11 September 1984 and 11 July to 12
September 1985 when the "typical" summer patterns were observed.
Bacterial abundances were remarkably similar in both years, particularly
below the euphotic zone (Table 8). Therefore, despite higher phytoplankton
biomass in 1984 than in 1985, there appeared to be sufficient organic carbon
input below the euphotic zone in 1985 to support a high bacterial biomass
151
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Table 8. Comparison of the means of key bacterial parameters measured
during summer conditions in 1984 (cruises 11-18, 8/20/84 to
9/11/84) and in 1985 (cruises 10-13, 7/11/85 to 9/12/85).
BACTERIAL PRODUCTION*
(cells L"1 h-1)
EUPHOTIC ZONE
2.92 x 108 ('84)
5.32 x 108 ('85)
BELOW EUPHOTIC ZONE
1.92 x 108 ('84)
3.08 x 108 ('85)
BACTERIAL ABUNDANCE
(cells L"1)
1.37 x 1010 ('84)
1.09 x 1010 ('85)
8.44 x 109 ('84)
7.02 x 109 ('85)
BACTERIAL TURNOVER
(d_1)
0.52 ('84)
1.17 ('85)
0.55 ('84)
1.05 ('85)
Ow CONSUMPTION
(mg L~* d~*)
1.48 ('84)
1.31 ('85)
1.04 ('84)
1.02 ('85)
02 CONSUMPTION
(mg L'1 d"1)
1.64 ('84)
1.34 C85)
1.07 ('84)
0.92 ('85)
18 13
~Based upon 2x10 cells produced mol H thymidine incorporated.
Based upon bacterial abundance according to the relationship given in
Figure 72.
152
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nearly equal Co chat found in 1984.
Bacterial production increased in summer 1985 compared to 1984
(Table 8). The magnitude of this increase was 82% within the euphotic zone
and 60% beneath it, resulting in doublings of summer bacterial turnover rates
(biomass specific growth rates) in 1985 compared to 1984. We suggest that
this increase in bacterial production and turnover may have been caused by
increased predation pressure on the bacterial community in 1985 as appears to
have occurred with phytoplankton. Thus, widespread anoxia, such as in 1984,
may exert a major influence on predation of both phytop 1ankton and bacterial
'coramun it ies .
Directly measured oxygen consumption rates were also similar in 1984 and
1985, particularly below the euphotic zone (Table 8). Within the euphotic
zone where oxygen consumption is unimportant to deep water oxygen depletion
since oxygen is replenished by phytoplankton, directly measured oxygen con-
sumption averaged only 11% lower in 1985 than in 1984. Comparable oxygen
consumption values were found by calculating oxygen consumption from average
bacterial abundances (Table 10, Fig. 73). In this case, oxygen consumption
rates decreased 19% within the euphotic zone and 14% below it.
We conclude that despite major differences in climatic conditions and in
the oxygen regime between 1984 and 1985, the bacterial parameters, including
oxygen consumption rates, were remarkably similar under summer conditions in
both years. These similarities, in concert with nearly identical phytoplank-
ton production in both years, suggest that a major ecological shift has
occurred in the mesohaline portion of Chesapeake Bay during the summer (and
perhaps the spring as well) such that a large portion of phytoplankton
production is processed directly by pelagic microheterotrophs rather than by
153
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higher organisms. This implies that the extent and duration of anoxia in the
Bay is determined not by major differences in biological parameters but
rather by oxygen replenishment into deep waters which in turn is a function
of physical factors such as water colunn stability, freshwater flow, pycno-
cline tilting, and wind nixing. The existing biological conditions are such
that we expect widespread anoxia to occur whenever the appropriate physical
conditions obtain.
The Role of SuIfur Cyc1 ing
As we have recounted earlier in this report, sulfur cycling can also be
a major mechanism for oxygen depletion in the nesohaline reaches of
Chesapeake Bay. Although not a part of the Sea Grant/EPA study, we feel it
appropriate to discuss briefly the results of ongoing sulfur cycling studies
which have been conducted during 1984-1986 (Tuttle, unpublished) within the
study area of the 1985 Sea Grant/EPA investigation.
