EPA-R3-73-039
July 1973 Ecological Research Series
Rates of Photosynthesis
and Photoplankton Growth
in Shagawa Lake, Minnesota
Office of Research and Monitoring
U.S. Environmental Protection Agency
Washington, D.C. 20460
-------
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EPA-R3-73-039
July 1973
RATES OF PHOTOSYNTHESIS AND PHYTOPLANKTON
GROWTH IN SHAGAWA LAKE, MINNESOTA
by
Robert 0. Megard
Department of Ecology and Behavioral Biology
University of Minnesota
St. Paul, Minnesota 55101
Research Grant No. 16010-DEG
Program Element 1B1031
Project Officer
Charles F. Powers
Pacific Northwest Environmental Research Laboratory
Corvallis, Oregon 97330
Prepared for
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D. C. 20460
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
-------
EPA Review Notice
This report has been reviewed by the Environmental Protec-
tion Agency and approved for publication. Approval does
not signify that the contents necessarily reflect the
views and policies of the Environmental Protection Agency,
nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
ii
-------
ABSTRACT
Average concentrations of chlorophyll a. at 6 localities in
Shagawa Lake were between 15 and 25 mg m ^ during June and
July, 1970, they increased to 150 mg m-3 in late August,
and decreased rapidly in early September.
Daily maximum specific photosynthesis and stored phosphorus
in the plankton both decreased while population densities
increased most rapidly. Rapid growth probably stopped
because the rate of chlorophyll synthesis exceeded the rate
of phosphorus uptake.
The theoretical limits of daily integral photosynthesis
imposed by the attenuation coefficients for photosyntheti-
cally active radiation are calcualted for the observed
ranges of daily maximum specific photosynthesis and
chlorophyll concentrations. High attenuation coefficients
probably prevented population densities from becoming high
enough to utilize all the dissolved phosphorus in the
water during the period of most rapid growth.
Specific rates of production and loss of particulate organic
carbon in the mixed layer (epilimnion) were both highest
just before rapid growth began. Specific rates of loss
decreased more rapidly than specific rates of production
to produce high specific rates of change during August.
Loss rates probably decreased during August because the
dominant alga, Aphanizomenon, became more buoyant. The
rapid decrease of population densities in early September
occurred because loss rates suddenly increased, probably
because the populations's buoyancy regulation became less
efficient.
A balance equation for the quantity of particulate organic
carbon in the mixed layer beneath a unit surface area is
used to estimate steady states for integral photosynthesis
and chlorophyll concentrations from the optical conditions,
rates of maximum specific photosynthesis, and rates of loss
that prevailed in the lake.
This report was submitted in fulfillment of Project No.
16010 DEC under sponsorship of the Office of Research and Monitoring,
Environmerital Protection Agency.
-------
CONTENTS
Abstract
List of Figures
List of Tables
Sections
I Conclusions
II Recommendations
III Introduction
IV The Lake
V Methods
VI Phytoplankton, Chlorophyll,
and Photosynthesis
VII Phosphorus
VIII Extractable Phosphorus and
Phytoplankton Growth
IX Nutrient Limitation
X Integral Photosynthesis and
Lake Productivity
XI Population Regulation
XII Steady States for Population
Densities and Integral
Photosynthesis
XIII Discussion
XIV List of Symbols
XV Acknowledgements
XVI References
Page
iii
vi
vii
1
3
5
7
13
15
27
33
39
41
49
55
59
63
65
67
-------
FIGURES
Page
1 Hydrographic map of Shagawa Lake 8
2 Mean concentrations of chlorophyll ja,
total phosphorus, and mean maximum
photosynthesis 16
3 Chlorophyll a_ and depth profiles of
photosynthesis at Station A 22
4 A, maximum specific photosynthesis;
B, the ratio of total phosphorus to
chlorophyll a.; C, the ratio of soluble
to total phosphorus; and D, mean
concentrations of total phosphorus
and chlorophyll a_ in surface waters 24
5 The relationships between maximum speci-
fic photosynthesis and chlorophyll
a_ concentrations 25
6 The relationships between concentrations
of chlorophyll ai and total phosphorus
in surface waters 31
7 Relationships among chlorophyll a.
concentrations, z., c. , and Zp 43
8 Regression of chlorophyll a_ on dry
weight of particulate material 50
9 Steady state population densities 56
10 Relationships among P , X, Zp, and w' 61
max
VI
-------
TABLES
Page
1 Hydrographic features of Shagawa Lake 9
2 Major dissolved ions in Shagawa Lake 10
3 Temperature, oxygen, and pH at depth
intervals at Station D 11
4 Chlorophyll a concentrations, photosyn-
thesis , phosphorus concentrations,
and temperatures at 6 localities 17
5 Coefficients of variability for chloro-
phyll concentrations 20
6 Concentrations of chlorophyll ja
and photosynthesis in the morning
and afternoon on 1 August, 1968 21
7 Concentrations of total phosphorus
at Station A in 1970 28
8 Phosphorus concentrations before and
after a storm 29
9 Extractable phosphorus in net plankton 34
10 Chlorophyll content of the phytoplank-
ton 36
11 Parameters for the photosynthetic
system of the phytopiankton 44
12 Means and standard deviations of
chlorophyll concentrations,
photosynthesis, and phosphorus
at 6 localities 48
13 Specific rates of change of production,
and loss for particulate organic
carbon in the mixed layer 53
VI1
-------
SECTION I
CONCLUSIONS
1. Population densities of phytoplankton in Shagawa
Lake were relatively constant during early summer,
1970. Population densities of Aphanizomeno n increased
most rapidly during a bloom that occurred in August,
and they decreased rapidly during the first week of
September.
2. The phytoplankton were stratified at the surface of
the epilimnion during the bloom but not at other times.
3. The photosynthetic capacity of the phytoplankton as
measured by the specific photosynthesis at the depth
where illumination is optimal increased during the month
before the bloom but decreased during the bloom.
4. Population densities increased rapidly during the
bloom because specific loss rates decreased more rapidly
than specific production rates. The stratification of
the phytoplankton during the bloom suggests that specific
loss rates decreased because the algae became more buoyant,
5. Phosphorus is a limiting nutrient for the phytoplank-
ton in Shagawa Lake, but phytoplankton production
appeared to be limited not by nutrients but by high
attenuation coefficients for photosynthetically-active
radiation.
6. Steady state population densities under the physical
conditions that prevailed are estimated with a balance
equation for the quantity of particulate organic carbon
in the mixed layer beneath a unit of lake surface.
-------
SECTION II
RECOMMENDATIONS
Although maximum population densities of phytoplankton
in Shagawa Lake during 1970 appeared to be limited by
high attenuation coefficients for photosynthetically
active radiation rather than by nutrient concentrations,
phosphorus could be made to limit population densities
by reducing the influx from the Ely Municipal Sewage
Treatment Plant, which contributed about 70% of the lake's
phosphorous.
A monitoring program should be continued after the new
treatment plant begins operation in 1972 in order to
ascertain the nature of the lake's response. A minimal
program should include measurements of the following:
integral photosynthesis, chlorophyll concentrations,
concentrations of particulate organic material, total and
soluble phosphorus, extractable phosphorus in the net
plankton, and algal counts. These should be sampled at
ten-day or two-week intervals during the open-water season,
-------
SECTION III
INTRODUCTION
Photosynthesis and nutrient uptake have been studied inten-
sively in order to understand fluctuations of phytoplank-
ton abundance, but studies of these processes cannot
indicate whether population densities are increasing,
decreasing, or stationary. Population densities depend
upon rates of production and loss; the effects of photo-
synthesis on population densities therefore can be defined
only if photosynthesis and population densities are measured
in compatible units or if both production and losses are
measured. Rates of flux from nutrient pools also depend
upon rates of production and loss of phytoplankton (Dugdale
1967). Since loss rates are usually neglected, studies
of relationships between nutrients and phytoplankton are
usually inconclusive (Fogg and Walsby 1971).
Most pelagic photosynthesis is accomplished by phytoplank-
ton circulating in the epilimnion. Thus the epilimnetic
phytoplankton functions as a unit. If photosynthesis is
measured in terms of daily carbon assimilation beneath a
unit surface, then the quantity of particulate organic
carbon associated with the phytoplankton beneath a unit
of surface in the epilimnion is an equivalent measure of
population density. There is no satisfactory way to
determine how much of the particulate organic carbon is
associated with phytoplankton, but the phytoplankton is
only one constituent of a larger pool of reduced particulate
carbon that also contains zooplankton, bacteria, and
organic detritus. Organic detritus may comprise 20-80%
of thei particulate organic material in lake water (Wright
1959), and it, like zooplankton and bacteria, is ultimately
produced by photosynthesis. Because each group of
organisms participates in the degradation of organic material
produced by algal photosynthesis and because many of the
chemical changes that occur in lakes depend more upon
rates of production and destruction of particulate organic
material than upon population dynamics of individual
organisms, it is more appropriate from the limnological
point of view to assess the dynamics of particulate
organic carbon than of phytoplankton or zooplankton alone.
Thus our inability to distinguish the quantities of
particulate organic carbon associated with each category
of living and dead organisms is not a serious disadvantage,
because the total quantity of particulate organic carbon
in the epilimnion is also a functional ecological unit.
Particulate organic carbon is lost from the epilimnion by
-------
three separate and unrelated mechanisms -- respiration,
excretion, and sinking. It is difficult to measure the
rates of these processes individually, but the aggregate
loss of particulate organic carbon may be calculated from
the rate that it is produced and the rate that its concen-
tration changes.
