oEPA
United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development ~. - : ./.
Zooplankton
Grazing and
Population Dynamics
Relative to Water
Quality in Southern
Lake Huron
LIBRARY
U.S. EKVUtt'.o-;
EDISOF, B...T,
LP 600/3
80-069
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RESEARCH REPORTING SERIES
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EPA-600/3-80-069
July 1980
ZOOPLANKTON GRAZING AND POPULATION DYNAMICS
RELATIVE TO WATER QUALITY IN SOUTHERN LAKE HURON
Donald C. McNaught
Marlene Buzzard
David Griesmer
Michele Kennedy
State University of New York at Albany
Department of Biological Sciences
ikOO Washington Avenue
Albany, New York 12222
Grant No. 803178
Project Officer
Nelson A. Thomas
Environmental Research Laboratory
Large Lakes Research Station
Grosse lie, Michigan H8138
ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
DULUTH, MINNESOTA 5580^
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DISCLAIMER
This report has been reviewed toy the Environmental Reserach Laboratory-
Duluth, U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views
and policies of the U.S. Environmental Protection Agency, nor does mention
of trade names or commercial products constitute endorsement or recommenda-
tion for use.
ii
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FOREWORD
Our nation's freshwaters are vital for all animals and
plants, yet our diverse uses of waterfor recreation, food,
energy, transportation, and industryphysically and chemically
alter lakes, rivers, and streams. Such alterations threaten
terrestrial organisms, as well as those living in water. The
Environmental Research Laboratory in Duluth, Minnesota develops
methods, conducts laboratory and field studies, and extrapolates
research findings
--to determine how physical and chemical pollution
affects aquatic life
--to assess the effects of ecosystem on pollutants
to predict effects of pollutants on large lakes
through use of models
to measure bioaccumulation of pollutants in aquatic
organisms that are consumed by other animals,
including man
This report, as part of the Large Lakes Research Program,
examines the relationship of zooplankton to water quality in
one of the Great Lakes. The knowledge of the relationship
between zooplankton and phytoplankton is essential when
attempting to understand a Great Lakes ecosystem. This report
details the relationship between water quality and zooplankton
populations in Lake Huron. The effects of various phytoplankton
forms of zooplankton grazing is also discussed in relation to
water quality.
Norbert Jaworski, Ph.D.
Director
Environmental Research Laboratory
Duluth, Minnesota
iii
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ABSTRACT
Planktonic herbivores controlled the growth of phytoplankton
In the open waters of Southern Lake Huron during the late summer
and fall; larger algae remained untouched during the spring dia-
tom bloom. Over a growing season these crustaceans grazed 0.3
to 4l.9% of the standing crop of phytoplankton. Immature cope-
pods (nauplii and copepodites) were the most effective grazers.
Selective grazing by crustaceans centered on nannoplankton (< 22
ym diameter), whereby 92 to 2840% of the daily productivity was
grazed by large cladocerans and immature copepods. In contrast,
netplankton was harvested to a much lesser degree. Laboratory
experiments reinforced this picture of selective cropping.
Presently in Lake Huron such selective grazing accounts for an
effective biological control over algal production; such control
will be lost if these phytoplankton populations are allowed to
shift to ones dominated by large blue-green algae. Populations
of herbivores were also used to. characterize water quality.
Generally outer Saginaw Bay and most inshore waters were of lower
water quality than offshore surface waters. These inshore areas
also had the highest productivity of zooplankton.
This report was submitted in fulfillment of Grant 803178 by
the State University of New York at Albany under the sponsorship
of the U.S. Environmental Protection Agency. This report covers
the period 15 May 1974 through 14 May 1976.
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CONTENTS
Foreword iii
Abstract iv
Figures vi
Tables x
Acknowledgement xiii
1. Introduction 1
1.1 Purposes 1
1.2 Role of grazers 1
2. Conclusions 3
3. Recommendations 6
4. Experimental Procedures 7
A.I Methods for total grazing 7
4.2 Methods for field estimates of
selective grazing 10
4.3 Methods for laboratory 10
4.4 Methods for plankton collection 11
4.5 Methods for acoustic biomass estimates . . H
5. Results and Discussion 12
5.1 Grazing by zooplankton upon natural
phytoplankton populations: seasonal
and diel estimates of total grazing ... 12
5.2 Grazing by zooplankton on natural
assemblages of netplankton and
nannoplankton 28
5-3 Experimental manipulation of food
resource characteristics 56
5.4 Horizontal distributions of dominant
organisms 66
5-5 Estimates of column biomass of
zooplankton 95
References 113
Appendix
A. Lakewide abundance of zooplankton (mean density). .115
B. Water quality sections, mean zooplankton density. .120
C. Graph of lakewide zooplankton density 128
v
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FIGURES
Number Page
1 Map of station locations in Southern Lake Huron ... 8
2 Relative contribution to total phytoplankton
grazing by important species and life stages
of crustacean zooplankton in Southern Lake
Huron (Limn.:Limnocalanus; Bos.:Bosmina
longirostrls) 19
3 Phytoplankton (as carbon) grazing rate by crus-
cacean zooplankton contrasted to standing crop
of phytoplankton carbon and rate of fixation of
carbon by phytoplankton in Southern Lake Huron. . . 20
4 Filtering and ingestion rates of common crusta-
cean zooplankton relative to water temperature. . . 21
5 Ingestion rate of zooplankton species relative
to food concentration in oligotrophic waters
of Southern Lake Huron 25
6 Mass ingestion rates of dominant zooplankton
species by season (symbols in text) 26
7 Mass ingestion rates (mgC piant day"-*- mgC animal)
for dominant herbivores feeding on nannoplankton. . 48
8 Mass ingestion rates (mgC pian-(- day~l mgC"-*- animal)
for dominant herbivores reeding on netplankton. . . 4 9
9 Control of phytoplankton by herbivorous zooplankton;
ratio plant carbon grazed to carbon fixed 55
10 Selective ingestion of beads by Diaptomus sici11s
adults (DA), copepodites (DC), and nauplii (N).