Hydrogen sulfide is produced during the metabolism of organic matter by
obligately anaerobic sulfate-reducing bacteria. In the mid-Bay, this process
appears to be most intense in deep channel anoxia sediments. Rates of sedi-
ment sulfate reduction measured during 1984 and 1985 indicate that sulfide
production in the channel sediments increased throughout the late spring and
summer, reaching a peak in mid-September in both years. This implies not
only that sulfate reduction is highly temperature-dependent (maximum bottom
water temperatures peaked during the August-September period of both years),
but that it may be strongly influenced by the accumulation of organic matter
from the annual summer phytoplankton maximum in the sediments. During April
to November 1984, sulfide production in deep Bay sediments averaged 75 mmol
154
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21 o 1
m d * and in 1985, the average rate (68 mmol m ~ d ) was comparable.
Therefore, we conclude that despite the decreased phytoplankton biomass
during summer 1985, sufficient organic natter reached the sediments to
support a rate of sulfate reduction similar to that found the previous year.
Oxygen consumption results when sulfide is oxidized (see eq. 2). If the
water column is not anoxic, as was the case during much of 1985 (Fig. 9),
then sulfide oxidation in surficial sediments contributes to sediment oxygen
demand. When the water column becomes anoxic, sulfide rises to the pycno-
cline where it is oxidized within as narrow depth band (ca. lm) in which
oxygen and sulfide coexist. Rates of water column sulfide oxidation measured
in 1984 and in 1985 near the pycnocline during anoxic events gave average
oxygen consumption rates of 9 mg O2 1~^ h~^. When the water column is not
anoxic, mean microheterotroph oxygen consumption below the euphotic zone is
about 1 mg O2 1 ^ h ' (Table 8). However, microheterotrophic oxygen consump-
tion is not limited to a narrow band, but occurs throughout the deep water.
If we integrate over an average hypoxia thickness layer of 10 m (Fig. 15),
then water column oxygen consumption by microheterotrophs is comparable to
that due to sulfide oxidation.
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REFERENCES
1. Azan, F. and Fenchel, T. The ecological role of water-column microbes
in the sea. Mar. Ecol. Prog. Ser. 10:257-263, 19 S 3 .
2. Biggs, R.B. and Flemer, D.A. The flux of particulate carbon in an
estuary. Mar. Biol. 12:11-17, 1972.
3. Boynton, W.R., Kemp, W.M. and Keefe, C.W. A comparative analysis of
nutrients and other factors influencing estuarine phytoplankton
production. In: Estuarine Comparisons, V.S. Kennedy, ed. Academic
Press, New York, 1982, pp. 69-90.
4. Carpenter, J.H. The Chesapeake Bay Institute technique for the Winkler
dissolved oxvgr-n method. Linnol. Oceanogr. 10:141-143, 1965.
5. Chervin, M.B., Malone, T.C. and Neal, P.J. Interactions between
suspended organic matter and copepod grazing in the plume of the Hudson
River. Estuar. Coast. Mar. Sci . 13:169-184. 1981.
6. D'Elia, C.F. Nutrient enrichment of Chesapeake Bay: an historical
perspective. In: Chesapeake Bay Program Technical Studies: A
Synthesis. U.S. Environmental Protection Agency, 1982, pp. 45-102.
7. Ducklow, H.W. Chesapeake Bay nutrient and plankton dynamics.
I. Bacterial biomass and production during spring tidal destratification
in the York River, Virginia, estuary. I.imnol. Oceanogr. 27 :651-659 ,
1982.
8. Ducklow, H.W. The production and fate of bacteria in the oceans.
Bioscience. 33:494-501, 1983.
9. Flemer, D.A. and Biggs, R.B. Particulate carbon : nitrogen relations in
northern Chesapeake Bay. J. Fish. Res. Bd. Canada 28:911-918, 1971.
10. Fuhrman, J.A. and Azam, F. Bacterioplankton secondary production
estimated for coastal waters of British Columbia, Antarctica, and
California. Appl. Environ. Microbiol. 39:1085-1095, 1980.