A balance equation for particulate organic carbon in the
epilimnion is formulated in this report, and data from
Shagawa Lake are used to calculate specific rates of
production, specific rates of change, and specific rates
of loss. It is often assumed that phytoplankton blooms
occur because production rates increase, but specific
rates of production decreased during a bloom that occurred
in Shagawa Lake. Population densities increased because
the decreased production rates were offset by decreased
loss rates. The balance equation is also used to calculate
steady state population densities for phytoplankton.
Mathematical models of plankton dynamics tend to be one
or the other of two types: time-related models concerned
with growth dynamics or depth-related models concerned
with productivity in water columns (Patten 1968). However,
depth-related models are instantaneous components of
time-related models, and an important requirement of a
time-related model must therefore be that it incorporate
a depth-related model. Also, if a time-related model is
to be realistic, it must be formulated in such a way
that data can be obtained to test and use it. Tailing
(1957b) and Vollenweider (1965, 1970) have developed a
realistic depth-related model for the daily production of
particulate organic carbon by phytoplankton. This may be
used as a basis for a realistic time-related model for
the dynamics of particulate organic carbon in the mixed
layer because the existing quantity of particulate
organic carbon may be estimated easily. Thus all the
data for evaluating the parameters of a time-related model
for particulate organic carbon may be obtained with
chemical methods.
-------
SECTION IV
THE LAKE
Phytoplankton are usually much more abundant in Shagawa
Lake (Lat. 47° 55' N, Long. 91° 25' W) than in most other
lakes in northeast Minnesota. About 80% of the water
entering Shagawa Lake comes from the Burntside River which
drains Burntside Lake, a large unproductive lake located
several km to the west. However, about 70% of the phos-
phorus and 25% of the total nitrogen entering Shagawa
Lake via surface waters come from a municipal sewage
treatment plant at Ely on the south side of the lake
(Powers et. al. 1972; Fig. 1).
£ O
The area of the lake is 9.6 x 10 m , its volume is 59.5
x 106 rn^, its maximum depth is 14.7 m, and its mean depth
is 6.2 m (Table 1). Many small, rocky islands are scattered
throughout the basin.
The salinity of Shagawa Lake is 1.6 meq liter , which is
similar to the salinity of other lakes in northeast Minne-
sota (Table 2; Bright 1968), but higher than the mean
salinity (0.4 meq liter ) of 40 lakes in the Experimental
Lakes Area in nearby northwest Ontario (Armstrong and
Schindler 1971), The dominant dissolved cations are
calcium and magnesium, and the dominant anions are
bicarbonate and sulfate.
The effects of the abundant plankton on water chemistry
are indicated by the differences between the oxygen
concentrations and pH in the epilimnion and those in
deeper waters (Table 3). Oxygen is usually present in
the deep waters of lakes in northeast Minnesota, but not
in the deep waters of Shagawa Lake. It is also notable
that the pH may exceed 10 in surface waters but it is near
neutrality below 6 m. The pH of surface waters in most
lakes of the region is typically neutral or slightly
acid (Bright 1968).
-------
Figure 1. Hydrographic map of Shagawa Lake. Six sampling
stations are indicated by the letters A, B, C, D, E, and F.
-------
Table 1. Hydrographic features of Shagawa Lake.
Depth
Interval
(ft)
0-10
10-15
15-20
20-30
30-40
40-48
Area
(106 m2)
9.64
7.64
6.66
5.76
1.10
1.01
(%>
79
69
60
11
10
Volume
(106 m3)
26.30
10.88
9.49
9.54
3.22
.05
(%)
44
18
15
16
5
fi p
Surface Area 9.64 x 10 m
Volume 59.5 x 106m3
Mean Depth 6.2 m
Maximum Depth 14.7 m
59.5
-------
Table 2. Major dissolved ions (mg liter" ) in Shagawa Lake,
19 northeastern Minnesota lakes (Bright 1968) and
for a group of 40 lakes in Ontario just north of
Minnesota (Armstrong and Schindler 1971).
Calcium
Magnesium
Sodium
Potassium
Bicarbonate
Chloride
Sulfate
Salinity (meq/1)
Shagawa
Lake
9.90
3.36
2.0
0.6
26.8
0.1
13.8
1.62
Mean fo:
nor the ai
Minneso'
lakes
10.2
3.0
1.5
0.8
39.0
0.1
5.7
1.7
Mean for 40
Ontario
lakes
1.6
0.9
0.9
0.4
3.8
1.4
3
0.4
10
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-------
SECTION V
METHODS
Population densities of the phytoplankton, rates of photo-
synthesis, and phosphorus concentrations were measured
during the spring and summer of 1970 in water samples from
6 localities (Fig. 1).
Gross photosynthesis by the phytoplankton was measured
with the oxygen light-dark bottle method (Strickland and
Parsons 1968, Tailing 1957a). The daily total gross
photosynthesis (integral photosynthesis) beneath a unit
surface was measured in. situ at Station A. Water samples
were collected at depth intervals, and portions of each
sample were transferred immediately to transparent and
opaque B.O.D. bottles (300 ml capacity). The bottles were
placed in holders, and the holders were suspended at the
depths from which the samples were collected (usually 0,
0.2, 0.5, 1, 1.5, 2r and 3 m). The bottles were suspended
for 6-hr incubation periods, from 1200 to 1800 hrs. Thus
almost 50% of the daily gross photosynthesis was measured.
Water samples were also collected at the surface from each
of the other 5 localities immediately before each experi-
ment. These samples were also transferred to pairs of
light and dark bottles, placed in holders, and the holders
were attached to a bar between two buoys so that the bottles
were suspended at a depth of 0.5 m, where the average light
intensity was optimal for photosynthesis. This depth was
ascertained from previous depth profiles of photosynthesis
obtained from samples incubated in. situ at depth intervals.
Oxygen concentrations in the transparent and opaque bottles
were determined immediately after each experiment; 203 ml
samples from the B.O.D. bottles were titrated with 0.025 N
Na2S203 delivered from 10-ml automatic burets, with which
oxygen concentrations may be estimated to the nearest 0.02
mg oxygen liter~l. Daily gross carbon assimilation was
computed from the oxygen changes with the equation,
2 (LB - DB)
P = F P.O.
= .625 (LB - DB)
where p is gross photosynthesis (mg carbon liter dy );
LB and DB are the oxygen concentrations (mg liter""!) in
the transparent and opaque bottles; PQ is the photosynthe-
tic quotient (moles oxygen produced/moles carbon assimilated),
13
-------
which was assumed to have a value of 1,2 (Westlake 1963,
Strickland 1965); and F is the ratio of the molecular
weights of carbon and oxygen (0.375) . It should be
emphasized that the estimates of daily gross photosyn-
thesis are somewhat low because the days were longer than
twice the 6-hr incubation interval.
2 1
Daily integral photosynthesis (Ip; mg carbon m dy ) at
Station A was estimated by determining the area enclosed
by the depth profiles with a planimeter. The photosyn-
thesis that occurred in the bottles incubated at the
average depth of optimal illumination was a measure of
daily maximum photosynthesis at the localities from which
the samples were collected. Maximum specific photosynthesis
(p ; mg carbon [mg chlorophyll]""' dy~^) was estimated as
th^ ratio of maximum photosynthesis to chlorophyll concen-
trations (p :c) in the samples incubated at optimal illumin-
ation. max
Concentrations of chlorophyll a_ in the water samples used
for the photosynthesis experiments were determined by
the SCOR/UNESCO procedure (Strickland and Parsons 1968).
Between 250 and 500 ml of sample were filtered through
Millipore type HA membrane filters. The filters were
macerated in a tissue grinder that contained 90% acetone,
and the absorbancie of the acetone extracts were deter-
mined after 1/2 hr.
Concentrations of total and soluble phosphorus were
determined with the ammonium molybdate method, using
stannous chloride as the reductant (Amer, Public Health
Association 1965). Concentrations of total phosphorus
were determined in unfiltered samples after they were
digested with K2S20g in a pressure cooker (Menzel and
Corwin 1965). Concentrations of soluble phosphorus were
determined on samples that were not filtered or digested.
The concentrations of soluble phosphorus are probably
overestimated somewhat because the reagents may have
hydrolyzed some of the particulate phosphorus. The
extractable phosphorus content of the net plankton
was determined by the procedure described by Fitzgerald
and Nelson (1966) .
14
-------
PHYTOPLANKTON, CHLOROPHYLL, AND PHOTOSYNTHESIS
The planktonic algae in Shagawa Lake were not examined
intensively during the summer of 1970, when the lake was
studied most intensively, but periodic examinations
indicated that Anabaena was dominant in June. Aphani2?o~
menon appeared in late June, it became conspicuous during
July, and very abundant in August, when it represented
virtually the entire algal biomass. A rapid decrease
occurred in early September,
Mean chlorophyll concentrations in surface waters fluctuated
between 11 and 21 mg m~3 during June and July, increased
to 146 mg m~3 on August 31, and decreased again during
September and October (Fig. 2, Table 4).
Chlorophyll concentrations at the 6 stations were more
similar to each other on the sampling days in June, July,
and early August than in late August, when the coefficient
of variability increased to 69% (Table 5). Although the
variance increased during August, concentrations were
not consistently higher at any one locality than at others.
Chlorophyll was probably concentrated by local water
movements at three localities on 6 August, 31 August, and
5 September, so the data from these localities were
excluded from the computations of the means and variances
for those dates (Table 4). The most extreme example was
on 31 August, when the concentration at the surface was
977 mg m~3 at Station A but between 67" and 321 mg m~3
at the other localities.
