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VIII-72

-------
Table D-3.   Specific leaf area (cm2/gdw)  of Potamogeton  perfoliatus  and  Ruppia
martima in some

Pond
1

2

3

7

8

Date
15 June
1 July
16 June
1 July
16 June
15 July
18 June
15 July
18 June
of the experimental ponds.
Section
Tip
Tip
Tip
Tip
Tip
Tip
Base
Whole
Tip
Tip
Base
Whole
Tip
Potamogeton
X
124.5
193.8
252.3
213.6
318.9
273.5
210.1
241.8
216.0
251.1
192.8
217.0
194.2
SD
35.4
52.4
54.2
59.4
49.0
43.1
45.1
53.9
54.0
115.1
102.6
108.0
80.0
%c
28
27
21
28
15
16
22
16
25
46
53
50
41
Ruppia
x 5D *L
12.4 5.3 43
71.0 38.3 54
49.3 9.1 19

40.4 6.1 15

58.9 14.0 24

48.8 13.7 28
                                    VIII-73

-------
1
• Table D-4. Chlorophyll a and b concentrations ( g/cm^) and ratios for
1 Potamogeton perfoliatus in a low nutrient dosed pond in relation to the
! dates nutrients were applied
Date o*" Sampling
Application
July 8
July 14
July 16
July 17
July 23
July 28
July 29 July 29
July 31
August 4 August 4
August 5
August 6
August 10
to the pond.
Chlor a
13.11
9.74
10.54
9.35
10.90
10.92
8.36
7.37
9.04
10.99

Chlor b^
2.01
2.25
2.35
2.83
1.53
2.36
1.87
2.39
2.68
1.73

a/b
6.53
4.33
4.48
3.31
7.15
4.62
4.47
3.08
3.38
6.34
VIII-74

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-------
Table E-2.   Epiphyte biomass and net productivity, and productivity of the host
     plant  Ruppia maritima for the upper (0-15 cm) and lower (+15 cm) sections
     of the plant. Total  production is expressed per gdw of the macrophyte.
 Plant      Epiphyte        Ruppia
Section      Biomass      Production
              (gdw)     (mg C gdw^h-
                                           Epiphyte Production
                                             (mg C
                                          Per Epiphyte  Per S
                                                         Total

                                                      Production
                                                   (mg C
 0-15
  15+
0.37

0.48

0.63



0.32

0.40

0.45
                              3.82

                              2.13

                              2.31

                              3.23

                              2.56

                              0.62

                              0.91

                              0.66
0.51

0.80

0.57



0.78

0.86

0.64
0.19

0.38

0.36




0.25

0.34

0.29
2.32

2.69

3.59



0.87

1.25

0.95
                                 VIII-77

-------
1
       Table  E-3.   Epiphyte biomass and productivity, and productivity of the host plant
           Potamogeton perfoliatus for the upper (0-15 cm) and lower (+15 cm) sections of
the
Plant
Section
(cm)
0-15






15+






plant. Total production is expressed
Epiphyte
Biomass
(g/gdw(SAV))
0.29
1.73
1.90
2.39
3.76
5.22
6.13
0.27
0.77
1.25
1.39
1.68
3.84
5.44
Potamogeton
Production
(mgC gdw1 h"1)
5.09
5.31
3.44
4.67
4.25
5.44
1.52
2.32
2.00
3.12
0.96
2.58
0.85
2.08
per gdw of the
macrophyte.
Epiphyte Production
Per Epiphyte
(mgC gdw~i h"1)
0.15
0.97
1.59
1.03
0.94
0.57
0.92
0.08
1.45
0.94
1.02
1.02
0.56
0.71
Per SAV
(mgC gdw1 h"1)
0.04
1.68
3.02
2.46
3.53
2.98
5.64
0.02
1.12
1.18
1.42
1.71
2.15
3.86

Total
Production
(mgC gdw1 h'1)
5.13
6.99
6.46
7.13
7.78
8.42
7.16
2.34
3.12 £
4.30
2.38
4.29
3.00
5.94
                                                 VIII-78

-------
                                            on
Treatment
Date
5 June
16 June
23 June
24 June
16 July
17 July
23 July
30 July
31 July
10 August
11 August
Control
1
7.16
5.89
4.69
(0.32)
4.53
(0.54)
3.09
(0.65)
7.83
(2.11)
8.89
(0.23)
9.47
(0.96)
8.35
(1.47)
7.36
(0.56)
6.52
5
6.34
6.31
9.19
(1.88)
4.65
(1.22)
3.84
(0.18)
8.03
(1.21)
7.43
(1.71)
7.95
(0.53)
7.17
(1.25)
4.71
(0.32)
6.92
Low
3
5.53
6.53
6.39
(0.84)
5.35
(0.34)
3.75
(0.38)
8.59
(0.66)
6.14
(0.50)
8.59
(1.01)
7.49
(0.39)
2.64
(0.33)
2. 6 ">
6
4.62
4.53
6.38
(0.71)
4.92
(0.36)
2.06
(0.67)
7.52
(1.40)
5.70
(0.96)
7.87
(1.37)
f.4s
5.13
(1.26)
5.24
Mid
4
5.19
4.33
5.77
(0.36)
6.55
(0.41)
3.18
(0.27)
10.26
(2.09)
6.23
(0.40)
10.21
(1.50)
8.83
(0.16)
9.29
(0.51)
6.80
8
5.50
6.48
6.28
(1.41)
5.35
(0.46)
2.22
(0.16)
11.31
6.64
(0.70)
11.88
(2.19)
8.75
(0.98)
9.77
(0.62)
5.63
Hi
2
-
5.94
5.35
(0.23)
7.05
(0.26)
4.33
(0.39)
10.06
(0.67)
9.18
(1.09)
8.57
(0.70)
3.17
1.09
(0.44)
1.46
gh
7
5.17
3.40
7.80
(0.70)
4.30
(0.15)
3.55
(0.95)
9.52
(1.85)
9.42
(0.39)
17.28
(4.02)
12.46
(1.96)
1.92
(0.17)
3.10
VIII-79