Composition of gut (middle) relative to bead
population available (upper); selectivity expressed
as Ivlev's index (lower) 65
11 Selective ingestion of beads by Cyclops blcuspldatus
(adults). For description see Fig. 10 67
vi
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Number Page
12 Horizontal distribution of nauplii in Southern
Lake Huron, 16-21 June 1974 (1 x 103) 69
13 Horizontal distribution of calanoid copepodites
in Southern Lake Huron, 8-12 October 1974
(1 x 103) 70
14 Horizontal distribution of Diaptomus minutus in
Southern Lake Huron, 10-14 November 1974 (1 x 103). 71
15 Horizontal distribution of Diaptomus oregonensis
in Southern Lake Huron, 10-14 November 1974
(1 x 103) 72
16 Horizontal distribution of Diaptomus sicills in _
Southern Lake Huron, 10-14 Novembe'r 1974 (1 x 103). 74
17 Horizontal distribution of cyclopoid copepodites
in Southern Lake Huron, 10-14 November 1974
(1 x 103) 75
18 Horizontal distribution of Cyclops bicuspidatus
in Southern Lake Huron, 16-21 June 1974 (1 x 103) . 76
19 Horizontal distribution of Tropocyclops prasinus
in Southern Lake Huron, 10-14 November 1974
(1 x 103) 77
20 Horizontal distribution of Daphnia retrocurva in
Southern Lake Huron, 26-31 August 1974 (1 x 103). . 79
21 Horizontal distribution of Eubosmina coregoni in
Southern Lake Huron, 16-21 June 1974 (1 x 103). . . 80
22 Horizontal distribution of jBosmlna longirostris
in Southern Lake Huron, 16-21 June 1974 (1 x 103) . 81
23 Horizontal distribution of Holopedium gibberum in
Southern Lake Huron, 26-31 August 1974 (1 x 103). . 82
24 Horizontal distribution of Chydorus sphaericus in
Southern Lake Huron, 16-21 June 1974 (1 x 103) . . . 84
25 Mean abundance (with standard deviation and range)
of Bosmina longirostris in 7 segments of Southern
Lake Huron during the spring (Sp), summer (S),
and Fall (F) of 1974 86
26 Mean abundance of Eubosmina coregoni in 7 segments
of Southern Lake Huron during 1974 87
VI1
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Number Page
27 Mean abundance of total cladocerans in Southern
Lake Huron during 1974 88
28 Mean abundance of the exotic invader Eurytemora
affinis in Southern Lake Huron during 1974 '. 7 ... 89
29 Mean abundance of cyclopoid copepods in Southern
Lake Huron during 1974 91
30 Mean abundance of oligotrophic calanoid copepods
(predominantly copepodites) in Southern Lake
Huron during 1974 . . . 92
31 The ratio of calanoids to cyclopoids (adults and
cops) plus cladocerans for the months April
through October 1974 in Southern Lake Huron.
The higher values are indicative of relatively
oligotrophic conditions commonly found in
segment 10 (not corrected for net efficiency) ... 93
32 A comparison of populations of Bosmina longirostris
and total calanoids for 1971 and 1974 for similar
33
34
35
main
Column
Apri
Column
May
Column
June
lake st
biomass
1-3 May
biomass
1974
biomass
1974 .
atio
of
1974
of
of
ns (data from
zooplankton (m
zooplankton
zooplankton
(m
(m
CCIW)
gC m~2
gC m~
-2
gC m
)
)
)
on 28
on 14-17
on 4-8
. 94
. 96
Q7
. Q8
36 Column biomass of zooplankton (mgC m ) on 16-21
June 1974 99
_2
37 Column biomass of zooplankton (mgC m ) on 17-22
July 1974 101
38 Column biomass of zooplankton (mgC m~ ) on 26-31
August 1974 102
39 Column biomass of zooplankton (mgC m ) on 8-12
October 1974 103
40 Column biomass of zooplankton (mgC m~2) on 10-14
November 1974 104
4l Acoustic column biomass (mgC m~ ) on 16-21 June
1974 109
viii
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Number Page
n
42 Acoustic column blomass (mgC m ) on 8-12 October
1974 no
43 Acoustic column blomass (mgC itT^) On 10-14 November
1974 Ill
44 Percentage composition of crustacean zooplankton
in Southern Lake Huron in 197^-75 (im.: immature,
ad.: adult) 128
ix
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TABLES
Number Pag
1 Cruise schedule (a) and stations (b) considered
within water quality management areas 9
2 Grazing budget for daytime, 23-26 July 197^, for
Southern Lake Huron 15
3 Grazing budget for daytime and nighttime, 26-30
August 1974, for Southern Lake Huron (A = adult,
J = juvenile, C = copepodite) 16
4 Grazing budget for daytime, 10 November
for Southern Lake Huron 17
5 Community filtering rate, net primary productivity
(NPP), grazing rate and ratio of carbon grazed:
carbon fixed, during daytime and nighttime for
cruises on Southern Lake Huron (D = daytime,
N = nighttime) 22
6 Relative biomass (% volume) of diatoms (Bacillario-
phyta) and blue-green algae (Cyanophyta) at
feeding stations in Southern Lake Huron in 197^ . . 27
7 Mean and range of algal ingestion rates based on
mass (mgCd-^mgC"1) for zooplanktonic crustaceans
of Southern Lake Huron 29
8 Assimilation as percentage of total algal carbon
ingested by various crustaceans 30
9 Standing crops (mgC m~3) of nannoplankton (< 22 ym)
and netplankton (> 22 ym) used in calculations
of percentage of carbon grazed (calculated from
Stoermer et_ al. 1978) 32
10 Grazing budget on nannoplankton, 12 April 1975. ... 33
11 Grazing budget on netplankton, 12 April 1975 3^
12 Grazing budget on nannoplankton, 5-9 May 1975 .... 35
13 Grazing budget on nannoplankton, 28 May-2 June 1975 36
x
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Number Page
14 Grazing budget on netplankton, 28 May-2 June 1975 . . 37
15 Grazing budget on nannoplankton, 5 August 1975. ... 38
16 Grazing budget on netplankton, 5 August 1975 39
17 Grazing budget on nannoplankton, 19 October 1975- 40
18 Percentage of net primary productivity grazed
within size groups of algae and by day and night. . 44
19 Mass ingestion rates on nannoplankton 46
20 Mass ingestion rates on netplankton 51
21 Percentage assimilation (mgC an"-*- hr~l assimilated/
mgC an"-'- hr"-'- ingested) on nannoplankton resources
during October 1975- One standard error is indi-
cated ' 52
22 Percentage assimilation (mgC an"1 hr assimilated/
mgC an"1 hr"-1- ingested) on netplankton resources
during October 1975. One standard error is indi-
cated 53
23 Variations in filtering rate on green (G) versus
blue-green (BG) algae by common herbivores of
Lake Huron. Significantly (**a = .01; *a = .05,
NS = not significant) higher filtering rates on
one member of pair indicated; (- = lack of signi-
ficant number precluded analysis) 58
24 a) Matrix of resource characteristics and b) ranked
food preferences for Diaptomus sicilis (E =
electivity) 60
25 a) Matrix of resource characteristics and b) ranked
food character preferences for cyclopoid copepo-
dites (E = electivity) 6l
26 Two level ANOVA with unequal sample size summarizing
effects of sheath and food size on filtering rates
of Diaptomus and cyclopoid copepodites (a = .001
(***)'; a = .05 (*)) 63
27 Biomass (mg Cm~-^) of zooplankton and other particles
per unit volume (m~3) determined acoustically
within size-class (0,8 to 2 mm and 2 to 3 mm) . . . 106
xi
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Number Page
28 Comparative biomass of zooplankton (0.8 to 2 mm)
estimated by acoustics and from net samples
in Southern Lake Huron, 197^-7 5 .......... 10?