The variation that may occur at a locality during a single
day is exhibited by the chlorophyll concentrations at
Station D on 1 August, 1968 (Table 6). Chlorophyll
concentrations above 4 m at 1200 hr (34-38 mg m~3) were
50% higher than at 0600 hr (24-26 mg m~3). Furthermore,
the highest specific photosynthesis (P) was measured at
the surface for the samples incubated in situ during the
morning, but at 0.5 m for the samples incubated in the
afternoon. As a result of these variations, daily integral
photosynthesis on 1 August, 1968, would be 1.9 g carbon
m~2 dy~l if calculated from the morning experiment and
2.5 g carbon m~2 dy~l if calculated from the afternoon
experiment. The skies were cloudless the entire day?
photosynthesis is apparently less vulnerable to inhibition
by high light intensities in the morning than afternoon.
Thus, the low rates of photosynthesis at the surface that
are evident in the depth profiles for 1970 (Fig. 3) are
perhaps largely photoperiodic responses that are character-
istic features of incubations performed after noon (1200-
1800 hrs).
15
-------
Oct
Figure 2. Mean concentrations of chlorophyll a_s total phos-
phorus, and mean maximum photosynthesis (p ) in surface
waters of Shagawa Lake at Stations A-F during 1970. The
standard deviation for tnch date is indicated by the length
of the line through each datum point. Concentrations of
soluble phosphorus at Stations A and B and temperatures of
surface water at Station A are also indicated.
16
-------
Table 4. Chlorophyll concentrations (c, mg m ), maximum
photosynthesis (pmax/ 9 carbon m~3 dy~l), maximum
specific photosynthesis (Pmax/ m9 carbon (mg
chlorophyll)~1 dyl), integral photosynthesis
(Zp, g carbon m~2 dy~l), the depth z^ (meters),
concentrations of total phosphorus (F^/ nig m~3)
and temperatures in surface waters (T, °C) of
Shagawa Lake in 1970. Numbers in parenthe_ses were
excluded from computations of the means (x) and
standard deviations (S.D.).
Experiment # Station
and Date
1, 3 June A 8 0.29 41 0.88 3.0 23 14
it ion
A
B
C
D
E
F
X
S.D.
A
B
C
D
E
F
X
S.D.
c
8
9
10
11
21
9
11
5
16
20
20
15
25
30
21
6
p
max
0.29
0.23
0.28
0.34
0.53
0.28
0.33
.11
0.31
0.40
0.36
0.28
0.42
0.58
0.39
.11
P Ep z .
max r i
41 0.88 3.0
25
29
31
25
31
30
6
20 0.45 1.4
20
18
18
17
19
19
1
F,_
t
23
27
26
22
32
25
26
4
35
39
38
31
35
36
36
3
2, 17 June A 16 0.31 20 0.45 1.4 35 18
17
-------
Experiment #
and Date
3, 2 July
4, 22 July
5, 6 Augus t
6, 17 August
ition
A
B
C
D
E
F
X
S.D.
A
B
C
D
E
F
X
S.D.
A
B (
C
D
E
F
X
S.D.
A
B
C
D
E
F
X
S.D.
C
17
14
16
13
15
16
15
1
18
27
22
19
20
21
21
3
25
120)
44
27
28
16
28
10
108
54
75
42
50
72
67
24
P
*max
0.46
0.47
0.52
0.37
0.49
0.51
0.47
0.05
0.88
1.08
0.84
0.87
0.85
0.75
0.88
.11
1.23
(2.26)
1.96
1.48
1.35
0.99
1.40
0.36
2.45
1.52
2.17
1.32
1.36
1.46
1.71
.48
P Zp
max F
27 0.94
34
33
28
33
33
31
3
50 1.04
39
38
46
43
36
42
5
52 1.35
(19)
45
54
47
60
52
6
32 2.14
28
29
31
27
19
28
5
Z. F.
l t
2.0 32
27
26
25
24
24
26
3
1.2 97
86
70
73
79
58
77
14
1.1 116
(180)
118
109
110
104
111
6
0.9 157
110
110
123
108
142
125
20
T
22
22
25
22
18
-------
Experiment # Station
and Date
max
max
1, 25 August A
B
C
D
E
F
S
8, 31 August A
B
C
D
E
F
S.
9, 5 Sept A
B
C
D
E
F
S
10, 17 October A
B
C
D
E
F
S
40
198
87
71
121
27
X 91
.D. 62
(977)
102
117
321
124
67
X 146
D. 100
29
29
37
60
(185)
31
X 37
.D. 13
16
18
16
14
18
26
X 18
.D. 4
1.72
4.75
2.80
2.73
4.06
1.16
2.87
1.36
0.87
0.47
0.92
1.83
(3'. 41)
0.87
0.99
0.50
0.25
0.23
0.26
0.28
0.32
0.33
0.28
0.04
43
24
32
39
34
43
36
7
30
10
25
31
(18)
28
25
9
16
12
17
19
18
13
16
3
z.
2.13 1.2
2.70
1.41 1.6
0.52 2.1
123 21
193
129
133
158
161
150
26
(433) 21
123
142
223
131
102
144
46
85
98
87
94
(153)
70
87
11
92
56
51
44
42
41
54
19
21
19
-------
Table 5. Coefficients of variability (S.D./X) for chloro-
phyll concentrations at 6 localities in Shagawa
Lake.
Date Experiment Mean
(X)
3
17
2
22
6
17
25
31
5
17
June
June
July
July
August
August
August
August
September
October
1
2
3
4
5
6
7
8
9
10
11
21
15
21
28
67
91
146
37
18
Standard
deviation
(S.D.)
4
5
1
3
10
23
62
100
13
4
.7
.7
.5
.2
.1
.9
.4
.2
.2
.2
S.D
X
43
27
10
15
36
36
69
69
36
23
20
-------
Table 6. Concentrations of chlorophyll ai at 0600 hrs and
1200 hrs and rates of photosynthesis measured in
the morning (0600-1200 hrs) and in the afternoon
(1200-1800 hrs) at Station D on 1 August 1968.
0600 hrs
1200 hrs
Depth Chlorophyll a
-3
(m)
rag m
Photosynthesis
P
g carbon
mg carbon
(mg chl)~1 dy~l
0.0
0.5
1.0
2.0
4.0
0.0
0.5
1.0
2.0
4.0
26
24
24
24
25
36
38
35
36
34
1.63
1.18
0.78
0.18
0.02
1.29
1.60
1.25
0.32
0.07
63
49
33
18
1
36
43
36
9
2
21
-------
SHAGAWA LAKE, Station A-1970
Chlorophyll a (mg/m3) Photosynthesis (g carbon/m3-dy)
rbon/mz-dy ©
0 20O 400 6OO 80O 0
£p = 2.7
£p= 1.4
Zp =05
Aug
5 Sep
(D
©
Figure 3. Concentrations of chlorophyll a_ in the upper
3-meters and depth profiles of photosynthesis during 10
experiments at Station A during 1970. Integral photosyn-
thesis (E p) is indicated for each experiment.
22
-------
Chlorophyll concentrations during August were typically
higher at the surface than at 2 or 3 m (Fig, 3). This
stratification was probably related in part to a weak
thermal gradient between 2 and 3 m that may develop on
calm, sunny days (Table 3) and in part to the buoyancy of
the Aphanizomenon colonies.
Maximum photosynthesis (Pmax) a^ each station was measured
by incubating surface samples from each locality simultan-
eously at Station A at the depth where the average daily
illumination was optimal for photosynthesis, which was
usually 0.5 m. Maximum photosynthesis, which is a simple
product of the chlorophyll concentration in a sample (c)
and maximum specific photosynthesis (pmax)> varied
approximately 10-fold in correspondence with the 10-fold
variations of chlorophyll, from 0.33 g carbon m dy~^
for experiment 1 to 3.22 g carbon m~3 dy~l for experiment
7 (Table 4, Fig. 2). (Maximum photosynthesis could not
be evaluated for experiment 8 because chlorophyll concen-
trations at Station A where the samples were incubated
were so high that light intensities were suboptimal at all
depths.)
Mean maximum specific photosynthesis (Pmax) decreased from
30 to 19 mg carbon (mg chlorophyll) -1 dyl during June
(experiments 1 and 2), increased to 52 mg carbon (mg
chlorophyll)~1 dy"1 in early August (experiment 5),
and decreased again during August and September (Fig. 4-A).
Maximum specific photosynthesis varied with temperatures
in lakes studied by Tailing (1957a, 1966) and in Lake
Minnetonka (Megard 1972) and with population densities of
the phytoplankton in other lakes (Wright 1960, Findenegg
1965) . However, there was no consistent relationship
between maximum specific photosynthesis and either temp-
eratures or population densities in Shagawa Lake. There
was apparently an inverse relationship between maximum
specific photosynthesis and chlorophyll concentrations
during experiments 1, 5, and 7, but not during the others
(Fig. 5). Maximum specific photosynthesis decreased but
the temperatures increased in June (Figs. 2 and 4-A).
Maximum specific photosynthesis decreased during August,
when population densities fluctuated and temperatures
were virtually constant. The most consistent relationship
was between maximum specific photosynthesis and the
extractable phosphorus content of the phytoplankton (Figs.
4-A and 4-C).
The decrease of maximum specific photosynthesis during the
time when chlorophyll concentrations increased most
rapidly demonstrates that growth (i.e. chlorophyll synthesis
23
-------
.1?
o ;
;-o 50-|
o.;o6
0-
.4'
s » 5 *'
J2 en
QL 6
01
(20-
(00-
"E eo-
2 E 60-
O
g
40-
.6
v-
\
-.2U."