11. Fuhrman, J.A. and Azam, F. Thymidine incorporation as a measure of
heterotrophic bacterioplankton production in marine surface waters:
evaluation and field studies. Mar. Biol. 66:109-120, 1982.
12. Heinle, D.R., D'Elia, C.F., Taft, J.L., Wilson, J.S., Cole-Jones, M.,
Caplins, A.B. and Cronin, L.E. Historical review of water quality and
climatic data from Chesapeake Bay with emphasis on effects of
156
-------
enrichment. Final Report, ITMCEES Ref . No. S0-15CBL. Univ. of Md. Center
for Environmental and Estuarine Studies, Chesapeake Biological
Laboratory, Solor.ons, Maryland, 1980.
13. Hobbie, J.E., Daley, R.J. and Jasper, S. Use of Nuclepore filters for
counting bacteria by fluorescence microscopy. Appl. Environ. Microbiol.
34:1225-1228, 1977.
14. Malone, T.C., Kenp, V.M., Ducklow, H.W., Boynton, W.P., Tuttle, J.H. and
Jonas, R.B. Lateral variation in the production and fate of
phytop lankton in a partiallv stratified estuary. Mar. Ecol. Prop. Ser.
32:149-160, 1986.
15. Muller, P.J. C/N ratios in Pacific deep-sea sediments: effect of
inorganic ammonium and organic nitrogen compounds sorbed by clays.
Geochim. Cosmochim. Acta 41:765-776, 1977.
16. Officer, C.B., Biggs, R.B., Taft, J.L., Cronin, L.E., Tyler, M.A. and
Bovnton, W.F. Chesapeake Bay anoxia: origin, development , and
significance. Science 223:22-27, 1984.
17. Schneider, E. 1987. The Maryland aquaculture project: project summary
(Draft)
18. Seliger, H.H., Boggs, J .A. and Biggley, W.H. Catastrophic anoxia in
Chesapeake Bay in 1984. Science 228:70-73, 1985.
19. Steele, J.H. The Structure of Marine Ecosystems. Harvard University
Press, Cambridge, Massachusetts, 1974.
20. Strickland, J.D.H. Production of organic matter in the primary stages
of the marine food chain. In: Chemical Oceanography, J.P. Riley and G.
Skirnow, eds. Academic Press, New York, 1965, pp. 478-610.
21. Taft, J.I.. Nutrient processes in Chesapeake Bay. Tn: Chesapeake Bay
Program Technical Studies: A Synthesis. U.S. Environmental Protection
Agency, Annapolis, Maryland, 1982, pp. 103-149.
22. Taft, J.L., Taylor, W.R., Hartwig, E.D. and Loftus, R. Seasonal oxygen
depletion in Chesapeake Bay. Estuaries 3:242-247, 1980.
23. Tuttle, J., Malone, T., Jonas, R., Ducklow, H. and Cargo, D. Nutrient-
dissolved oxygen dynamics: roles of phytoplankton and microheterotrophs
under summer conditions. Final Report to U.S. Environmental Protection
Agency, Chesapeake Bay Program, Annapolis, Maryland, 1985, 121 pp.
24. Williams, P.J. LeB. Incorporation of raicroheterotrophic processes into
the classical paradigm of the planktonic food web. Kieler Meeresforsch.
5:1-28, 1981.
157
-------
25.
Williams, P.J. LeB. and Askew, C. A method of measuring the
mineralization of micro-organisms of organic compounds in spawater.
Deep-Sea Res. 15:363-375, 1968.
26. Williams, P.J. LeB., Ber-.an, T., and Holm-Hansen, 0. Amino acid uptake
and respiration by marine heterotrophs. Mar. 3iol. 35:^1-47, 1976.
27. Wright, R.T., and Coffin, R.B. Factors affecting bacterioplankton
density and productivity in salt marsh estuaries. Tn: Current
Prespectives in Microbial Ecology. C.A. Reddy and M.J. K lug , eds. Am.
Soc. Microbiol., Washington, D.C., 1984, pp. 485-494.
158
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