29 Lakewide abundance of zooplankton population
densities (# m~~3) fOr the water column five meters
to surface ..................... 11 6
30 Lakewide abundance of zooplankton population
densities (# m~3) for the water column ten meters
to surface ..................... 117
31 Lakewide abundance of zooplankton population
densities (# m~3) for the water column twenty
meters to surface ................. 118
32 Lakewide abundance of zooplankton population
densities (# m~3) for the water column forty
meters to surface ................. 119
33 Seasonal water quality section summary for the
water column less than ten meters ......... 121
3^ Monthly water quality section summary for the
water column less than ten meters ......... 123
xii
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ACKNOWLEDGEMENTS
The study was an integral part of the Southern Lake Huron
project. Logistic support was provided by the men of the RV
Roger Simons under the command of Captain James Murphy. Mr.
Nelson A. Thomas, the Project Officer, was of help in many as-
pects including arrangement of field operations. Mr. Robert Zeh
of the S.U.N.Y.A. Electronics Shop designed the acoustical
equipment. The quality line drawings were produced by Mr. Ryland
Loos. Ms. Linda Welch typed the manuscript with unusual preci-
sion and care.
xiii
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SECTION 1
INTRODUCTION
PURPOSES
The objectives of this investigation involved a) determin-
ing the magnitude of grazing on natural phytoplankton popula-
tions in Southern Lake Huron, b) collecting experimental field
evidence for selective grazing on specific size-groups of phyto-
plankton, c) providing experimental laboratory evidence concern-
ing the mechanisms of selective herbivory, d) formulating a
measure of biological control by herbivorous crustaceans, and e)
determining the abundance of the crustacean zooplankton in
Southern Lake Huron, both by net sampling and modern acoustical
techniques.
ROLE OF GRAZERS
Detailed estimates of grazing by the herbivorous zooplank-
ton are necessary to understand the degree to which biological
control of algal populations occurs. Likewise, since the con-
trol of a size-class or taxonomic grouping of algae may mean the
stimulation or release from competition of another group of pri-
mary producers, numerous interactions may exist between the
grazers, their predators, and their algal prey. Each of these
must be understood in itself before we may truly understand the
functioning of aquatic systems.
The impact of the grazers upon a natural assemblage of
phytoplankton is controlled by a series of interactions affect-
ing grazing both quantitatively and qualitatively. These in-
clude the interaction between the ingestion rate of the zoo-
plankton and the abundance of algae; ingestion by zooplankton
often saturates at 1CP cells ml~l for freshwater organisms
(McMahon and Rigler, 1963); the relationship between grazing
selectivity by zooplankton and the relative abundance of pre-
ferred algal foods (Bogdan and McNaught, 1975); the differential
resistance to digestion of algal foods (Porter, 1973); tempera-
ture of the environment and Q-jn for ingestion; and assimilation
of a particular species and daily and seasonal fluctuations in
grazing behavior, related either to apparent endogenous rhythms
in grazing (Chisholm, Stress, and Nobbs, 1975) or to the inhibi-
tion of grazing by substances released into the environment.
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In designing a study of an aquatic ecosystem, attention
should be focused on three basic processes, including seasonal
and daily patterns of grazing by zooplankton, remineralization
of algal nutrients by zooplankton, and selective cropping of
large zooplankters by fish predators. These factors imply that
a considerable amount of self-regulation occurs in aquatic eco-
systems. As algal production increases with the input of nu-
trients from an urbanized society, not only does zooplankton
grazing increase, but the remineralization or recycling of im-
portant nutrients changes correspondingly.
The purpose of this report is to examine the first of these
three important interactions: the seasonal and diel (daily)
patterns of grazing by the crustacean zooplankton of Lake Huron.
Due to technical difficulties, we did not attempt to directly
measure the component assigned to the rotifers, nor did we esti-
mate grazing on bacteria and detritus, since this component is
of lesser importance, even in eutrophic ponds (Saunders, 1969)3
and bacteria are probably not the major food source for the
herbivorous zooplankton in mesotrophic Lake Huron.
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SECTION 2
CONCLUSIONS
Planktonic herbivores have many functions in Southern Lake
Huron, a natural unperturbed aquatic system. They feed upon
both algae and detritus. In doing so they may remove very lit-
tle of the standing crop of phytoplankton at the upper extreme
of sizes, whereas at the lower extreme they may selectively re-
move entire groups of algal species. Thus, the impact of graz-
ing rarely affects the entire assemblage, but may result in the
inhibition or stimulation of specific algal groups. Grazers not
only selectively influence the phytoplankton through consumption,
but also remineralize important amounts of algal nutrients,
thereby stimulating ungrazed algae. Ultimately successful
groups of herbivores provide important food resources for larval
fishes. Thus, herbivore dynamics must be understood to model
and manage aquatic ecosystems like Southern Lake Huron. In this
study we have determined the level of both total and size-selec-
tive grazing by crustaceans in Southern Lake Huron.
Crustaceans graze from 0.1 to 37-8% of the net primary pro-
ductivity and 0.3 to 4l.9$d~-'- of the standing crop of phyto-
plankton in Southern Lake Huron. Grazing is generally high in
early summer and fall and low during both the spring diatom
bloom and the mid-summer blue-green bloom. The immature stages
of copepods were the most effective grazers, including popula-
tions of nauplii, calanoid copepodites, and cyclopoid copepc-
dites. At the organismal level, mass ingestion rates (% body
weight ingested day-1) were highest for nauplii (100$), cyclo-
poid copepodites (100%), Cyclops bicuspidatus (103%), and
DIaptomus adults (100$) and lowest for Limnocalanus (0.5$).
Thus, it was not surprising that the percentage assimilation of
ingested food was highest for rapidly growing nauplii and cope-
podites abundant in this ecosystem and lowest for a predaceous
calanoid copepod. Grazing rates appeared to be closely related
to temperature and the presence of large diatoms and blue-green
algae. Grazing was not limited by populations of algae exceed-
ing the saturation level for crustacean grazing, since Southern
Lake Huron is oligotrophic and its waters were characterized by
phytoplankton populations well below 10^ cells ml"-'-.
The nannoplankton (algae < 22 ym diameter) were selectively
grazed by zooplankton. From 92$ to 2839$ of the net primary
productivity of nannoplankton was removed each day of the 6
3
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months constituting the growing season. This exceptional har-
vest was due to selective grazing by the cladocerans Holopedium
gibberum, Eubosmina coregonl, and Daphnia retrocurva, and cope-
pod nauplii, cyclopoid copepodites, and calanoid copepodites.