-------
Table F2.  Nighttime respiration (Rn) for total  communities in experimental  ponds based
     on dawn/dusk readings of dissolved oxygen (g02 m~3 d"1).
Treatment
Date
15 June
23 June
16 July
23 July
30 July
10 August
Control
1
3.2t
(0.77)
2.75
(0.36)
4.85
(0.43)
6.55
(0.22)
5.37
( )
2.48
(0.57)
5
3.05
(0.47)
3.31
(1.58)
4.07
(0.33)
5.17
(0.75)
4.33
(0.53)
-
Low
3
3.74
(0.67)
3.20
(0.46)
5.01
(0.07)
4.86
(0.63)
4.55
(0.38)
0.63
(0.22)
6
3.45
(0.44)
3.13
(0.26)
4.62
(0.92)
6.04
(0.31)
3.35
(0.27)
3.65
(0.69)
Mid
4
3.80
(0.36)
3.35
(0.13)
5.05
(0.31)
5.94
(0.25)
4.32
(0.78)
3.78
(0.89)
8
4.36
(0.11)
3.66
(0.80)
5.66
(0.30)
6.26
(0.18)
6.09
(1.47)
3.76
(0.18)
High
t
4.26
(1.14)
5.64
(0.33)
6.93
(0.15)
8.60
(0.69)
7.10
(0.13)
3.05
(0.20)
7
3.24
2.71
(0.61)
6.15
(0.54)
7.25
(0.24)
6.87
(2.00)
3.06
(0.11) i
                                            VIII-80

-------
f
           Table F-3.  Ratios of apparent productionrnighttime respiration (ParRn) for total communitit
                in experimental  ponds based on dawn/dusk readings of dissolved oxygen.
Treatment
Date
23 June
24 June
16 July
17 -July
23 July
30 J'dy
31 July
10 August
11 August
Control
1
1.71
1.65
0.64
1.61
1.36
1.76
1.55
2.97
2.63
5
2.78
1.40
0.94
1.97
1.44
1.84
1.66
-
-
Low
3
2.00
1.67
0.75
1.7i
1.26
1.89
1.65
4.19
4.22
6
2.04
1.57
0.45
1.63
0.94
2.35
1.93
1.41
1.44
Mid
4
1.72
1.96
0.63
2.03
1.05
2.36
2.04
2.46
1.80
8
1.72
1.46
0.39
2.00
1.06
1.95
1.44
2.60
1.50
High
2
0.95
1.25
0.62
1.45
1.07
1.21
0.45
0.36
0.48
7
2.88
1.59
0.58
1.55
1.30
2.52
1.81
0.63
1.01
                                                     VIII-81

-------
                                   CHAPTER IX
                       THE DECLINE OF SUBMERGED VASCULAR
                    PLANTS IN UPPER CHESAPEAKE BAY:   SUMMARY
                     OF RESULTS CONCERNING POSSIBLE  CAUSES1
                                W.  Michael  Kemp2
                               Walter R.  Boynton3
                               Robert R.  Twilley2
                               J. Court Stevenson
                                 Jay C. Means3
                                      1983
^Contribution No. 1431          University of Maryland
 Center for Environmental and Estuarine Studies (UMCEES),

2Horn Point Environmental Laboratories, UMCEES, Box 775,
 Cambridge, MD.   21613 USA

3Chesapeake Biological Laboratory, UMCEES, Box 38,
 Solomons, MD.   20638 USA
                                     IX-i

-------
                                ACKNOWLEDGEMENTS
     We would like to thank our many colleagues, students and research associ-
ates for their contribution to the conduct of this program: S. Bollinger, J.
Cunningham, D. Flemer, W. Goldsborough, T. Jones, M. Lewis, D. Marbury, L.
Murray, S. Nixon, R. Orth, P. Penhale, M. Shenton, K. Staver, R. Wetzel,  R.
Wijayaratne.  Special thanks are due to Al Hermann for his major contribution
to the numerical simulation modeling.  This reserach was supported by grints
from the US Environmental Protection Agency No. R805932010 and X00324801D and
the MD State Dept. Natural Resources, Tidewater Administration No. C18-8U-430
(82).
                                     IX-ii

-------
                                    ABSTRACT


     This paper provides a summary and synthesis  of research conducted to
investigate possible causes of the decline in abundance of submerged aquatic
vegetation (SAV) in upper Chesapeake Bay beginning in the late 1960's.  Three
factors potentially were emphasized in this study:  runoff of agricultural
herbicides; erosional  inputs of fine-grain sediments; nutrient enrichment and
associated algal growth.  Widespread use of herbicides in the estuarine water-
shed occurred contemporaneous with the SAV loss;  however, extensive sampling
of estuarine water and sediments during 1980-81 revealed that typical  bay
concentrations of herbicides (primarily atrazine) rarely exceeded  2 ppb.   On
two occasions relatively high values (20-45 ppb)  were observed for brief (2-4
h) periods in a small  cove following runoff events.  Short (2-6 h) and long
(4-6 wk) term experiments indicated that ephemeral phytotoxic effects  would be
expected in response to these highest herbicide concentrations followed by
rapid recovery.  However, normal concentrations (< 5 ppb) had little measurable
effect on plants.  Historical increases in turbidity have been documented for
some bay tributaries since the 1940's.  During our study light (PAR) attenua-
tion by suspended fine-grain sediments contributed more to total turbidity in
bay shallows (< 1.5 m) than did phytoplankton chlorophyll £.  Diel cycles of
PAR available in SAV beds indicated that plant photosynthesis was  light-limited
for much of the day, and PAR often fell below the compensation level (Ic)
needed for minimal plant growth.  Although some SAV species exhibited  consider-
able ability to adapt to reduced light by such mechanisms as increased pigmenta-
tion and stem elongation, increased turbidity has probably reduced overall
depth distributions of SAV markedly.  Effects of  the continual increase in
nutrient enrichment of the bay (documented since  1930) were tested by  experi-
mentally fertilizing pond mesocosms at levels common to the upper  estuary.
Moderate to high nutrient loadings resulted in significant increases in growth
of epiphytic and planktonic algae and c Teases in SAV production, as  well  as
premature seasonal senescence of fertil ..ad plant populations.  Direct measure-
ments demonstrated the inhibitory effect of epiphytic growth on SAV photosyn-
thesis, due largely to light attenuation.  The results of these various experi-
ments were synthesized into an ecosystem simulation model which demonstrated
the relative potential contributions of the 3 factors to SAV declines, where
nutrients > sediments > herbicides.  Other factors and mechanisms  are  also
discussed along with possible resource managements options.
                                      IX-1