0
Enriched
Phosphorus
Jun
Jul
Aug Sep
"ES
|2
i
Figure 4. A) Maximum specific photosynthesis, B) the
ratio of total phosphorus to chlorophyll £ (Ft/c) and the
ratio of soluble to total phosphorus (FS/F£), C) the ex-
tractable phosphorus in net plankton, and D) concentrations
of total phosphorus and chlorophyll &_ in surface waters.
24
-------
40-
30-
20-
JO 20 30
JO 20 30 0 JO 20 30 4O
_L 60-
.c
o
50-
o>
g e
ex. 40-
Q- c
£ 30-
o
O
20
4
_
^
/
-^
5
_
0
1 I 1
6
.
*
III!
0> 0 JO 20 30 JO 20 30 40 40 60 80 JOO J2
E50l
40-
30-
20-
40-
7
*
*
9
-
*
vo
*f
*
50 100 450 ZOO
20 40 60
mg/m3
Chlorophyll g
JO 20 30 40
Figure 5. The relationship between maximum specific photo-
synthesis (Pj^ ) and chlorophyll for all experiments except
experiment 8, when P- could not be evaluated.
25
-------
in this case) and photosynthesis are separate processes.
A similar "uncoupling" of photosynthesis and growth occurs
at Windermere, where specific photosynthesis remains
high after cell division is limited by low silica concen-
trations (Tailing 1966).
Integral photosynthesis (£p) was measured at Station A by
incubating samples at depth intervals in situ (Fig. 3,
Table 4). Variations of integral photosynthesis generally
corresponded to variations of population densities, with
exceptions due to variations of maximum specific photosyn-
thesis. Integral photosynthesis at this station varied
from 0.45 g carbon m~2 dy~l for experiment 2 (17 June) to
2.70 g carbon m~2 dy~l for experiment 8 (31 August). If
a photosynthetic zone is defined to be the depth above
which photosynthesis occurs, then the depth profiles of
photosynthesis indicate that the photosynthetic zone was
somewhat more than 3-m thick at other times. It was
only 1.5 m thick at Station A on 31 August (experiment 8)
when chlorophyll concentrations at the surface reached
977 mg m~3.
The weighted mean value of integral photosynthesis at
Station A from early June through late October was 1.2
g carbon m~2 dy"1. The total gross photosynthesis at
this station for the 5-month period is therefore 180 g
carbon m~2.
26
-------
SECTION VII
PHOSPHORUS
Mean concentrations of total phosphorus in surface waters
were between 25 and 40 mg m~3 during June (Fig. 2, Table
4) . They increased to a maximum of 150 mg m~3 j_n late
August, and declined to 55 mg m~3 in October.
Concentrations of soluble phosphorus were less than 2 mg
m~3 in June. There was a sharp increase in early July,
and the highest concentrations were 50-80 mg m~3 in
August (Fig. 2).
Many tourists visit Ely during the summer and the increased
phosphorus concentrations in the lake after 4 July may
be due in part to an increased phosphorus influx from
sewage effluents, but preliminary calculations indicate
that the quantity contributed by sewage cannot account
for all of the increase in the lake. The sudden decrease
in August occurs when the number of tourists decrease,
but the low ratio of total phosphorus to chlorophyll
(Ft/c, Fig. 4B) indicates that much of the phosphorus was
incorporated in the phytoplankton at that time. Therefore
much of the phosphorus may have been transported out of
the water at the end of the bloom by algae sinking to the
lake sediments.
Mean concentrations of total phosphorus in surface waters
increased from 26 mg m~3 to 150 mg m~^ during the 54-dy
period between 2 July and 25 August, a net increase of
124 mg m~3. The increase was approximately linear (Fig.
4-D), and the average rate of increase was 2.3 mg m~3 dy-1.
Concentrations at Station A were about the same at all
depths above 6 m at any time during this period (Table 7).
If the mean subsurface concentration in the upper 6 m
at all stations increased as much as the mean surface
concentrations, then the total quantity of phosphorus in
the water above 6m (46.7 x lO^m^) increased from 1.3 x
109 mg to 7.0 x 109 mg, or 110 kg dy-1.
The effect of severe wind-induced turbulence on the quantity
of phosphorus in Shagawa Lake is illustrated by 2 series
of analyses on 1 and 3 August (Table 8), before and after
a severe storm on 2 August. The mean concentration of
total phosphorus in the water above 6 m was 19 mg m~3
higher after the storm than before it, a 21% increase.
The total quantity of phosphorus in the water above 6 m
increased 0.9 x 10° mg (900 kg). It should be noted,
however, that the rate that phosphorus increased was
accelerated only temporarily as a result of the storm.
27
-------
o
u
o
o
(N
CM
CJN
^t1
-------
Table 8.
1 August
Phosphorus concentrations (mg m ) at stations A-F
and the quantity of phosphorus above 6-m depth
before and after a storm on 2 August, 1970.
Quantity of
phosphorus
above 6 m
4.0 x 10 mg
Total Phosphorus
Depth
(m) A
0.5 93
3.0
5.0 69
6.0
10.0
11.0
0.5 36
3.0
5.0 35
6.0
10.0
11.0
3 August
B
89
68
67
409
20
19
32
409
Stations
C D E F
85
66
80
Soluble
23
24
45
64
80
107
218
87 64
104 80
127
107
X
81
79
97
108
Phosphorus
35
36
177
228
59 33
68 52
92
73
34
40
63
82
Total Phosphorus
0.5 102
3.0 107
5.0 107
6.0
10.0
11.0
0.5 52
3.0 57
5.0 55
6.0
10.0
11.0
94
92
95
456
37
31
27
482
115
119
82
Soluble
36
34
42
109
111
111
234
113 106
117 109
114 103
107
109
101
102
Phosphorus
71
73
73
246
78 68
83 68
90 67
57
58
63
50
4.9 x 10 mg
29
-------
The increase of phosphorus during July and August raises
the question as to whether there was a net deposition
or a net release of phosphorus from the sediments. The
issue may be resolved in this case by solving the equation,
= I + S - L, (2)
where I is the rate of influx from the watershed, primarily
from the sewage treatment plant, S is the rate of exchange
with the sediments (the sign of S will be positive if
phosphorus was released, and negative if phosphorus was
deposited), and L is the loss to the outlet. The rate of
change (dF/dt) was evaluated above (110 kg dy1) . The
equation may be solved for S because the influx from the
sewage treatment plant (I) and the loss at the outlet (L)
may be evaluated with data obtained by the staff of the
Shagawa Lake Eutrophication Project at Ely, but these
data are not yet available.
The relationships between concentrations of total phos-
phorus and chlorophyll are shown in Fig. 6. There was no
apparent correlation between phosphorus and chlorophyll
during June, July, and October (experiments 1, 2, 3, 4,
and 10), but there were linear relationships in August
and September, during and immediately after the period
when chlorophyll concentrations increased most rapidly
(experiments 5, 6, 7, 8, and 9). The slope of the
regression equation for experiments 5, 6, and 7 (1.33 4^
0.61 mg chlorophyll [mg phosphorus]""-'-) is less than but
not significantly different from the slope of the equation
for experiments 8 and 9 (2.01 + 0.32 mg chlorophyll [mg
phosphorus]~^).
30
-------
0
ZOO
8
£
C
O
Experiments 5, 6 and ?
C= (.33 F, -105
Experiments 5, 6, 7
_Eicperfmenfs 6 and 9
C=2.0I (F,)-(39
mg/m
Total Phosphorus (F, )
Figure 6. The relationships between concentrations of chlorophyll
a and total phosphorus in surface waters of Shagawa Lake.
31
-------
SECTION VIII
EXTRACTABLE PHOSPHORUS AND PHYTOPLANKTON GROWTH
Phosphorus that is extractable from algae by boiling water
represents surplus phosphorus stored by the algae as a
result of luxury consumption (Fitzgerald and Nelson 1966).
The fluctuations of extractable phosphorus described here
pertain to phosphorus in material retained by a #25-mesh
plankton net. This material includes zooplankton and
detritus as well as algae, but the data are presumed to
reflect changes in the extractable phosphorus of the
phytoplankton, particularly during August, when virtually
a7!! the algae were large enough to be retained by a net
and zooplankton were not abundant.
The extractable phosphorus in the net plankton was less
than 0.1 mg (mg chlorophyll)"1 during June and early July
(Fig. 4-C, Table 9). Thereafter, extractable phosphorus
increased to a maximum of 0.34 mg (mg chlorophyll)"! in
mid-August, indicating that the algae were storing
phosphorus. Extractable phosphorus then decreased during
the time that chlorophyll concentrations increased most
rapidly, and it fell to 0.13 mg (mg chlorophyll)""! in
early September.
The extractable phosphorus content expressed in terms of
the dry weight of the net plankton (Table 9) is approx-
imately the same as that of algae grown in culture media,
even though concentrations of soluble phosphorus in the
lake (1-50 \ig liter"!) are much lower than the concen-
trations that limit growth in cultures (1.6 mg liter"!).
The extractable phosphorus of net plankton in Shagawa
Lake increased from 0.17 mg (100 mg dry weight)~1 in
early July to a maximum of 1.2 mg (100 mg dry weight)"!
in August. In contrast the extractable phosphorus content
of blue-green algae is about 0.05 mg (100 mg dry wt)"! in
cultures that are phosphorus deficient (less than 1.6
mg liter"1) and about 0.4 mg (100 mg dry wt)~! in
cultures that contain surplus phosphorus (7.2 mg liter"!)