Relatively little harvest of nannoplankton was attributable to
adult Diaptomus. In contrast, only 30% to 367% of the netplank-
ton net primary productivity was ingested. The harvest of net-
plankton, in contrast, was due in part to the same cladocerans
(Holopedium, Eubosmina), but with much less netplankton inges-
tion accomplished by copepods. During August, the cladocerans
grazed 297% of netplankton production, as opposed to 26% by
calanoids and 44% by cyclopoids. Considering the size of these
herbivores, Chydorus sphaericus (21.9%d-1) and nauplii (21.2^d~1)
exhibited the highest mass ingestion rates (% body weight d~l)
and Diaptomus spp. (adults), the lowest (2.4%d~1) on nannoplank-
ton. On netplankton, Chydorus sphaericus (12.7%d~l) had the
highest mass ingestion rate. Generally those animals which in-
gested large amounts also egested large amounts, such that as-
similation was inversely related to ingestion. Diaptomus minu-
tus and Cyclops bicuspidatus adults assimilated most of the
small volume of netplankton they ingested, whereas Holopedium
and calanoid copepodites, while efficient ingestors, were inef-
ficient assimilators of nannoplankton. Generally, assimilation
rates on nannoplankton (mean 54.3 + 13-4%; range 5-100%) were
much higher than on netplankton (mean 29.3 +. 8.0%; range 3-4-
83.3%), due in part to the protective gelatinous sheath sur-
rounding many blue-green algae.
Grazing exerted a significant biological control upon the
growth of algal populations in Southern Lake Huron. Biological
control exists when the daily production of a resource is
totally grazed. Control through the natural populations of
crustacean herbivores existed from early May through late Octo-
ber 1975, with up to 25.3x the daily production of nannoplankton
harvested during August. In contrast, biological control over
netplankton growth existed over the same period, but with a
maximum of 367% harvested. We emphasize that the primary pro-
ductivity of oligotrophic waters of Lake Huron is In balance
with the grazers, due to the apparently rare Inhibition of
grazing by blue-green algae. Algal growth must be kept under
control.
Laboratory experiments permitted enumeration of those
algal characteristics which limited grazing. Both Diaptomus
slcilis and cyclopoid copepodites were highly selective grazers.
They both selected large plain cells, consumed small plain
cells in the proportion offered, and rejected small and large
sheathed cells, as well as small spined cells. It is not sur-
prising that blooms of nuisance algae are characterized by
blue-greens which have sheaths. Size-selective grazing was
further investigated in the field using micro-beads made of
polystyrene, added to natural algal foods. Diaptomus (nauplii,
4
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copepodites, and adults) and Cyclops bicuspldatus (adults) both
selected very small beads, less than 1 jam in diameter, illus-
trating again the intense selective pressures which occur upon
those small phytoplankton characteristic of oligotrophic lakes,
such as Lake Huron.
Eutrophic waters are characterized by communities of crus-
taceans peculiar to warm waters and most likely associated as-
semblages of algae and groups of predatory fishes. Cyclopoid
copepods and cladocerans are often considered eutrophic indica-
tors and were found in abundance in inshore waters of Lake Huron
and particularly in the mouth of Saginaw Bay. Calanoid copepods
are more oligotrophic organisms. All were found offshore, but
the most oligotrophic, Diaptomus slcills, was most abundant in
the midlake region.
Planktonic ratios (calanoids/cyclopoids and cladocerans)
and indicator species were utilized to structure water quality
within the eight management segments of Southern Lake Huron.
The inshore segments (4, 5, 7, 8) and segment 6 offshore of
Saginaw Bay consistently showed lower water quality than segment
10 (northern open waters). Significant increases in pollution-
indicating crustaceans were not apparent between samples col-
lected by Canadian Center for Inland Waters (CCIW) in 1971 and
ourselves in 197^; historical trends must be examined over
longer intervals to be evident.
The horizontal distribution of total crustacean biomass
was examined, both by net sampling and through acoustical inte-
gration of echoes from these small targets. Net sampling
showed that zooplankton biomass was greatest in the mouth of
Saginaw Bay. Frequently, large densities moved counter clock-
wise from the bay and drifted southward along the Michigan
shoreline. Inshore densities were also of importance, with
notable areas of high standing crops near Goderich, Canada.
Acoustic estimates of particle abundance likewise showed maxima
in the mouth of Saginaw Bay. However, in contrast to net sam-
pling, deepwater maxima of biomass were also observed through
the use of acoustic estimates. This discovery is explained by
the suggestion that these large deepwater particle densities
are aggregations of detritus.
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SECTION 3
RECOMMENDATIONS
The waters of Southern Lake Huron are of high water quality,
as evidenced by populations of oligotrophic, cold water calanoid
copepods. Dominant herbivores today maintain a desirable bal-
ance between grazing (removal) and production of phytoplankton.
That is, a natural biological control exists upon phytoplankton
growth (both nannoplankton and netplankton). Zooplankton selec-
tively graze small phytoplankters. Most species of zooplankton,
however, ignore certain large sheathed blue-green algae. Such
colonial blue-greens occur in nutrient enriched environments;
their growth also may show lesser acute effects from chlorinated
hydrocarbons. Since Intense biological control of nannoplankton
is a natural ecosystem characteristic, .any combination of nanno-
plankton inhibition (by grazing) and netplankton stimulation (by
nutrient enrichment and selective poisoning of nannoplankton)
will lead to a shift in dominance by blue-green algae and
troublesome blooms.
Currently the high water quality of Southern Lake Huron,
reflected In the presence of dominant populations of calanoids,
can be maintained by shrewd ecosystems management. We recom-
mend the usual reduction of inorganic limiting nutrients (P),
coupled with a reduction in toxicant loadings. Also, the man-
agement of planktonphagous fishes (chiefly the alewife) Is
necessary to maintain the endemic herbivore population. If
these four objectives are met, biological control by crustaceans
upon algal populations should continue in Southern Lake Huron.
Warnings of potential changes are evident. The largest
standing crops of zooplankton, dominated by the eutrophic
Bosmlna longirostris, were found in the mouth of Saginaw Bay.
Bosmina was shown to be an ineffective grazer, even when abun-
dant. In addition, other eutrophic forms including the calanoid
Diaptomus siclloides were found in the bay. Thus, zooplankton
populations in the bay were typical of eutrophic waters. We
suspect that such herbivore populations do not control algal
production in the bay (a matter of current Investigation).
While the waters of the main lake may never resemble those of
the shallow bay, the trend, unless reversed, will be toward re-
duced water quality.
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SECTION 4
EXPERIMENTAL PROCEDURES
4.1 METHODS FOR TOTAL GRAZING
A modification of the original methods of Nauwerck (1959),
who determined grazing through the use of tagged (-^C) phyto-
plankton, was employed. The radioactivity of zooplankton was
measured after a brief feeding period (15 to 30 min) too short
for the production of radioactive faeces at low food levels.
Basic to this method is the estimation of the shortest time in
which a particle of food can pass the gut, or in which signi-
ficant amounts of COp can be respired. Previous investigators
have estimated this period to be 10 to 20 min (Bogdan and
McNaught, 1975). Before being given radioactive food, the ani-
mals were acclimated for 1 hr in non-radioactive cells of the
same natural assemblage tagged with Na ^CO^. Approximately
2000 animals were acclimated in 600 ml of natural algae in lake-
water. After 1 hr, 400 ml of tagged food was added. The ani-
mals were then allowed to feed on the hot mixture for 15 to 30
min. They were relaxed with COp water, killed with boiling
water, and preserved. Each experiment was run in quadruplicate.