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                                   INTRODUCTION


      It is widely recognized that  submerged vascular plants play an important
role  in the ecology of littoral regions of lakes, estuaries and oceans (1, 2,
3).   While a number of studies have noted the Ability of these plant communi-
ties  to attenuate variability of nutrient, sediment and production cycles (4,
5, 6), several such communities have themselves undergone extreme fluctuations
in distribution and abundance.  For example, in the mid 1930's a widespread
die-off of the seagrass, Zostera marina, was well documented throughout the
North Atlantic coastal regions (7JITFTe cause of this occurrence has never
been  unequivocally established, although recent suggestions have pointed to
subtle climatic shifts (8).  Other reports of regional declines in abundance
of submerged aquatic vegetation (SAV) have indicated the possible influence of
human activities (9, 10).

      Few of the  reported SAV declines have occurred in estuarine environ-
ments and most have involved 1-2 plant species.  However, in one of the world's
largest estuaries, Chesapeake Bay, a major loss of SAV has continued since
the mid 1960's to the present (11, 12, 13).  More than 10 species have experi-
enced significant decreases in abundancce, including Potamogeton perfoliatus,
_P.  pectinatus, Valisneria americana, Zannichellia palustris, Ruppia mantima
as well as the marine species Z. marina (12).In~the upper estuary this decline
in native species was proceeded" by an invasion of the exotic, Myriophyllum
spicatum (Fig. la) which eventually also died-back (11).  Studies of seed and
pol len distribution in sediment cores T'rom the upper bay have demonstrated
that  this dimunition in plant abundance is unprecedented (Fig. la) for at
least the last century (13).  In general, it appears that the recent decline
occurred first and with greatest intensity in the brackish waters of the estu-
ary,  with 1L. marina communities in the lower bay being affected less and some-
what  later (l"3JT

      Numerous mechanisms have been cited as possible causes of this occurrence
(12).  The concept that natural entrained population cycles or global  climatic
events might be responsible seems  unlikely in view of the range of biological
and physiological characteristics  for the numerous species invoked.  In addi-
tion, there is no parallel trend in plant abundance apparent in nearby coastal
regions (13).  Other factors including animal foraging and grazing and major
storm events are probably of occasional and local importance, but these are
part  of the normal milieu to which SAV are exposed and hence are insufficient
to explain this abnormal decline.  The absence of correlations between distri-
bution of SAV and industrial pollutants renders such anthropogenic wastes an
unlikely cause; however, more general changes in water quality associated with
diffuse sources (e.g. runoff) do represent a potential explanation(12).

      In an extensive review concerning SAV in Chesapeake Bay and elsewhere
(12), it was suggested that three  categories of environmental changes deserved
                                       IX-2

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                         IX-3

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?!
                 Fig. 1.  Summary of long-term trends  (1930-1980)  in  selected  variables for
                          Chesapeake Bay and  its  tributaries:   a)  submerged aquatic vegetation
                          abundance in the upper  bay (1958-1976 (11),  prior to 1958 (14)); b)
                          use of atrazine in  coastal  plain  counties draining into the bay (12);
                          c) Susquehanna River flow (69); d) idealized sediment yield for Patux-
                          ent River basin (28, 29); 3)  fertilizer  sales 1n Maryland (35); f)
                          nitrogen in sewage  discharge  from Washington, D.C. into Potomac River
                          estuary (35, 69).
                                                        IX-3a

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               further attention:  increased  fine-grain  sediments  from  land erosion;  increased
               algal  growth from nutrient enrichment  of  estuarine waters; and aqueous concen-
               trations of herbicides  arising  from  agricultural runoff.

                    This  paper  summarizes the  results from  recent studies examining  possible
               causes of  the SAV decline in  upper Chesapeake  Bay.   Specifically, we  report
               the  salient findings  of investigations concerning: estuarine distributions,
               degradation, sorption,  and SAV  phytotoxicity of selected herbicides;  light
               availability and vertical attenuance by  suspended material, distribution of
               this suspended matter,  and photosynthetic responses  of  SAV to different
               light  regimes; effects  of nutrient enrichment  on algal  growth, and responses
               of SAV growth and production  to elevated  levels of planktonic and epiphytic
               algal  biomass.  We also synthesize these  results toward reconstructing a plaus-
               ible scena, io vis a vis this  loss of submerged vascular plants, and we
               provide comments on resource  management  options and  the future of SAV in upper
               Chesapeake Bay.


                                            APPROACH TO PROBLEM


                    In 1978 we  initiated a 3-year study  to  investigate various aspects of the
               ecology of SAV communities in Chesapeake  Bay.  While intensive research was
               conducted  at several  locations  along the  estuarine salinity gradient, our work
I               focussed on communities located in the low salinity  (5-15°/oo) region.  The
i               research was designed to address three general questions:  (1) factors poten-
               tially responsible for  the observed  decline  in SAV distribution and abundance;
               (2)  SAV community interactions  and characteristics;  and (3) resource  management
               options.

                    This  research program was  organized  in  a  hierarchical fashion to deal
               with the complexity of  the ecosystems  studied  in addressing these questions.
               Depicted in Fig. 2 1s the manner in  which (c)  specific  research approaches
               were Integrated  into  (b) broad  research objectives which were, in turn, related
               to (a) research  questions (15,  16).  General objectives of this reserach pro-
               gram involved:  the development of mechanistic relationships among ecological
               factors through  controlled experimentation;  the interpretation of these results
               in terms of actual  conditions in nature;  and the establishment of a framework
               to extend  specific results in to a general context.   The specific research
               approaches employed ranged from shortterm laboratory  experiments to seasonal
               mesurements in the estuary and  experimental  ponds, with ecological modeling at
               all  levels for analysis, interpretation and  generalization.  In the following
               sections we review the  results  of our  field, laboratory and modeling  studies
               to evaluate the  possible roles  of herbicides,  suspendable sediments and nutri-
               ents 1n the SAV  decline in the  bay.