(Fitzgerald and Nelson 1966) .
Mean chlorophyll concentrations began to increase rapidly
34 days after phosphorus concentrations began to increase.
During the 34-day period before the rapid increase of
chlorophyll, the ratio of total phosphorus to chlorophyll
(Ft/c) in the water increased from 1.7 to 4.4. Maximum
specific photosynthesis (P^ax) also increased, perhaps
because the extractable phosphorus content of net plankton
increased (Fig. 4).
33
-------
Table 9. Extractable phosphorus in the net plankton from
Shagawa Lake during 1970.
Date
12 June
15 June
27 June
mg phosphorus
mg chlorophyll
0.02
0.09
0.08
mg phosphorus
100 mg dry wt
0.06
0.32
0.28
4 July
10 July
25 July
0.05
0.08
0.21
0.17
0.29
0.74
14 August
19 August
24 August
29 August
0.34
0.19
0.15
0.14
1.2
0.66
0.52
0.49
4 Sept
0.13
0.46
34
-------
During the 25 days that chlorophyll increased most rapidly
(6 Aug-31 Aug) there were decreases in the ratio of phos-
phorus to chlorophyll in the water (Ft/c), in maximum
specific photosynthesis (Pmax), and in extractable phos-
phorus in the net plankton. However, total phosphorus
and the fraction of the total phosphorus in the water that
was soluble (Fg/Fj-) increased slightly. Thus the phosphorus
content of the algae decreased while they grew most
rapidly even though concentrations of dissolved phosphorus
in the water remained high.
An experiment was performed during the period that
population densities were decreasing that provides direct
evidence that the phytoplankton retained the ability to
take up phosphorus. A water sample was enriched with
2 mg liter"! phosphorus on 3 September and incubated 24-hr
at the depth where illumination was optimal. The extrac-
table phosphorus increased from 0.13 mg (mg chlorophyll)~!
before to 0.20 mg (mg chlorophyll)-! after enrichment
and incubation (Fig. 4-C).
The data suggest that the duration of rapid growth, and
hence the chlorophyll concentrations at the time when
rapid growth stops may have depended upon the time required
for the extractable phosphorus content of the phytoplankton
to decrease from a maximal to a minimal value. The
phytoplankton became phosphorus-deficient because the
rate of uptake was less than the rate of loss. The rate
of uptake may have been low because the phosphorus in the
water may have been in a form that was unavailable, but
uptake also may have been suppressed because subsurface
light intensities decreased as population densities in-
creased (Kuhl 1962, 1968). Low subsurface light inten-
sities associated with high population densities would
tend to suppress phosphorus uptake, but chlorophyll
synthesis would be enhanced because of shade adaptation.
Tailing (1966) observed that the chlorophyll content of
algae below the photosynthetic zone in Windermere is about
50% higher than of algae at the surface during the period
when population densities of Asterione11a increase .
This type of shade adaptation is also exhibited by
phytoplankton in Shagawa Lake. The chlorophyll content
of phytoplankton in water deeper than 1.5 m was higher
than that of phytoplankton in shallower water during
July and August 1967 (Table 10). The chlorophyll content
was 3 mg (10° algae)~1 in all samples from the surface
on 14 August, despite the wide range of population
densities, but it was 12 and 15 mg (10^ algae)-1 at depths
of 6 and 10 m. The photosynthetic zone was 3-m thick
35
-------
Table 10. Algal population densities and chlorophyll
concentrations in Shagawa Lake on 29 July and
14 August, 1967.
Q
Date
29 July
Station D
14 August
Station A
B
E
F
D
Depth
(m)
0.5
1.5
3.0
6.0
10.0
12.0
0
0
0
0
0
2.0
4.0
6.0
10.0
Temp mg chl.
(°C) 10~3
21.2 104
110
20.8 69
19.6 4
17.3 3
17.2 1
24
138
30
56
21.3 26
21.3 30
19.7 5
19.3 5
18.8 5
10 algal mg (
filaments 10^
per m3 fil<
21.9
21.2
10.1
.2
.1
9.8
46.9
9.5
16.7
8.4
6.7
1.6
.4
.2
5
' 5
7
16
20
3
3
3
3
3
4
3
12
15
36
-------
on this date and less than 3 m during August, 1970 (Fig.
3). Hence, more than 50% of the algae in the epilimnion
experience light intensities which are insufficient for
photosynthesis but which apparently stimulate chlorophyll
synthesis. The contrasting response of chlorophyll
synthesis and phosphorus uptake to low light intensities
apparently induces unbalanced growth, and blooms may be
self-limiting because phosphorus uptake cannot keep pace
with chlorophyll synthesis.
The cellular level of extractable phosphorus is a dynamic
equilibrium involving the following rates: uptake of
mineral phosphorus, losses of the same through excretion,
and transformation to other types of cellular phosphorus
via biosynthetic processes. A substantial phosphorus
demand would exist to sustain a bloom of the magnitude
observed. Considering that phosphorus uptake and storage
requires an energy expenditure, it would be reasonable
to expect that algae would preferentially utilize ortho-
phosphate available from intracellular pools (here
assessed as extractable phosphorus). The rapid decline
of extractable phosphorus during the'phase of most rapid
growth most probably reflected a rapid transformation of
stored mineral phosphorus to cellular organic matter.
37
-------
SECTION IX
NUTRIENT LIMITATION
Powers gjb al. (1972) added phosphorus, nitrogen, and carbon
singly and together to phytoplankton enclosed in 300-
liter bags that were suspended in Shagawa Lake during
July 14-30 and August 5-24, 1970. Enrichment with
phosphorus alone, nitrogen alone, and nitrogen and
phosphorus together stimulated chlorophyll production
during both experiments, but there was no response to
carbon enrichment. Thus it was concluded that the phyto-
plankton were limited by both phosphorus and nitrogen.
Although these experiments suggest that nitrogen and
phosphorus were deficient relative to carbon, Aphanizomenon
is able to fix nitrogen (Stewart et. al. 1967, Stewart
1968, Granhall and Lundgren 1971) , and concentrations
of soluble phosphorus in the lake were high when growth
rates were highest. Thus phosphorus and nitrogen may
have been the limiting nutrients, but algal production
probably was limited not by nutrients but by other factors,
probably those affecting light attenuation, as described
below.
39
-------
SECTION X
INTEGRAL PHOTOSYNTHESIS AND LAKE PRODUCTIVITY
Dally photosynthesis at the average depth where illumina-
tion is optimal (pmax^ depends upon chlorophyll concen-
tration (c) and maximum specific photosynthesis (Pmax) :
p = cP (1)
^max max
Daily integral photosynthesis (Ep) is the area enclosed
by a depth-profile of photosynthesis. This area may be
set equal to the area of a rectangle. If one side of the
rectangle equals the rate of photosynthesis at the average
depth where illumination is optimal (p ) , then
max
2i E ZP/Pmax '
where z^ is the length of the other side (Tailing 1957b) .
If the average intensity of photosynthetically-active
radiation at the surface is IQ, and chlorophyll is uniformly
distributed in the photic zone, then
where IZi is the average intensity at the depth zj., kc is
the attenuation by chlorophyll, c is the chlorophyll
concentration, and kw is the attenuation by the water
itself and by substances other than chlorophyll dissolved
or suspended in it. Therefore,
zi = k c + k ' (4)
1 kcc + w
The depth z^ increases to a maximum as chlorophyll concen-
trations decrease,
limit _
c+0 zi
w
and it decreases as chlorophyll concentrations increase:
liltlit z± = 0 . (6)
41
-------
Thus In
/ o
k c + k max
c w
= 2iCPmax
= zi Pmax
cP * (?)
"
The product of Zj_ and c is the quantity (mg m ) of
chlorophyll above the depth z±. If this quantity is
designated as Cj_f then
^ = ciPmax
Thus maximum photosynthesis (Pmax) increases but the
depth Zj_ decreases as chlorophyll concentrations (c)
increase, provided that maximum specific photosynthesis
(Pmax) and the component optical terms of Zj_ are constant
or less variable than chlorophyll concentrations. These
constraints evidently pertain to Shagawa Lake as to Lake
Minnetonka (Megard 1972), so that the ratio £p/Pmax (=zi'
equation 2) decreases systematically as chlorophyll con-
centrations increase. Consequently the depth z^ may be
estimated from the chlorophyll concentration without
measuring light intensities (Fig. 7-A) . This makes it
possible to estimate integral photosynthesis at any
locality with equation (9) from chlorophyll concentration
(c) and maximum photosynthesis (pmax) e which are both easy
to measure.
The instantaneous values of kw and kc are difficult to
measure independently but their mean values may be evaluated
from the regression of £p/Pmax on cnlor°phyll' The
relationship between £p/Pmax (=zi) an<^ chlorophyll
concentrations for the depth profiles of integral photo-
synthesis at Station A is shown in Fig. 7-A. Inspection
of the datum points indicates that the value of z-^ at
zero chlorophyll (limit c+0 z^) is approximately 3 m.
Mean daily values of In (IO/IZ^) range between 2 and 3.5,
but the most frequent value is 2.7 (Rhode et. al . 1958;
Vollenweider 1965, 1969, 1970). If the latter value is
chosen, then the mean value of kw [= In (IO/IZ^) /lim c+0 z-jj
in Shagawa Lake is 0.9 m~l. The curve in Fig. 7-A is that
obtained by solving equation (4) at different chlorophyll
concentrations (c) for the caae where In (IO/IZ^) is 2.7,
kw is 0.9 nT1 and kc is 0.03 m2 (mg chlorophyll) ~1 (Table 11),
These values of kw and kc arc both higher than in Lake Minne-
tonka, where kw and kc derived in the sam way are 0.7 and
0.015 respectively (Megard 1972).