In the laboratory, animals were picked using a 1 mm diameter
metal loop, carefully avoiding the removal of large filamentous
algae. Approximately 50 animals were placed in a scintillation
vial and digested with Protosol (New England Nuclear); after 24
hr at 60ฐC, the vials were filled with 10 ml of cocktail (49$
toluene and 49$ ethylene glycol monomethyl ether and 2% Liqui-
fluor [NEN]). All samples were placed in the dark for 24 hr to
eliminate chemoluminescence and then counted in a Beckman Model
133 liquid scintillation counter. Typically, zooplankton sam-
ples were counted with an efficiency of 95$, as contrasted to
phytoplankton foods counted at 65$ efficiency.
Samples of the natural phytoplankton assemblage of
southern Lake Huron were collected at two nearshore stations,
one near the outlet at Port Huron (Station 63) and one in the
mouth of Saginaw Bay (Station 47). Samples of the natural zoo-
plankton community were taken by a vertical tow (0 to 5 m) using
a net of 64 ym aperture at these close-by stations (Pig. 1)
(Table 1).
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TABLE 1. CRUISE SCHEDULE(A) AND STATIONS(B) CONSIDERED
WITHIN WATER QUALITY MANAGEMENT AREAS (4-10)
A. Cruise Number Cruise Dates
11 April 28-May 3, 1974
12 May 14-17, 1974
13 June 04-08, 1974
1^ June 16-21, 1974
15 July 17-22, 197^
16 August 26-31, 1974
17 October 08-12, 1974
18 November 10-14, 1974
20 April 02-13, 1975
21 May 28-31, 1975
22 August 06-10, 1975
23 October 17-23, 1975
Area Stations
Area 4 14, 15, 26, 46, 52, 63
Area 5 36, 37, 38, 39, 40, 4l, 42, 43, 44, 45
Ar>ea 6 16, 23, 24, 25, 47, 48, 49, 50, 51, 53, 67
Area 7 6, 64, 65
Area 8 7, 9, 10, 56, 57, 58
Area 9 11, 13, 54, 55, 60, 66
Area 10 20, 21
-------
4.2 METHODS FOR FIELD STUDIES OF SELECTIVE GRAZING
Techniques similar to the above for total grazing were em-
ployed, except that food resources were size-fractionated. The
natural assemblage of phytoplankton was passed through a fine
net (22 ym), leaving the nannoplankton (< 22 ym) in the lake-
water and removing the netplankton (> 22 ym), which was resus-
pended in filtered (0.45 ym) lakewater. Tagged (1^C) nanno-
plankton, following a 4 hr in_ situ incubation, were mixed with
cold netplankton to reconstitute the natural assemblage. In a
similar manner, tagged netplankton were mixed with cold nanno-
plankton just before feeding. The natural assemblage of herbi-
vores, already acclimated to their food assemblage, was reaccli-
mated for 1 hr in watercooled shipboard tanks. Animals accli-
mated in 400 ml lakewater were fed 600 ml hot food. After
feeding for 12 to 15 min, the animals were killed and preserved.
Assimilation was calculated by feeding animals for 1 hr,
as above, and then removing them and placing them in cold food
for 4 hr, during which time any hot food would be eliminated
from the gut. Assimilated food was measured as above, using
liquid scintillation techniques. Percentage assimilation
(Sorokin, 1968) was calculated from the ratio mgC assimilated
hr~l to mgC ingested hr~l.
4.3 METHODS FOR LABORATORY STUDIES OF SELECTIVE GRAZING
Paired-food Selection Experiments
Seven common herbivorous crustaceans, including five adult
stages (DIaptomus sicills, Cyclops blcuspidatus, Cyclops
vernalis, Tropocyclops prasinus, and Eubosmina coregoni) and two
of their larval stages (copepod nauplii and cyclopoid copepo-
dites), were fed mixtures containing 50% by volume one of two
blue-green algae (Gloeocapsa sp., Anacystis nidulans) and one
of three green algae(Scenedesmus quadricauda, Ankistrodesmus
faleatus, and Pediastrum sp.). These experiments Involved two
treatments with replicates, one treatment in which the blue-
green was tagged (50 yd ^C) and the green cold, and the other
where the green algae was tagged (-^C) and the blue-green cold.
A minimum of six internal replicates for each of these combina-
tions were available. Thus, 24 independent treatments resulted
In 144 samples containing 7 herbivores. Approximately two sub-
samples of 50 animals were picked by hand from each sample;
thus, approximately 1 x 10^ animals were handpicked to provide
data on selective feeding at the species/stage level. Differ-
ences in selectivity were detected, using the t-statistic for
unpaired means and a 2-level ANOVA. In addition, electivities
were calculated according to the method of Ivlev (1955).
10
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Bead Selection Experiments
Herbivorous zooplankters rely not only on size, but shape,
taste, and other factors in selectively feeding. A food source
characterized only by size would be of immense experimental use.
We selected polystyrene beads which are sphaerical, tasteless,
and vary in size. These specific beads were formulated by Dr.
M. Gliwicz in the Laboratory of Hydrobiology at Warsaw Univer-
sity. They varied in size from 0 to 10 ym diameter, with a
peak in frequency at 3 yrn. Beads were mixed with the natural
food assemblage on shipboard and fed to the natural zooplankton
populations. After 30 min animals were relaxed and killed.
The zooplankton were mounted in a Turtox medium (CMCP) and the
beads measured under oil immersion, using a calibrated ocular
disk. The bead population in the gut was then compared to that
of the environment in determining selectivity and electivity.
4.4 METHODS FOR PLANKTON COLLECTION
The crustacean zooplankton were collected in a large cone
net (0.8 m diameter, 3 rn hang) of 64 ym aperture nitex. Hauls
were made from 0 to 5 m, 0 to 10 m, 0 to 20 m, and 0 m to bottom
at each survey station. Animals were concentrated and stored
in buffered formalin. In the laboratory, animals were concen-
trated to 25 to 100 ml, and three 1 ml aliquots were counted.
Enumeration for adults was to species and sometimes sex (diapto-
mid copepods). Immature copepods were lumped as nauplii and
cyclopoid and calanoid copepodites.
4.5 METHODS FOR ACOUSTIC BIOMASS ESTIMATES
Large scale sampling to determine zooplankton biomass is
expensive relative to vessel time required, and even more so
with regard to processing samples. We have developed an acous-
tical technique for estimating the biomass of particulate organ-
ic material in suspension (McNaught, 1975). Estimates are
normally categorized within three size classes (0.8 to 2 mm,
2 to 3 mm, 3 to 5 mm) by utilizing the increased signal return
within envelopes created by the 500 and 200 kHz, 200 and 120
kHz, and 120 and 80 kHz frequency transmitter-receivers. This
technique readily illustrates the distinct advantage of han-
dling large amounts of data. However, acoustical methods have
the disadvantage that targets, while placed into size categories,
cannot as yet be classified by biological category (order,
species, etc . ) .