                                   HERBICIDE INPUTS,  FATE AND PHYTOTOXICFY


                    For the last two decades the most widely  used herbicide in the Chesa-
               peake Bay  watershed (and particularly  in  the surrounding coastal plain) has
                                                     IX-4

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Fig.  2.   Schematic flow chart illustrating the relationships  between  specific
         research approaches, broad research  objectives  and overall research
         questions.  Three thicknesses of arrows  suggest levels  of  influence
         for these items.
                                    IX-5a

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                been atrazine (12, 17).  Since its introduction to the region in the early
                1960's, its use, which is particularly important for corn crops, has grown
                steadily to the present (Fig. lb).  Atrazine was thus selected as the primary
                compound for intensive investigation.  Our research also considered the distri-
                bution and phytotoxicity of a second important compound, linuron, which is
                used for wetd control in soybean crops.


                                     Herbicide Distribution and Degradation

                     In general about 0.2-2.0% of atrazine applied to agricultural  fields is
                transported into adjacent waters in runoff, and resulting concentrations in
                the estuary depend on dilution, dispersion, adsorption and degradation (17,
                21, 27).   In the open waters of Chesapeake Bay observed concentrations of
                atrazine and linuron rarely have rarely exceeded 1 ppb (18, 19, 20).  In major
                tributaries such as the Choptank and Rappahanock Rivers concentrations of 5
                ppb may occur following a major spring runoff event.  Such events can generate
                transient (2-6 h) concentrations up to about 40 ppb in secondary tributaries
                and small cc-.cs (20, 21, 22).

                     Our data indicate that atrazine and linuron degrade rapidly in estuarine
                conditions with half-lives of 1-6 wk (17, 23).  In addition,  these  herbicides
                exhibit strong tendency for sorption to particles and colloids (20, 24), and
                degradation of particle-bound atrazine appears to be even more rapid (25).  As
                a result of dilution and degradation, concentrations of atrazine on suspended
                and deposited sediments in the estuary were seldom greater than 5 vg/kg
                suggesting little accumulation potential, (20), and moreover, once  bound to
                particles atrazine becomes virtually unavailable for plant uptake (25).


                                       Herbicide Uptake and Phytotoxicity

                     Using 14C-ring labelled atrazine Jones et al. (in 25) found that uptake
                by £. perfoliatus was rapid, reaching equilibrium within 1 h.  The  observations
                that atrazine concentrations were low in sediments (20), combined with the
                relatively slow translocation of the herbicide measured within plant vascular
                tissues (25) suggest the primary mode of uptake is foliar. A strong correla-
                tion was observed between atrazine uptake and short-term depression of photo-
                synthesis.  After a sequence of washes in atrazine-free water, a portion of
                the incorporated atrazine was released and photosynthesis returned  to control
                levels within 4-8 h (25).

                     In addition to short-term (2 h) experiments relating photosynthetic
                depression to atrazine exposure, microcosms containing estuarine water and
                sediments with £. perfoliatus or M. spicatum were used to test longer duration
                (4-6 wk) effects of atrazine and Tinuron on plant growth and  production.  At
                concentrations less than 100 ppb of either herbicide, both plants exhibited a
                significant trend of photosynthetic recovery within 1-2 wk after initial  ex-
                posure despite the continued presence of herbicides in microcosm waters (25).
                Using a linear dose-response model  for apparent photosynthesis (or  net biomass
                accumulation) versus the logarithm of initial herbicide concentration, signifi-
                cant (r2x).93) relationships were observed for all  combinations of  herbicides
5
                                                      IX-6

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and plants (Fig. 3).  Values of IJ.Q and *50 (concentrations resulting in
1% and 50% inhibition of photosynthesis) ranged from about 2-4 ppb and 70-120
ppb, respectively, for the 4 combinations of herbicides and plant species
tested (25).

     At 5-10 ppb (the upper limit of herbicide concentrations typically occur-
ring in most of the estuarine shallows) a 10-20% loss of photosynthesis would
be expected.  However, the ephemeral nature of such elevated herbicide levels
and the tendency toward rapid photosynthetic recovery in these plants  would
render such effects short in duration.  Overall, atrazine phytotoxicity has
been tested for 7 species in Chesapeake Bay in various experiments,  and some
differences in sensitivity were apparent (25,  26,  27).  For example, the exotic
species, M. spicatum, exhibited greater tolerance  than most native plants,  and
the seagrass. Z. marina, appeared also to be less  sensitive, while the fresh-
water member oT the Hydrocharitaceae, ±. americana, was among the least toler-
ant species tested.

     Some concern about the possible phytotoxicity of atrazine metabolites
formed during degradation was allayed in recent experiments, again using 14C-
labelled compounds (provided by Cipa-Geigy Corporation, Greensboro,  NC), where
150 values were found to be at least 10 times  greater than for the parent
compound (25).  Thus, while conditions certainly occur sporadically  in the
estuary where herbicide stress could induce some loss in SAV production, the
possibility that herbicides led to the SAV decline saems remote.


                 SUSPENDABLE SEDIMENTS AND SAV LIGKT RESPONSES
     Substantial changes in both sediment yield to and turbidity in Chesapeake
Bay and some tributaries have occurred over the last several  decades (Fig. Id).
In one major tributary (Patuxent River), there have been periods of increasing
and decreasing sediment yields, punctuated by occasional  major input events
during the last 50 years (14, 28, 29, 30).  Overall, it seems that  turbidity
in this tributary is higher now than 30 years ago (33) even though  inputs  of
fine-grain sediments have recently decreased (29), indicating there is not a
simple relationship between these factors.