42
-------
0. O
W -P
(00
mg/m3
chlorophyll concentration (c)
200
Figure 7. The relationship^, among chlorophyll concentrations
(c) and the depth coefficient z^ (panel A); the quantity of
chlorophyll above the depth z^, c^ (Panel B); and integral
photosynthesis, Ip (Panel C).
43
-------
(0 -H
tn N
rd H
en o
H
H G
rH
G
O T3
-P G
g
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44
-------
If In (IO/I2£) is relatively constant, then the limits of
integral photosynthesis by a population with a particular
density are imposed by the prevailing values of k , kw,
and Pmax- '^ne numerical values of z^, c^, and Ep for two
values of Pmax that correspond to the values of kw and kc
estimated for Shagawa Lake are shown in Table 11, and
the resulting curves are shown in Fig. 7. It is notable
that the attenuation of photosynthetically-active
radiation by chlorophyll (kcc) exceeds attenuation by
the water alone (kw) where chlorophyll concentrations
exceed 30 mg m~3 (Table 11, column 2). Where chlorophyll
concentrations are 100 mg m~3, the total attenuation
coefficient, a (= kw + kcc) , is 3.9 m~l, Zj_ is only 0.7 m,
and c^ is only 69 mg chlorophyll m~2.
_ -3
Large increments of chlorophyll above 50 mg m produce
small increments of ci and L'p; a four-fold increase of
chlorophyll, from 50 to 200 mg m~3 increases c^ and Zp
only 50% (Fig. 7). Integral photosynthesis ranges between
2 and 4 g carbon m~2 dy~l where chlorophyll concentrations
exceed 100 mg m-3, depending mostly upon variations of
Pmax; variations of population densities have little
effect.
The highest integral photosynthesis measured at station A
was 2.7 g carbon m~2 dy~l on 31 August, when chlorophyll
concentrations ranged from 977 mg m~3 at the surface to
220 mg m~3 at 1 m, but integral photosynthesis was almost
that high (2.1 mg m~3) on 17 August and 25 August, when
chlorophyll ranged between 40 and 105 mg m~3 (Fig. 3).
The range of maximum specific photosynthesis in Shagawa
Lake is about the same as in Lake Minnetonka (Megard 1972) ,
but the upper limits for integral photosynthesis are
lower in Shagawa because the attenuation coefficients for
photosynthetically-active radiation are larger than in
Lake Minnetonka. The high attenuation coefficients not
only limit integral photosynthesis, they also prevent the
population densities from becoming high enough to utilize
all the dissolved phosphorus.
Equations 7-10 are strictly applicable only where phyto-
plankton is uniformly distributed in the photic zone.
However, algal population densities at a particular
locality in Shagawa Lake may change radically during a
day, and the phytoplankton are stratified in the photic
zone during August. Thus even measurements of photo-
synthesis at depth intervals in situ cannot provide
reliable estimates of integral photosynthesis at a
particular time or place, because both algal abundance
45
-------
and the optical properties of the water may vary substan-
tially during the period of measurement. However, the
effects of these ephemeral variations will tend to
cancel during long time intervals, so that the mean daily
value of integral photosynthesis for the lake as a
whole may be estimated from the mean chlorophyll concen-
trations (c) and mean specific photosynthesis (P )
with equation (8). max
The area under curves for chlorophyll and specific rates
of photosynthesis vs_. time in Fig. 4 were determined with
a planimeter, and the weighted mean value of c for the
period from June through October was found to be 34 mg
chlorophyll m~3 and that of Pmax to ^e 29 mg carbon (mg
chlorophyll)"1 dy"1. The mean value of Zi may be
estimated from the regression of £p/Pma °n chlorophyll
(Fig. 7-A) , which indicates that the va₯ue of z-[ where
chlorophyll is 34 mg m~3 ±s about 1.5 m. Thus, by
substitution in equation (8), the weighted mean value of
integral photosynthesis for the entire lake is 1.5 g
carbon m~2 dy"1 or about 220 g carbon m~"2 for the 150-
day period. The ice usually melts in mid-April and the
lake usually freezes in late November. Integral gross
photosynthesis is probably about 0.5 g carbon m~2 dy"1
during spring and November. If the total open water
season is 75 days longer than the study period, then the
additional gross photosynthesis is about 40 g carbon m~2,
and the total gross photosynthesis for the open-water
season is approximately 250-300 g carbon m~2.
The daily influx of organic material from the sewage
effluent is very much smaller than the daily production
of organic carbon by the phytoplankton in the lake. If
average daily integral photosynthesis is 1.5 g carbon m~2,
then the average total production for the entire lake
(9.6 x 106 m2) is 14,000 kg carbon dy"1. The rated
capacity of the treatment plant is 4 x 106 liters per
day. If the effluent contains 55 mg.liter"1 organic
material, as is typical for a secondary sewage effluent
(Amer. Chem. Soc. 1969) , and if the organic material is
50% carbon, then the influx from the treatment plant is
about 100 kg carbon dy"1. Thus, although the treatment
plant contributes about 70% of the phosphorus that
enters the lake, it contributes less than 1% of the
organic carbon.
The Burntside River at the west end of the lake contributes
most of the water, but the sewage outfall at the south
side contributes most of the phosphorus. The basin also
has an irregular shape with many islands that might
46
-------
partition the water into discrete masses with somewhat
different nutrient concentrations and productivities.
However, the mean chlorophyll concentrations, specific
rates of photosynthesis, and total phosphorus concentra-
tions at the sampling stations are not significantly
different from each other, and there are no gradients
with respect to the major sources of water or nutrients
(Table 12). The highest average phosphorus concentration
is at Station B, near the lake's outlet, not at Station
A, where the sewage effluents enter. On the other hand
the lowest average phosphorus concentration is at Station
Cr in mid-lake, not at the station near the entrance of
the Burntside River. Although a more intensive sampling
program might reveal consistent differences between the
stations, it appears that the water and dissolved sub-
stances entering the lake are distributed uniformly
throughout the basin by currents and that the lake is a
well-mixed system despite the contrasting water quality
of the inflowing streams and irregularities of the
basin itself.
47
-------
Table 12. Means and standard deviations for chlorophyll
concentrations (c), maximum specific photosynthe-
sis (Pmax) anc^ total phosphorus concentrations
in Shagawa Lake, June through October, 1970,
max
Station A 31+30
B
C
D
E
F
27 + 15
36 + 28
30 + 22
37 + 35
28 + 18
34 + 12
24 + 10
29 + 9
33 + 11
30- + 11
31 + 14
78 + 42
83 + 55
74 + 39
75 + 43
78 + 47
76 + 50
48
-------
SECTION XI
POPULATION REGULATION
Algae, bacteria, zooplankton, and organic detritus are
all composed of particulate organic carbon. Particulate
organic carbon produced by algae in the photosynthetic
zone is distributed throughout the epilimnion by turbulent
mixing. The particulate organic carbon may be incorporated
into zooplankton, bacteria, and detritus after it is .
produced by algae. It may be degraded to soluble carbon
compounds by respiration and excretion, or it may be lost
by sinking. Thus the particulate organic carbon in the
mixed layer beneath a unit of lake surface is a functional
unit, with gains due to photosynthesis and losses due to
respiration, excretion, and sinking, as indicated in the
following diagram.
Integral photosynthesis
Phytoplankton Respiration
Zooplankton sinking'"
Bacteria ^
_. , . , Excretion
Detritus
The organic carbon contributed to Shagawa Lake by sewage
is much smaller than that produced by the phytoplankton,
and the quantity contributed by other sources in the
watershed is probably negligible also. It should be
emphasized that predation by zooplankton is not a loss of
particulate organic carbon from the mixed layer; it is
only a transfer between groups of organisms.
Strickland (1960) has indicated that it is neceasary to
measure both the density of a population and its rate of
production in order to assess the fertility of a water
mass. If both population densities and rates of production
are measured in units of particulate organic carbon,
then the specific rate of change, specific rate of
production, and specific rate of loss may be calculated.
Although concentrations of particulate organic carbon were
not measured in Shagawa Lake, they may be estimated from
the relationship between the concentrations of chlorophyll
and particulate material in the water (Fig. 8). The
slopes of the equations for the regression of chlorophyll
on particulate material indicate that chlorophyll
concentrations during August are about 2% as large as
the concentrations of particulate material. Hence the
ratio of particulate material to chlorophyll is about 50:1.
The particulate material probably contains about 10%
inorganic material and 40% carbon (Strickland 1965, Table
49
-------
o.
o
o
0
ff
100
80
6O
40
20
O
^
./*
s^ %
* 27 Auq 69
till
4000
2000
mg/m
dry weight of participate material (w)
Figure 8. Regression of chlorophyll concentrations on the
dry weight of particulate material in water from the mixed
layer during August, 1969. Data provided by the staff of the
Shagawa Eutrophication Project.
50
-------
3) . Therefore the ratio of particulate organic carbon to
chlorophyll (f) in Shagawa Lake is probably about 20:1.