11
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SECTION 5
RESULTS
5.1 GRAZING BY ZOOPLANKTON UPON NATURAL PHYTOPLANKTON POPULA-
TIONS: SEASONAL AND DIEL ESTIMATES OF TOTAL ZOOPLANKTON GRAZING
Formulations for Grazing Estimates
Students of the grazing process have traditionally relied
upon two measures of grazing, the species or community filtering
rate, and the community grazing rate. We will modify the second
concept and add a third, the relative impact of grazing or the
amount of productivity grazed.
Filtering Rate (F.R.)
The most commonly used index of grazing is the filtering
rate (Rigler, 1961). Actually filtering rates are backcalcu-
lated from ingestion rates, which all investigators measure.
Using isotope (lj^C) techniques, if the ratio of radioactive to
cold cells in the feeding suspension is known, and the activity
of animals is measured, ingestion can be calculated. But since
ingestion efficiencies may vary from 0.5 to 2.0% (Bogdan and
McNaught, 1975), actual filtering rates are higher than the
minimal filtering rates calculated by most investigators from
the following formulation:
(I) PR = Activity animals/No, animals
(Activity phytoplankton/Volume phyto)x% hotx time
ป
= cpm an~^ = ml an""-'- hr~^
cpm ml"1 hr
Grazing Rates (G.R.)
Once the minimum filtering rate has been determined, only
the density of grazers must be known to calculate the grazing
rate. Grazing rates have units of relative water volume grazed
per time (% day~l) or relative amount of algal carbon grazed per
unit time (% day"-'-). Both of these indices are useful, but
since the grazing efficiency has often been ignored, those
grazing rates based upon volume (IIA) may be less ecologically
meaningful than those based on algal carbon grazed (IIB). In
either case, they are calculated from a knowledge of filtering
rates as follows:
12
-------
(IIA) Based on water volume:
GR .. = No. animals x FR = m3 day"1 = % day"1
volume - ^
unit water volume mj
(IIB) Based on carbon:
GRcarbon = algal carbon ingested = (% day~i)(mgCm~3) = % day"1
algal carbon available (mgCm-3)
Thus, the grazing rate involves the loss of carbon (dC)
with regard to the standing crop (C), or the integral,
1 dฃ
C dt
Relative Grazing Impact (G.I.)
In contrast to the grazing rate, we can express the amount
of carbon grazed with regard to the amount fixed the same day by
the primary producers. Immediately this notation tells us at
what times the grazers are overharvesting the primary produc-
tion. The impact upon productivity is calculated using the
filtering rate (Eq. I), but the denominator is the rate of
primary productivity (mgC m~3 hr"1) Instead of the standing
crop of algal C (mgC m~3), as in the grazing rate calculation.
Impact on primary productivity is thus determined as follows:
(III) G.I. = algal carbon ingested = (% day"1)(mgCm-3) = %
algal carbon fixed (mgCm-3 day"1)
Note that the units (%} refer to the percentage of productivity
(carbon fixed) which was grazed, where both the processes are
calculated over the same time interval.
In contrast to the grazing rate, this measure of the im-
pact of grazing on primary productivity Involves the Integral
of change (decrease) In plant carbon with time, due to inges-
tion by zooplankton, divided by the integral of the change (in-
crease) in plant carbon fixed with time, where:
t0/t:LdC/dt
t0/tldC/dt
If all the species of G.I. terms are summed, we have the total
community grazing impact (C.G.I.) discussed later in this sec-
tion.
Grazing Budgets
Monthly Budgets
Grazing budgets have been compiled for each monthly cruise
13
-------
on Southern Lake Huron. Three basic Indices have been employed,
including the filtering rate (Eq. 1) for each species, life
stage, and the total community; the grazing rate (Eq. 2b) based
on carbon, and the percentage of gross primary production (GPP)
grazed (Eq. 3) both during daylight and darkness. Detailed
budgets have been provided for periods of limited grazing, in-
cluding August 1974 (Table 3), as well as periods of intense
grazing during July 1974 (Table 2) and November 1974 (Table 4).
During July 1974, the relatively rare Holopedium gibberum
constituted only 1.4$ of the crustacean density, but due to a
high filtering rate (0.4l6 ml an~l hr"1) and high ratio of food
intake per unit biomass, it accounted for 26.2% of the C grazed
(Table 2). The most important grazers were the calanoid cope-
pods (69.5% numbers), which did 60.5$ of the grazing. Abundant
copepcd nauplii (63906 m~3) (5.2$) and the cyclopoid copepodites
(16369 m~3) (5-8$) were a distant third and fourth respectively.
As expected, relatively little grazing was done by rare species
which constituted an insignificant percentage of total zooplank-
ton biomass. Bosmina longirostrls (0.3% numbers) contributed
only 0.7% grazing, while Tropocyclops prasinus, a small cyclo-
poid (0.8% numbers), contributed only 0.2% grazing.
Grazing was greatly depressed during August 1974. The
most abundant organisms, the calanoid copepodites (35-2% num-
bers), accounted for 26.5$ grazing during daylight, as opposed
to 0.3$ during darkness. Copepod nauplii, second in abundance
(29.7$ numbers), by virtue of higher filtering rates at night,
did 49.5$ grazing during daylight and 62.0% at night. Diel
differences in grazing are significant. Most grazing is accom-
plished with populations feeding nearer the surface at night.
Grazers actually overcropped phytoplankton production (GPP)
during October 1974 (Table 5). During daylight, abundant calan-
oid copepodites (26.3$ numbers), although characterized by low
filtering rates (.005 ml an"1 hr"1), did 39-9$ of the grazing;
nauplii grazed 18.5$ of GPP to account for much of the remain-
der. However, Daphnla galeata (6.6$ numbers), with a high fil-
tering rate (.059 ml an~l hr~l), and Epischura lacustris (6.0$
numbers), with a very high filtering rate (.526), accounted for
5.2 and 12.6$ of the grazing, respectively. October was the
only month other than April when an adult calanoid (Epischura)
accounted for more than 10$ of grazing during daylight. Adult
calanoids must be chiefly predatory in nature. In contrast,
grazing was slightly increased during hours of darkness. Again
calanoid copepodites, with greatly increased filtering rates
(0.414), did most (38.5$) of the grazing, followed by the
nauplii (17.2$) and Holopedium. gibberum (10.4$). Also, in Octo-
ber a large number of minor forms were important. Bosmina
longirostris and Eubosmina coregoni both exhibited high filter-
ing rates during day and night, as did Daphnia retrocurva.