                  Light Attenuation due to Suspended Sediments

     Laboratory and field experiments were conducted to assess the  relative
contributions of suspended sediments and chlorophyll-^ to diffuse,  down-welling
light attenuation.  Diffuse attenuation of phytosynthetically active radiation,
PAR (400-700 nm), due to different concentrations of suspended materials was
measured in stirred, opaque-sided, cylindrical  chambers (60 cm dia, 120 cm 'it)
by observing PAR at 10 cm vertical intervals.  Known amountd  of dried,  sieved
(62 v) sediment or algal culture suspensions were added to the chambers to
develop relations between attenuation coefficient (k) and suspended concentra-
tions.  The slope of k versus algal biomass (in terms of dry  wt.) was found to
be about twice that for k versus inorganic suspended sediments due  to the  high
absorbance of algal pigments at PAR wavelengths (31).  However, the relative
ranges of these two suspensoids typical of shallow estuarine  environments
                                      IX-7

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                                          IX-8

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Fig.  3.   Reduction in apparent  photosynthesis  (%) for two  species of  submerged
         aquatic  vegetation  resulting  from treatment with  the herbicide atrazine
         at 5 initial concentrations.   Three degrees of  shading  represent
         (from heavy  to  light)  maximum in situ atrazine  concentrations observed
         in:   open Chesapeake Bay waters; main bay  tributaries;  and a small
         cove within  one tributary (17, 25).
                                        IX-8a

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suggest that suspended solids are a far greater component to total turbidity.
Concentrations of suspended sediments (seston) often increase from 20-100
mg/i within 1-2 h due to moderate (5-10 m/s) summer winds (25, 31), while
chlorophyll-a concentrations typically range from 5-15 vg/t over a tidal
cycle (Fig. T) and are not markedly influenced by wind events.  These common
excurisons in seston and chlorophyll-^ would result in about 50% and 8% reduc-
tions in PAR at 50 cm depth, respectively.

     To assess jn-situ light conditions, measurements of seston, chlorophyll-
a, attenuation coefficient, water depth and PAR were made over tidal cycles in
Tittoral areas with and without SAV communities (25, 31).  Data from a typical
cycle (Fig. 4) indicate that both chlorophyl!-£ and suspended particulate
concentrations were lower within SAV communities than in non-vegetated littoral
areas, and the differences were greatest at low slack tide, probably due  to
settling of particulates in the quiescent waters of the bed.  It i* evident
here that PAR attenuation followed seston consistently at both sites, while
not so for chlorophyll-a.  Consistent with laboratory data, most (> 95%)  light
attenuation could be attributed to suspended solids other than chlorophyll-a.
Sediments may also be important in light attenuation when they settle from The
water column onto SAV leaves.  For example, Staver et al. (31) found 80-90%
of total epiphytic material (attached to leaves) to be inorganic sediments as
opposed to algae or other organic materials in a Chesapeake Bay SAV community.
However, there is a continuous cycle of deposition and resuspension of these
loosely attached sediments, and the integrated, long-term effect of these
processes on photosynthesis is not known.


                             Light Response of SAV

     In laboratory chambers the responses of P. perfoliatus and M. spicatum
photosynthesis to light intensity followed a "Basic rectangular hyperbolic
form.  Values of IK (intersection of initial slope and Pmax) ranged from
200-300 uEm^s'1 and were similar to values reported for other SAV
species (31).  A second important parameter of the photosynthesis-irradiance
(P-I) relationship is light compensation point (Ic), which is the PAR intens-
ity at which net production (in excess of respiration) approaches zero.  Values
of Ic at about 50 uEm^s"1 were reported for apical 10 cm growing
tips of £. perfoHatus (25).  However, Ic values for whole plants (including
non-photosynthetic root-rhizome tissue) would be substantially higher, probably
in the range of 75-150 yEjn~2s~1.  Given the ambient turbidity
levels generally observed in the bay (k > 2/m) in addition to light reduction
associated with epiphytic materials, it would appear that SAV are generally
exposed to light regimes approaching Ic.  Unless these plants can adjust  to
such light stress, net growth would not be possible.

           In fact, Goldsborough (32) found that at least one species (£.
perfoliatus) has considerable ability to adapt to reduced light conditions via
morphological changes (such as stem elongation and increased leaf area:weight
ratio) and biochemical changes (such as increased chlorophyll-£ per leaf  area)
which influence P-I relationships.  However, under extreme shading (< 75  yEm~2s"1)
the capacity of adaptive mechanisms was exceeded, resulting in reductions in
new shoots, number of flowering plants, and root storage nodules (which appear
                                      IX-9

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                          IX-10

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Fig.  4.   Time-course observations  for planktonic  chlorophyll £, total  seston,
         water depth, light  attenuation  coefficient and  light available at 10
         cm depth and at the bottom of a 1.0 m  (mean  depth) water  column for a
         bed of submerged plants (£. perfoliatus) and for an adjacent  unvege-
         tated area  on 22 September 1980.   Shaded regions represent typical
         ranges for  1^ (intersection of  initial slope and maximum  photosyn-
         thesis in a photosynthesis-irradiance  function) and Ic (compensation
         light level  for 10  cm  apical  growing tips of plants) (31, 32).
                                    IX-lOa

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to be overwintering organs, 1).


                   Overall  Effects of Suspendable  Sediments  on  SAV

     In the Patuxent Estuary, which formerly contained extensive SAV communi-
ties, chlorophyl l-a_ concentrations increased from  about 10 to 40pgA and
secchi  disc depths decreased from about 0.8 m to 0.35 m (or  k = 1.8 -
4.U/m,  assuming k = 1.4/secchi) for spring-summer  during the 1960's decade
(33).  Partitioning these attenuation coefficient  values between chlorophyl1-a
and other particulate and dissolved materials (34)  shows about  7% of total
attenuation could be attributed to the algal pigment  in the  early 1960's,
while about 14% could be so partitioned for the 1970 data.   While the relative
importance of phytoplankton attenuation increased  over this  period, it  appears
that most light attenuation was still associated with inorganic particulates
and other non-chlorophyllous organics.