The ratio for marine diatoms was 60 in nutrient-poor
water and 25 in coastal water that contained excess
nitrogen and phosphorus (Strickland 1965, Table 6) . The
ratio in La,ke 227 in the Experimental Lakes Area of north-
west Ontario was 300 before the lake was artificially
fertilized, but it decreased after fertilization to 115
in 1969 and to 55 in 1970. Small flagellated algae
dominated the phytoplankton before it was fertilized,
but blue-green algae dominated afterward (Schindler 1971) .
The changing ratio in this lake suggests that blue-green
algae contain more chlorophyll than others, or that the
proportion of the particulate material that contains
chlorophyll increases in enriched lakes, or both. The
ratio apparently decreases as nutrients increase in both
the sea and lakes; perhaps continued fertilization of
Lake 227 would decrease the ratio to the value in Shagawa.
The existing quantity of particulate organic carbon in
the mixed layer beneath a unit of lake surface (w1) is
w1 = czmf , (11)
where c is the chlorophyll concentration, z^ is the thick-
ness of the mixed layer, and f is the ratio of carbon
to chlorophyll.
If the quantity of particulate organic carbon changes at
a constant rate during an interval between sampling dates,
then the specific rate of change (r) during an interval
may be computed from the quantity present at the beginning
(w0) and end (w£) of the interval with the equation
r »
L
0.5
where t is the duration (days) of the interval.
The specific rate of change depends upon the specific rate
of production (y) by photosynthesis and the specific rate
of loss (X) by respiration, excretion and sinking:
r = y - A
Specific rates of production may be estimated directly
from measurements of integral gross photosynthesis with
the oxygen method. Various studies have shown (cf.
Eppley 1968) that measurements of photosynthesis with
the 1*C technique are in fact measurements of the net
51
-------
increase of particulate organic carbon. The oxygen
method should then yield estimates of gross production
because it includes a correction for losses by respiration
As with the ^-^C method, losses by sinking are prevented
because the plankton is enclosed in bottles. There is no
correction for losses by excretion, but this error is
probably small and offset by increases of population
densities during the time that photosynthesis is measured.
If the average daily rate of production during a time
interval is an average of the rates at the beginning (po)
and the end (pt) of the interval and if the specific rate
of change is constant, then the specific rate of
production may be estimated with the equation
°-5 (^o + Pt> . (14),
0.5 (w^ + w£)
The specific rate of loss may be computed by substituting
the estimates of r and y in equation (13) .
The estimated specific rates of change, of production,
and of loss for particulate organic carbon in the mixed
layer are shown in Table 13. It was assumed that the
thickness of the mixed layer (zra) was constant at 5 m
and that the ratio of carbon to chlorophyll (f) observed
in August (20) also prevailed in June and July.
The quantity of particulate organic carbon increased
approximately 5-fold during August, but average daily
production increased only about 50%. The specific rate
of change was approximately 0.02 dy~l during July, it
ranged between 0.04 and 0.07 during August, and it
decreased to -0.22 during the rapid decrease in early
September. Specific rates of production and of loss
were high, 0.49 and 0.47 respectively in late July, before
population densities began to increase most rapidly. The
specific rate of production decreased from 0.50 to 0.16
during the rapid increase of population densities in
August, but the specific rate of loss decreased more
rapidly, from 0.48 to 0.08. The rapid decrease of
population densities in early September occurred when the
specific rate of loss increased to 0.37 without a
concurrent increase of specific production.
These calculations suggest that population densities
increased rapidly during August because specific rates
of loss decreased faster than specific rates of production,
Furthermore the rapid decrease in early September was
produced by an increase in the loss rate. Thus the
buoyancy implied by the stratification of chlorophyll
52
-------
rd
CO
tn
co rd <<
W ,d
O W
oo ^^* r** oo
OJ OO rj» ^J*
*
o o o o
rH
m
4H O
t [ ~]
T3 CU
£H K*1
co co o^ o
co co ^ in
o o o o
cd ctf
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"S Q)
, kj ^
rH
C B
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in CN r~ o
31 CN rH CN
O 0 0 0
« *
+ 1 + +
H 0)
-P ,C
0-4->
id tn -P
"^3 CH rH CUl
O -H CO]
rH 0 +
& C --H 1
O -P-P 0 in in o o
4-H ,£3 D rH fl O,
O M 3 rd O --'
cd ^ PI .n
> o QJ o rH m
M u cd cu o u o
CO (T> CO CO
in in co CN
rH
c
Q) rd
tn tn
C rH
rd O
O 0)
-P
MH cd - -P
OH £
en o -p ja +
jin
rH OJ > M C Cd
4H ,£ Cd O -H rH 0
0 0 0 O
3 o o o in
VD 00 00 ^f
rH rH rH CN
H -P
O Q)
Cu o cd
CO 4H A rj H rH tn
rj rj rj rj
CO rH rH
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£" K*
-------
during August (Fig. 3) appears to be a critical mechanism
of population regulation by Aphanizomenon; greater
buoyancy apparently reduced losses by sinking and permitted
population densities to increase rapidly even though
maximum specific photosynthesis decreased. Moreover,
the rapid decrease in September., probably occurred because
Aphanizomenon lost its ability to regulate its buoyancy.
In contrast to many other algae Aphani zomenon is neutrally
buoyant (Smayda 1969). The buoyancy of blue-green algae
depends upon the presence of gas vacuoles (Walsby 1972,
Fogg and walsby 1971). Oscillatoria redeckii produces
vacuoles at low light intensities (Whitton and Peat 1969).
The vacuoles of Anabaena flos-aquae collapse at high
light intensities if COp, is present (Walsby 1969) . The
vacuoles are located near photosynthetic membranes (Smith
and Peat 1967), and the collapse may be due to the turgor
pressure of the cells, which increases as sugars are
produced during photosynthesis. Therefore, blue-green
algae tend to produce gas vacuoles when they are in the
aphotic zone of the epilimnion and to lose them in the
photic zone. The production and collapse of vacuoles may
be fast enough to result in diurnal movements (Fogg 1969,
Fogg and Walsby 1971) such as those implied by the dis-
tribution of Anabaena in a Nile reservoir (Tailing
1957c), where the algae tend to be uniformly distributed
early in the morning and concentrated near the surface
in late afternoon.
In Shagawa Lake, high levels of extractable phosphorus
in early August may have enhanced the ability of Aphani-
zomenon to produce vacuoles, which reduced sinking rates
and permitted the population to become stratified. The
low levels of extractable phosphorus in early September
may have inhibited photosynthesis and vacuole regulation,
thereby increasing sinking rates, and contributing to
the rapid decrease of population densities.
Even though several very high population densities were
excluded from the calculation of average population
densities during August and September, the quantity of
particulate organic carbon in the mixed layer beneath 1
m^ of lake surface may have been overestimated because
the phytoplankton were stratified. If population
densities were overestimated, then the estimates of r
and X are too high and those of y are too low.
54
-------
SECTION XII
STEADY STATES FOR POPULATION DENSITIES
AND INTEGRAL PHOTOSYNTHESIS
The relationships between the quantity of particulate
organic carbon in the mixed layer, production, and loss
may be expressed as
w1 + Zp - Aw1 = w1 , (15)
O X
where A is the fraction of the particulate organic carbon
that is lost each day by respiration, excretion, and
sinking.
If the quantity of particulate organic carbon does not
change, then wo = wx and Zp = Aw'. Therefore,
in fl
^ cPmax
kw + kcc
Thus steady state population densities are defined by the
intersection of curves for the production (Zp) and loss
(Aw1) of particulate organic carbon in the mixed layer.
In Fig. 9, the quantity of particulate organic carbon
beneath 1 m2 of surface (w1) is computed for a mixed
layer (zm) 5 m thick where the carbon:chlorophyll ratio
(f) is 20. Integral photosynthesis is calculated for
two cases, one where Pmax is 50 and the other where Pmax
is 30 mg carbon (mg chlorophyll)~1 dy~l. In both cases
kw is constant at 0.9 m~l and kw at 0.03 ia- (ng chlorophyll)
If Pmax i-s 30 mg carbon (mg chlorophyll) ~1 dy~l and tne
loss (A) is 0.5 dy~l (Fig. 9-A), then the maximum specific
rate of change (r^ max) occurs where chlorophyll
concentrations (c) are 10 mg m~3 ancj the quantity of
particulate organic carbon in the mixed layer (w1) is
1000 mg m~2. However, integral photosynthesis and losses
are both 50% of the quantity of particulate organic
carbon in the mixed layer when chlorophyll concentrations
reach 25 mg m~3. Losses exceed production at higher
chlorophyll concentrations, so plankton populations with
these specified parameters will become stationary at
25 mg chlorophyll m~3, if the rate of loss decreases
to 0.25 dy~i then the maximum rate of change (r2 max)
occurs when chlorophyll concentrations reach 30 mg m~3,
and the stationary chlorophyll concentration is 80 mg m~3.
55
-------
4000
0 5000 <0000 (5000 20000
mg carbon/in2 (w1 =czmf)
0 50 (00 ^0 200
mg chlorophyll/m3 (c)
Figure 9. Steady state population densities defined by two
values of maximum specific photosynthesis (P ) and loss
rates.
56
-------
If, on the other hand, Pmax increases to 50 (Fig. 9-B),
then the maximum rates of increase (ry max and r^ max)
the stationary chlorophyll concentrations are correspondingly
higher. The stationary chlorophyll concentrations are
60 mg m~3 where the loss is 0.50 dy~l and 150 mg m~3
where the loss is 0.25 dy~^.
Thus it appears that the chlorophyll concentrations during
July (about 20 rag m~3) represent nearly steady state
population densities for the prevailing optical conditions
when maximum specific photosynthesis is approximately
30 mg carbon (mg chlorophyll)"-'- dy~l and loss rates are
approximately 0.5 dy~l.