14
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17
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Summary: Important Species and Stages by Month
Limnocalarms mac rums and nauplll of copepods (probably a
large percentage calanoid nauplii), as well as a few adults of
Dlaptomus sicills, were the Important grazers under the slush
ice on 2 April (Figure 2). This is not surprising, since Limno-
calanus is adapted to waters below 11ฐC (McNaught, 1966) and
exhibits its highest filtering rates at 0.6ฐC (Figure 4), as
discussed later. By 5 May, copepod nauplii were totally domin-
ant, although Diaptomus slcllis was still important and Holo-
pedium had made its first contribution to grazing. By late
July, calanoid copepodites, most likely those developed from the
previously dominant nauplii, significantly increased grazing,
along with Holopedium (Table 2). Grazing plummeted during Aug-
ust (Table 3). The small amount of production was grazed by
nauplii (mostly cyclopoids) and a decreasing population of cal-
anoid copepodites, Diaptomus minutus and D. ashlandi, Holopedium,
Daphnia galeata, Bosmina longirostris and B. coregonl. Thus,
the month of least grazing pressure was 1) the month of greatest
zooplankton density, and 2) the month of greatest zooplankton
diversity. In October, when grazing was influential in reducing
the amount of primary production, zooplankton such as nauplii,
calanoid copepodites, cyclopoid copepodites, Bosmina, and
Epischura lacustris were important. By November, Daphnia
galeata and Epischura had increased in importance at the expense
of the developmental stages of copepods.
Impact of Grazers upon Carbon Pool
The standing crop of algal carbon was greatest during
April 1975 and October 1974, although the flux to this pool
through primary production (GPP) was largest in July 1975 and
August 1975 (Table 5). This fixation (GPP) of carbon by the
algae was the only flux into the pool. Grazing upon the algae
by zooplankton, a major loss from the pool, was greatest in July
and especially October 1975 when carbon grazed was over one-
third the carbon fixed (Figure 3, Table 5). Thus, high fixation
rates and negligible grazing during August were at least in part
responsible for higher standing crops of phytoplankton in Octo-
ber. In a similar fashion, since grazing of C exceeded fixation
during October, standing crops showed a reduction in November
(Table 4).
Controls upon Grazing
Temperature, Filtering, and Ingestion Rates
At first analysis, both low and high water temperatures
apparently inhibit filtering rates and thus the ingestion of
algal C by the dominant cladocerans and copepods and their im-
mature life stages in Lake Huron (Figure 4). In analyzing this
figure and all the remaining figures throughout this report, the
-------
Cyc/opoi'd Copepodites
t~-'~~*M~,
Epischura
Figure 2. Relative contribution to total phytoplankton grazing
by important species and life stages of crustacean
zooplankton in Southern Lake Huron (Limn.:Limnocalanus;
Bos.:Bosmina longirostrls).
19
-------
2000 L
1000-
-2000
- 1000
ซ? 100
o
o
_Q
t_
O
o
x
3
Figure 3. Phytoplankton (as carbon) grazing rate by crustacean
zooplankton contrasted to standing crop of phytoplankton
carbon and rate of fixation of carbon by
phytoplankton in Southern Lake Huron.
20
-------
ro
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33 O O O. Cd Cd
-} < O "Z
-------
following species' codes will be necessary:
Holopedium gibberum (A) = ฎ Cyclopoid cops. =
Daphnia galeata (A) =O Cyclops vernalis = El
Chydorus sphaericus = 3 Cyclops bicuspidatus = D
Eubosmina coregoni = C
Bosmina longirostris = nauplii = A
Calanoid cops. = A
Diaptomus (A) = A
Limnocalanus = V
Filtering rates were maximum for the cladocerans at 12ฐC,
with adults of Daphnia galeata and Bosmina longirostris fil-
tering below 0.5 ml an"1 hr~1. Calanoid copepods presented a
similar pattern, with the important exception of Limnocalanus
macrurus. Diaptomus adults exhibited maximum rates of 0.24 ml
an-1 hr"1, while their copepodites were typically 58% higher
(0.38). For both, filtering was negligible at 0.6, 5, 10, and
20ฐC. Limnocalanus, the largest calanoid copepod in the Great
Lakes and a stenothermal relict specie occupying cold hypo-
limnetic waters during summer months, exhibited a distinct pat-
tern of temperature response. Filtering rates were highest at
0.6ฐC under slush ice and decreased at 10ฐC, most likely an
adaptation to its unusually cold environment.
The rate of ingestion of algal carbon by zooplankton (mgC
an"1 hr-1) showed a similar pattern, which was expected because
filtering rates are determined from measurements of ingestion
(Figure 4). From the viewpoint of community trophic dynamics,
it should be noted that the relatively small Bosmina longiros-
tris had a higher maximum ingestion rate than Daphnia galeata,
at about twice its biomass. Likewise, ingestion rates of
calanoid copepodites exceeded those of adult Diaptomus and even
the large Limnocalanus. Maximum ingestion rates for Cyclops
bicuspidatus and cyclopoid copepodites were lower than those of
the calanolds and cladocerans; cyclopoids are probably omniv-
orous, relying less upon plant foods.
The low optimum temperature for feeding (12ฐC) was not sur-
prising. The mean temperature (April-November) for the waters
(upper 10 meters) of Lake Huron is approximately 13ฐC. How-
ever, significant inhibition of feeding at 10 and 20ฐC most
likely occurs as a result of factors other than temperature,
as discussed below.
Effects of Food Concentration
McMahon and Rigler (1961) discovered a saturation feeding
level of 105 cells/ml (yeast) for large Daphnia magna.; ingestion
was constant at food levels higher than this. Diaptomus
oregonensis exhibits a similar level (Richman, 1966).Recent
examination of saturation feeding suggests that such rectilinear
models are suitable, although saturation levels vary by species
23
-------
and may be as low as 103 cells/ml for the marine calanoid,
Calanus (Frost, 1975). Plotting ingestion rates (mgC an~l d""1)
against algal concentration (mgC m3) suggested that important
grazers like Diaptomus, as well as the entire natural community
in Lake Huron, do not exhibit saturation feeding at algal carbon
levels of IxlO2 mgC m~3 (Figure 5). For Lake Huron, this level
of organic C corresponds to about 2x10^ algal cells per ml.
Grazing Versus Age Structure and Food Quality - Use of Mass
Ingestion Rates
The grazing effectiveness of a species or life stage is
best illustrated by calculating the ratio of algal carbon in-
gested to unit body carbon (mgC d~l mgC"1). This mass ingestion
rate, if unity, indicates that a species ingested algal carbon
equivalent to its own body carbon in one day (similar to in-
gesting its own weight per day). Both seasonal and taxonomic
trends are evident in these data (Figure 6).
The seasonal range of this ingestion ratio varied from
8.5 x 10 mgC d"1 mgC"1 for Llmnocalanus adults in May to
1.8 x 10ฐ for copepod nauplii in October. That is, Llmnocalanus
daily ingested algal C equivalent to 0.09% of its body C which
is equal to 180% of their body carbon in October. This inges-
tion ratio in effect provides an index to the efficiency of
utilization of the phytoplankton stock by zooplankton.