      In any case, the observed increase in turbidity would  cause a significant
reduction in the depth distribution of SAV.  For example,  if we take summertime
irradiance at the water surface to be 1200 uEm"2s~1  (a typical  value
at noon on a sunny Aug-Sep day) and Ic to be 100 uEirf^"1, the
depth to which sufficient light penetrates to support SAV growth would  be
reduced from 1.4 m to 0.6 m for the aforementioned  scenario  in  the Patuxent
between 1960 and 1970.  Similar turbidities were observed (25)  throughout
littoral zones in upper Chesapeake Ba\  suggesting  that the  depth distribution
of SAV has been restricted due to redb  -. PAR availability.


                     NUTRIENT ENRICWENT AND ALGA1    V RELATIONS


      Watershed inputs of nitrogen (N) and phosphorus (P)  to Chesapeake Bay
have increased several-fold in the last two decades  (Fig.  1  e,f), and aqueous
concentrations of inorganic nutrients have likewise increased appreciably  in
many areas of the estuary (33, 35).  Correspondly,  phytoplankton populations
(as measured by chlorophyl1-a) have also expanded  during this period (33,  35).
Since it appears that submerged vascular plants can  obtain their N and  P re-
quirements from sediment pore waters (36, 37) which are rich in nutrients,  it
is expected that nutrient additions to overlying waters would increase  SAV
growth to a limited extent only.  In fact, there are a number of mechanisms
where fertilization may lead to decreases in SAV production  and abundance.
Nutrient enrichment tends to promote algal growth,  e'ther phytoplanktonic  (38)
or epiphytic (10) which in turn can inhibit SAV photosynthesis  via reductions
in light and molecular transport across SAV epidermis (39, 40).  In addition,
it has been postulated that increased phytoplankton  populations can stimulate
growth of animal fouling communities on SAV leaves  and other firm, stable
substrates (25, 41) leading to similar photosynthetic stress for the plants.
In this section we consider this C'.csclon terms of correlations from field
observations and results of nutrient enrichment studies in experimental  ponds.
                                     IX-11

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                     Algal  Responses to Nutrient Enrichment

     The effects of nutrient enrichment on algal and SAV growth were investi-
gated in 8 experimental ponds (area = 350m2; depth = 1.2 m) treated in
duplicate with 3 nutrient levels (plus controls) at 8-10 day intervals for a
10 week period during the summer of 1981.   Initial  nutrient concentrations
after dosing were 120, 60 and 30 yM inorganic nitrogen (50% NH^; 50%
NOj), with N:P (atomic) ratios of 10:1.  Nutrient loading rates at medium
dosage were equivalent to runoff from a typical  watershed (26% agriculture,
11% residential) draining into a small Chesapeake Bay tributary (42).

     Phytoplankton biomass (as chlorophyll-^) generally increased with treat-
ment levels (significantly greater at low and high  doses), particularly at the
highest nutrient amendments (Fig. 5).  A sequence of phytoplankton bloom events
were evident in high dose ponds, where chlorophyll-^ increased by 1-2 orders
of magnitude on several occasions within a 1-2 d period.   This response was
similar to those previously reported for nutrient enrichment studies (43, 44).
Epiphytic algal biomass was significantly higher than controls in all treated
ponds (Fig. 5).  Levels of epiphytic material in nutrient treated ponds were
similar to those observed in the estuary on scenescent plants while control
levels were comparable to those occurring early in  the SAV growing  eason (32,
45).  Again, this pattern of increased epiphytic algal growth is consistent
with results from nutrient enrichment experiments elsewhere (10, 46, 47).


                         Effects of Algal  Growth on SAV

     Correlations between PAR attenuation in the water and phytoplankton chloro-
phyll -a were obtained from measurements in the experimental ponds using only
those (Tata where the estimated weight of algal cells comprised a major faction
(> 10%) of total seston weight (25).  The slope of this relationship (0.0146
n^mg"1; r2 = 0.95) Is quite consistent with observations  from deep
coastal waters (48) and slightly greater than those derived from laboratory
cultures (31, 49).  We also obtained a strong relationship between PAR attenu-
ance through a plane  (estimated by scraping epiphytes from leaves of known
area into a petri dish and measuring PAR attenuation, 50) and epiphytic biomass
(r2 = 0.87).  The slope of this relation was 0.36% (mg/cm2)'1, which
again is comparable to previous reports (10, 50, 51).  Direct measurements of
SAV photosynthesis (using both 1J»C uptake and oxygen evolution methods) over
a range of epiphytic colonization levels (0-5 gdw epiphyte/gdw SAV) yielded
significant correlations with about 50% reduction in photosynthesis at 4
gdw/gdw (25).  After 8 wk of nutrient treatment, SAV biomass in medium and
high fertilization systems had decreased significantly compared to controls
(Fig. 5).  While the contribution of the water column to total attenuation
(from water surface to plant leaves) was generally  small  compared to that
associated with epiphytic material, without such light reduction due to tur-
bidity, epiphytic growth would have been insufficient to reduce light below
compensation levels (Ic).  Similar observations on the relative roles of
phytoplankton and epiflora have been reported for Danish lakes (51).
                                     IX-12

-------
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    PHYTOPLANKTON
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   -EPIPHYTIC BIOMASS
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                   A
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   _ SAV  BIOMASS
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        Control   Low    Medium   High

              NUTRIENT DOSE

-------
!
             Fig.  5.   Summary of phytoplankton stocks (as  chlorophyll £),  weight  of  epiphytic
                      material,  and submerged aquatic vegetation  biomass  in  August  1981,
                      for experimental  ponds  treated with  4  levels  of  nutrient  enrichment
                      after 3 weeks.   Given are means   1  standard  error  (25).
                                                   IX-13a

-------
•I!
 •  -
 '.  '                                  Nutrient  Enrichment  and  SAV  Decline

   I                 While nutrient  fertilization  has  been occurring  1n  Chesapeake  Bay  over
   '            the last century or  more,  the upper bay  outbreak of M. spicatym  in  the  early
               1960's (just  proceeding the  general  SAV  decline) may~have marked the  onset  of
   ;            critical nutrient enrichment levels.   It appears that this  species  has  competi-
               tive advantage ov»r  other  SAV under conditions of  elevated  nutrient inputs
               (e.g,  52).  During the last  several years  numerous investigators have postu-
               lated that eutrophication  was responsible  for  observed losses of SAV  in fresh-
               water systems including:   Loch  Leven,  Scotland (38);  Lake Erie,  Chio  (53);
               Whitewater Lake, Ontario  (54);  and Norfolk Broads, England  (10).  Others have
               suggested that nutrient enrichment may be  one  of several factors contributing
               to dlmunition of seagrasses  in  su^h coastal ecosystems as the Dutch Waddenzee
               (9), the French Mediterranean (55) and Western Australia estuaries  (56).   In
               all  cases, these declines  have  occurred  progressively over  decade-long  periods,
               consistent with observations in Chesapeake Bay (13).