57
-------
SECTION XIII
DISCUSSION
The calculations described above indicate the degree to
which population densities may be influenced by changes
in loss rates without changing other components of the
system. It is frequently assumed that rapid increases
of population densities are caused by high rates of
specific photosynthesis induced by high concentrations
of limiting nutrients. However, specific rates of
photosynthesis decreased during rapid growth in
Shagawa Lake even though concentrations of dissolved
phosphorus were high. It appears that the high atten-
uation coefficient for photosynthetically-active radia-
tion together with high loss rates prevented the phyto-
plankton from utilizing dissolved phosphorus as rapidly
as it was delivered, and that population densities
increased during August because loss rates decreased
more than production rates. In this case rates of loss
appear to be regulated by changes in the buoyancy of
the dominant alga. Circumstantial evidence suggests
that the buoyancy of Aph an i z ome non does not depend only
upon nutrient concentrations in the ambient water but
also upon intricate relationships among the optical
conditions , rates of chlorophyll synthesis , and rates
of phosphorus uptake.
The data from Shagawa Lake suggest that both production
rates and loss rates are influenced by algal nutrition.
The relationship between loss rates and nutrients cannot
be defined because there are at least three different
kinds of losses. However, the primary production of
particulate organic carbon can only be accomplished by
photosynthesis. Photosynthesis and nutrient uptake
are separate processes, but if photosynthesis is a
function of nutrient uptake, then the maximum specific
rate may be expressed in terms of Michaelis-Menton
kinetics (Dugdale 1967, Thomas 1970):
P = ^ S M7)
max max k + S ' ^ '
D
where Pmax is maximum specific photosynthesis where the
concentration of the limiting nutrient (S) is saturating
and ks is the nutrient concentration where Pmax = ^
Thus , integral photosynthesis may be written
max k
kw + kcc s
59
-------
Experiments by Thomas (1970) on natural populations of
marine phytoplankton indicate that it will be difficult
to evaluate Pmax and ks for natural assemblages, but
the role of nutrients cannot be defined precisely unless
this is done. Fortunately Pmax may be evaluated easily
without measuring ^max or ks, although the highest
values of Pmax measured in Shagawa, 50-60 mg carbon (mg
chlorophyll) "J- dy"1 may approximate /^max for Aphanizomenon
in this lake.
The general relationship among variations of population
densities, maximum specific photosynthesis, and loss
rates are shown in Fig. 10. If nutrient enrichment
increases Pmax without changing losses then integral
photosynthesis increases from Zpi to Ep2 and the quantity
of particulate organic carbon in the mixed layer increases
from w;[ to w^>. However,. if nutrient enrichment increases
Pmax and simultaneously reduces A, then Zp3 and w^ are
higher than Ep2 and w£.
The intersection of the curves define steady states for
Zp and w1, but no steady state exists is losses exceed
integral photosynthesis at all population densities (Fig.
10-D) .
An algal species may become more abundant than others in
its environment if its mechanisms of nutrient uptake
and photosynthesis are more efficient, if it has smaller
losses, or both. Aphanizomenon belongs to an ancient group
of blue-green algae that occurs in the oldest fossili-
ferous rocks (Barghoorn and Tyler 1965, Whittaker 1969).
Its mechanisms of nutrient uptake and photosynthesis may
be less efficient than those of other algae that have
developed specialized organelles during their evolution.
Consequently, the ability to regulate buoyancy conferred
by its gas vacuoles may be not only an important mechanism
that enables Aphanizomenon to produce dense blooms in
eutrophic lakes but also the mechanism that has enabled
it to compete with less conservative algae since Precambrian
times.
60
-------
Figure 10. The general relationships among the quantity of
particulate organic carbon in the mixed layer (w*), integral
photosynthesis (xp)> maximum specific photosynthesis, and
loss rates.
-------
SECTION XIV
LIST OF SYMBOLS
c Concentration of chlorophyll a
p Maximum daily photosynthesis per unit volume of
max water at the average depth of optimal illumination
P Maximum daily photosynthesis per mg chlorophyll at
max the average depth of optimal illumination
Zp Daily integral photosynthesis
z. The depth given by the ratio Zp:p
l max
I The daily average intensity of photosynthetically-
active radiation at the lake surface
Iz. The daily average intensity of photosynthetically-
1 active radiation at the depth z±
e The base of natural logarithms
k Attenuation of photosynthetically-active radiation
by a unit chlorophyll concentration
k Attenuation of photosynthetically-active radiation
by the water and by substances other than chloro-
phyll dissolved and suspended in the water
<* The total attenuation coefficient for photosynthe-
tically-active radiation
c. The quantity of chlorophyll above the depth z.
w The concentration of particulate material in the
water
w1 The quantity of particulate organic carbon in the
mixed layer beneath a unit surface area
z The thickness of the mixed layer (epilimnion)
f The ratio of the concentration of particulate
organic carbon to the concentration of chlorophyll
in the water
r The specific rate of change for particulate organic
carbon in the mixed layer
63
-------
The specific rate of production of particulate
organic carbon in the mixed layer
The specific rate of loss of the particulate
organic matter in the mixed layer by respiration,
excretion, and sinking
64
-------
SECTION XV
ACKNOWLEDGEMENTS
Most of the data upon which this report is based were
obtained during 1970 by Paul Smith and Alberta Smith.
The Environmental Protection Agency generously permitted
us to use the laboratory facilities of the Shagawa Lake
Eutrophication Control Project at Ely. We are grateful
to Robert Brice, Marcus Schuldt, and to other staff
members of the Eutrophication Control Project who
provided invaluable cooperation and information,
particularly the data in Fig. 8.
Financial support for the initial research at Shagawa Lake
during 1967 and 1968 was provided by the Office of Water
Resources research (Project A-016 Minn) and by the
Minnesota Resources Commission.
65
-------
SECTION XVI
REFERENCES
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Amer. Chem. Soc. Washington, D.C. 249 p.
2. Amer. Public Health Assoc. 1965. Standard methods
for the examination of water and wastewater, 12 ed.
Amer. Public Health Assoc. New York. 769 p.
3. Armstrong, F.A.J. and D.W. Schindler, 1971. Pre-
liminary chemical characterization of waters in the
Experimental Lakes Area, northwestern Ontario. Jour.
Fish. Res. Bd, Can. 28: 171-187.
4. Barghoorn, E.S. and S.A. Tyler. 1965. Microorganisms
from the Gunflint chert. Science 147: 563-577.
5. Bright, R.C. 1968. Surface-water chemistry of some
Minnesota lakes, with preliminary notes on diatoms.
Univ. of Minn. Limnological Research Center Interim
Report no. 3. 58 p.
6. Dugdale, R.C. 1967. Nutrient limitation in the sea:
Dynamics, identification, and significance. Limnol.
Oceanogr. 12: 685-695.
7. Eppley, R.W. 1968. An incubation method for estimating
the carbon content of phytoplankton in natural samples.
Limnol. Oceanogr. 13: 574-582.
8. Findenegg, I. 1965. Relationship between standing
crop and primary productivity. Mem. 1st. Ital.
Idrobiol. 18. Suppl: 271-289.
9. Fitzgerald, G.P. and T.C. Nelson. 1966. Extractive
and enzymatic analyses for limiting or surplus
phosphorus in algae. Jour. Phycol. 2: 32-37.
10. Fogg, G.E. 1969. The physiology of an algal nuisance.
Proc. Roz. Soc. B. 173: 175-189.
11. and A.E. Walsby. 1971. Buoyancy regulation
and the growth of planktonic blue-green algae. Mitt.
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70
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SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
/. Report No.
2.
3. Accession No.
w
4. Title
RATES OF PHOTOSYNTHESIS AND PHYTOPLANKTON GROWTH
IN SHAGAWA LAKE, MINNESOTA
7. Autbor(s)
Megard, Robert 0.
Report Dtte
Performing Orgtaiittion
Report No.
9. Organization
Minnesota University, Minneapolis
Limnological Research Center
12. SponsoringOrgtait*tioa Environmental Protection Agency
,_ _ . , Office of Research and Monitoring
15. Supplementary Notes 3
Environmental Protection Agency report
number, EPA-R3-73-039, July 1973.
10. Project No.
11. Contract/Grant No.
16010 DEG
13. Type of Report *nd
Period Covered
16. Abstract
Integral photosynthetic rates and growth rates of phytoplankton
were measured before, during, and after a bloom of Aphanizomenon,
which occurred in August 1970. The size of the population in-
creased during the bloom despite decreasing specific production
rates, probably because the dominant algal became more buoyant
and decreased its sinking rate. The effects of changing the
specific production rates and specific loss rates are demonstrated
by comparing the integral photosynthetic rates and chlorophyll con-
centrations that would prevail under steady state conditions.
17a. Descriptors
phytoplankton photosynthesis, algal growth, algal nutrients, phosphorus,
eutrophication, aquatic algae*, algal blooms, nutrients , water pollution
effects
17b. Identifiers
lake pollution
c. COWRR Field A Croup 05C
IS. Avtilability
19. Security Cltss.
(Report)
20. Security Cltss.
(P*ge)
Abstr,ctor
21. No. of
P*ges
Send To:
22. Price
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
US DEPARTMENT OF THE INTERIOR
WASHINGTON. O. C 20240
Institution
UnivBr<;ity nf Minnesota
WRSIC 102 (REV JUNE 1(71)
-------
U S Environmental *ote<*on Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, Uwi
Chicago, IL 60604-3590
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