                    In our experimental pond studies, SAV biomass reductions in high dose
               ponds amounted to a  foreshortening of  the  normal growing sesaon; however, some
               effects on vegetative  reproduction were  also apparent.   For example,  shoot
               production from budding was  markedly reduced in experimental ponds  the  spring
               following a summer of  high nutrient treatment, and after a  second year  of
               identical treatment  vegetative  root nodules were also a  significantly reduced
               (25).  To interpret  these  results  in the context of long-term declines  in
               abundance, it may be necessary  to  relate premature seasonal scenescence to
               decreased reproductive success  in  a more quantitative fashion.


                                   SYNTHESIS  OF  FINDINGS AND IMPLICATIONS


                                 Ecosystem Modeling  as a Tool for Synthesis

                    The research discussed  in  this paper  has  identified and described  a wide
               range of detailed Interactions  between submerged vascular plants  and  their
               environment.   We used  a variety of numerical simulation models for  these SAV
               ecosystems to place  the complex interactions into  a unified ecological  frame-
               work.  Che of these  models was  designed  to address questions relating to man-
               agement of SAV resources in  the estuary, including both the role of these
               plants 1n secondary  production  and the environmental  factors influencing plant
               growth (57).   Generally, this model  consisted  of 14 simultaneous, non-linear
   {            differential  equations, with time  as the independent  variable.   Each  equation
   I            described the temporal behavior of a state variable,  with each term in  a given
   i            equation representing  a connection between that variable and another  internal
   |            or external variable.   The model was calibrated with  data collected from field
               and laboratory measurements  and was used to interpolate  between  discrete ob-
               servations in time and space and to both hindcast  and forecast consequences of
               changing external factors  (e.g. temperature, nutrient loading and flushing
               rate).

                    Digital  computer  simulations  were performed (using  finite difference
               numerical methods) to  consider  the effects of  herbicide, sediment and nutrient
                                                    IX-14

-------
  fl
.
inputs to the estuary individually and combined (Fig.  6).  Under simulated
conditions based on maximum observed herbicide concentrations in a shallow
embayment, annual mean SAV biomass (B) and total net production (NP)  decreased
by about 2% and 4% respectively (Fig. 6a), with greatest effects during May-
June runoff events.  A doubling of sedimer.. loading to the SAV bed and sur-
rounding waters resulted in J3% and 22% reductions in B and NP, and the effects
were most pronounced in the late summer and fall periods when heavy winds
stimualted sediment resuspension (Fig. 6b).  A two-fold nutrient errichment
resulted in marked reductions in B and NP (29% and 48%), with strongest depres-
sions occurring in June (Fig. 6c).  Apparently, relatively high nutrient and
light levels in spring promoted rapid growth of epiphytic algae which was
retarded subsequently in summer due to nutne-t depletion.  These simulation
results are similar to findings from pond fertilization experiments (25).
Combining these three stresses in a single simulation  resulted in an  average
of 35% and 56% losses of biomass and production, and the net effect of the
three factors thus appeared to be somewhat antagonistic (Fig. 6d).

     It is interesting to note that under combined stresses the growing season
(period of continuous biomass accumulation) was reduced from about 4.5 mo to
less than 2 mo.  In addition, plant biomass at the end of the simulated year
was considerably lower than initial levels under combined factors.  While this
might suggest an effect on reproductive success of this population, multi-year
simulations produced recurring patterns similar to that shown in Fig. 6.
However, this  is not surprising because reproductive biology, per se, was not
an explicit function in the model.


            Gradual Reductions in Abundance versus Population Demise

     Most of the species occurring in upper Chesapeake Bay undergo clear annual
cycles of above-ground structure, and new growth in spring may be generated
either from seeds or from underground storage organs.   The .nodel  simulations
indicated that under environmental stress, net growth  available for allocation
to sexual or vegetative reproduction is reduced.  Experimental observations
have demonstrated that extreme light stress, related either to turbidity or
epiphytic growth, results in significant decreases in  both flowering  and pro-
duction of underqround storage nodules (25, 32).  We hypothesize that the
observed gradual reductions in abundance of stressed plants in model  and field
experiments would eventually result in a local demise  cf these populations due
to reproductive failure.  This final outcome would require a repetitive se-
quence of annual stressed growth conditions, incrementally undermining repro-
ductive vitality.

     In general, SAV communities exhibit considerable  ability to modify their
local environment and thereby reduce stresses; however, this "buffering capac-
ity" can be exceeded and lead to premature seasonal die-back.  Two examples
involve nutrients ar I suspended particulates.  While SAV communities  are cap-
able of rapid uptake of nutrients from overlying water (4), nutrient  enrichment
beyond the assimilative capacity can promote epiphytic algal growth to the
detriment of vascular plar:t production (e,.g. 10, 25).   Secondly, the  well
established ability of seagrasses and other SAV to trap and bind suspended
particulate matter (e.g. 5, 58) is limited in silty estuarine environments
                                                    IX-15

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27


18


 9



36


27


18


 9
         A) HERBICIDE

            RUNOFF
                       Basel i ne
                       Simulation


                         BasalIne
             b Herbicides

               Added
                     I   I   I
B) SEDI MENT

   LOADING
Baseline
            Sediments

            Added
36


27


18


 9



36 -


27 -


18 '


 9 -
C) NUTRIENT  j$j%jBa,.lin.

ENRICHME
D)COMBINED
                       Base!ine
         FMAMJ  JASONDJ

                MONTH  OF YEAR
                      IX-16

-------
Fig.  6.   Results of numerical  simulations of submerged vascular plant  biomass
         over an annual  cycle  under conditions  of  maximum herbicide  runoff,
         doubled sediment loading,  doubled nutrient  enrichment, and  a  combina-
         tion of these three stresses.   In all  cases  the  upper  line  (shown for
         comparative purposes) represents the baseline conditions  which  are
         compared to means  of  field observations in  the upper panel  (57).
                                    IX-16a

-------
(25), and associated turbid waters tend to reduce light availability and plant
production.  It appears that while moderate levels of these stresses result in
little if any reductions in plant production and distribution,  extreme levels
can overwhelm the ability of such plant communities to ameliorate these factors.


                         Reconstructing the SAV Decline

     Historical trends in data for SAV distribution and abundance, sediment
and nutrient loading and herbicide use (Fig. 1) coupled with experimental
results and model simulations reported here allow development of a plausible,
although not conclusive, scenario concerning the observed decline of SAV in
Chesapeake Bay.

     Accelerated soil erosion and sediment loading associated with extensive
land development in the late 1950's apparently led to elevated  turbidity in
several regions of the estuary (28, 29, 30, 35).  Nutrient loading also in-
creased from point sources associated with population centers and from regional
diffuse sources associated with intensifying agriculture.  In the Patuxent
River estuary, marked declines in SAV populations (59) coincided with increased
turbidity and nutrient concentrations.  In the Potomac River, invasion by M.
spicatum followed by proliferation of blue-green algal mats, correlated wiTh
expanding sewage discharges and incipient declines in native SAV (60).  The
establishment of M. spicatum in brackish portions of Chesapeake Bay proceeded
a general SAV decTine (11), which may have been a response to eutrophication
(52).   In the late 1960's, it appears that strict erosion control practices
reduced sediment runoff inputs and associated turbidities in some regions;
however, nutrient inputs continued to rise.  The general use of herbicides for
agricultural weed control in the watershed in the mid-1960's may have contrib-
uted slightly to deteriorate SAV growth conditions, and the greater tolerance
of ^. spicatum to herbicides is consistent with the observed brief prolifera-
tion of this species.  Possibly further compounding these stressed conditions
was the drought of the 1960's (Fig. 1) which led to increased salinities (61)
and probably salinity stress for various freshwater species (62).  In the
summer of 1972 a 200 year storm event resulted in rates of sediment deposition
up to 25 cm  (63) in some locations, virtually burying local plant communities.
While this storm occurred well after the initial evidence of SAV decline, its
effects are apparent in plant abundance data (Fig. la).  There is also evidence
for considerable damage to SAV populations from intense grazing and foraging
episodes by waterfowl and fish (64, 65).

     While it is impossible to definitely demonstrate what caused the SAV
decline, data from controlled experiments, as well as recent field observations
and historical records all suggest that nutrient enrichment and increased
turbidity probably played major roles. Herbicide runoff may represent an
ephemerally  and  locally important stress, but our data indicate its contribu-
tion to this general die-back has been minimal.  Major meteorological events
and direct grazing certainly have contributed to the  rigors of SAV existence
and may have accelerated the recent decline, but historical records indicate
these factors probably also played a secondary role.
                                      IX-17

-------
r
                                       Management of SAV Resources

                  The potential  for improving environmental  conditions for propagation and
             growth of submerged vascular plants in the estuary can be considered in terms
             of the relative importance and controllability  of  key factors.  Herbicide
             losses from croplands to adjacent watercourses  can be reduced by such measures
             as alternative weed control  techniques and "buffer stripping" at the margins
             of fields;  however, the socio-economic costs of such strategies  would be con-
             siderable (66).  Research results presented here indicate herbicides exert a
             minor influence on  SAV, so that the wisdom of such measures  may  be questionable.

                  Three  major sources of suspendable sediments  to the estuary are runoff,
             shoreline erosion and resuspension of bottom sediments (67).   Both shore ero-
             sion, which is important in the middle bay and  eastern shore tributaries (67,
             68),  and resuspension, which dominates in most  estuarine shallows (25),  are
             largely uncontrollable.  Existing soil conservation practices have been  reason-
             ably  effective in reducing sediment runoff (29, 30) and probably represents a
             realistic management option, given the apparent contribution of  suspendable
             sediments to the SAV decline.

                  Although nutrient inputs  to Chesapeake Bay from sewage  effluents represent
             less  than 20% of the total  for nitrogen (69), in some areas  such as the Potomac
             and Patuxent Rivers, they are  a major source which can be controlled,  albeit
             at considerable cost.  Diffuse sources, dominated  by agricultural  runoff,
             represent the largest nutrient input to the estuary, and efforts to develop
             effective controls  are crucial.  The substantial experimental  and correlative
             data  available strongly suggest that nutrient enrichment has resulted in losses
             of submerged vascular plants in a variety of aquatic systems, including Chesa-
             peake Bay.   This, coupled with other documented consequences of  excessive
             fertilization (e.g. noxious algal blooms and anoxic bottom waters) point to a
             considerable need to focus on  nutrient waste management.

                  The role of SAV communities In maintaining water quality and promoting
             secondary production has been  well established  in  general and in Chesapeake
             Bay in particular.   For example, it has been estimated that  about 50% of the
             current sewage derived nutrient inputs to the bay  could have been assimilated
             by healthy SAV communities which existed in 1960 (70).  Kahn (71) has  calcu-
             lated that  previous SAV populations supported a fishing resource valued in
             excess of a million USA dollars per year.  Nevertheless, estuaries are char-
             acteristically stressed environments, and such  natural stresses  combined with
             anthropogenic factors tend to  make estuarine littoral regions only marginally
             compatible  with SAV physiological requirements.  If a decision were made to
             attempt a reversal  of this SAV decline, it appears that several  steps would be
             required,  first, nutrient inputs should be reduced and efforts  should be made
             to continue and improve soil erosion control practices.  Second, a better
             understanding of propagation and reproduction mechanisms and capabilities of
             these plants is needed.  Finally, a substantial program (possibly involving
             direct transplanting) might be required to accelerate restoration of these SAV
             communities.
IX-18

-------
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•I  I
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