Seasonally, three periods of greatly reduced ingestion, where
all species and life stages show reduced intake, are evident;
these are late May during the diatom bloom, late August during
the blue-green bloom, and November during the fall diatom pulse.
Two factors of physiological importance may be basic to
the limitation of ingestion. Many species of zooplankton
have specific size preferences. For example, Diaptomus prefers
small algal cells (< 22 ym) and others such as Daphnia feed
non-selectively over a wide range of sizes (Bogdan and McNaught,
1975). Thus, when large algal cells are abundant, the animals
must shift to larger forms, select rare small species in low
density, or shift to non-algal foods. Apparently, during late
May and June, when the majority of algal cells are diatoms
(92.5%) and mostly large (Table 6), these populations of crus-
taceans turned to other foods. Certainly they did not consume
those species which had fixed ^C, as indicated by the low
ratio of algal C ingested to body C.
Secondly, the antibiotic or inhibitory nature of algal
metabolic products has been extensively investigated, beginning
with the experiments of Ryther (1954) and continuing today.
During August, the Cyanophyta or blue-green algae were abundant
in the open lake (76% biomass) (Table 6). While many small
forms were present (Chroococcus, Gloeocystis), algal toxins and
food size combined with other factors may have been responsible
24
-------
40
*T
i
o
> 30
'ซ=
o
O
cr>
E
o>
DC
20
CO
CD
crป
10
0
25" 50 75 100 125
Concentration Algal Carbon [mg Cm"3]
150
Figure 5. Ingestion rate of zooplankton species relat
to food concentration in oligotrophic waters
of Southern Lake Huron.
25
-------
0.001
APR
MAY
JUNE
JULY
AUG
SEPT
OCT
NOV
Figure 6. Mass ingestion rates of dominant zooplankton
species by season (symbols in text).
26
-------
CjJ
EH
ffi
O H
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PS CO '~-
H S
H PS PS
EH C5 H
rH OS
Q. 0)
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27
-------
for reduced ingestion (Figure 4). Certainly temperature was not
the cause of decreased feeding.
Species trends were evident regarding the rate of C in-
gested during any month. Within taxonomic groups, the cladocer-
ans exceeded the calanoid and cyclopoid copepods. However,
larval calanoids (copepodites) and nauplii were often the fast-
est ingestors (Figure 6). Over the growing season, from April
through November, two or three groups of organisms were evident,
based on their rate of ingestion of algal C (Table 7). These
have been termed fast ingestors (greater than 21% body weight
per day) and slow ingestors (less than 5% body weight ingested
per day). Fast ingestors are most likely herbivores, whereas
slow ingestors are omnivores or carnivores. For the first time
we have identified the trophic position of all of the crusta-
ceans in the Lake Huron ecosystem.
Assimilation of Ingested Phytoplankton
In modeling the dynamics of ecosystems of the Great Lakes,
not only is the rate at which phytoplankton is ingested impor-
tant, but also the relative amount which is ultimately assimi-
lated. Obviously the assimilated component leads to growth; the
remainder is in part egested or remineralized. Cladocerans,
copepod nauplii and copepodites showed the highest mass inges-
tion rates (Figure 6). If a species or life stage combines high
ingestion with high assimilation, growth should be rapid.
Assimilation was highest for nauplii and cyclopoid cope-
podites (Table 8). These immature forms assimilated 53-55% of
the food they ingested on a diel basis. It was necessary to
measure assimilation both day and night, since feeding rates
vary greatly on a diel basis. Since nauplii may ingest 180%
of their body weight and assimilate 50% of this or 90% of their
weight, they can double their size in a day, truly an example
of "r-selection" growth characteristic of the zooplankton.
Thus, we now have critical evidence, not only for fluxes of C
into zooplankton but also for the assimilation of C, vital to
modeling efforts on Great Lakes ecosystems.
5.2 GRAZING BY ZOOPLANKTON ON NATURAL ASSEMBLAGES OF NETPLANK-
TON AND NANNOPLANKTON
Grazing budgets on nanno- and net-plankton were calculated
for each of six cruises on Southern Lake Huron (October, 1974;
12 April, 1975; May, 1975; 28 May - 2 June, 1975; 5 August,
1975; and 19 October, 1975). Budgets were determined for both
daylight and nighttime feedings, since the amount of food grazed
has been shown to vary cyclically throughout the day and is usu-
ally higher during the hours of darkness (Haney and Hall, 1975).
28
-------
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o
CO
1
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CO K
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30
-------
Estimates of standing crops of both nannoplankton and net-
plankton were necessary to make grazing computations. These
were determined from counts and cell volumes for each species of
phytoplankton, which were kindly provided by Dr. E. Stoermer of
the University of Michigan (Table 9).
Each budget (Table 10-17) separates the herbivorous zoo-
plankton into the Cladocera and Copepoda. The cladocerans are
further listed by species, and in the case of Holopedium
gibberurn and Daphnia galeata, by age (juvenile or adult).
Adults were invariably parthenogenic females with eggs, or in
very unusual cases, males. The remainder of the cladocerans
were lumped by species and included all age groups which were
present. The copepods were treated somewhat differently. All
nauplii were treated as a group. The calanoids were classified
generally as copepodites and specifically as adults by species.
The cyclopoids were grouped as copepodites and adults of four
prominent species (3 genera). Thus for many groups, information
on selective grazing is available at both the immature and adult
level for a number of species.
During October 1974, the nauplii of all copepods and the
calanoid copepodites were the most active grazers. The nauplii
grazed 25-3? of the nannoplankton productivity (25-3% during
daylight and none at night), while the calanoid copepodites
grazed 83-9$ (61.35? during daylight and 22.6% at night). The
only other important grazer on nannoplankton was Holopedium
gibberum, which grazed 39-9% of the nannoplankton productivity
(36.6$ during daytime and 3.3% during nighttime). The crusta-
cean community thus harvested 231-2$ of the daily nannoplankton
productivity (157-6% during daylight and 73.6% at night). In
contrast, they grazed 78.4% d~^- (53-5% d~l during daytime and
24.9% d~l at night) of the nannoplankton standing crop. These
figures illustrate how the zooplankton community may take over
100% of the productivity, but less than 100% of the standing
crop. More importantly, at these exceptionally high grazing
rates, it is obvious that the nannoplankton would be depleted
below threshold grazing levels within a few days. As we will
see later, during October, 1975 the nannoplankton was nearly
depleted (only 56.9% mg Cm~3) when observed.
During October 1974, the netplankton were not as severely
grazed as the nannoplankton. Calanoid copepodites were the
principle active grazers, exceeding the harvest of netplankton
accomplished by all other forms. Calanoid copepodites harvested
45.2% of the netplankton productivity (13-1% during daytime and
32.1% at night). Epischura (9-8%) and Holopedium (10.4%) were
important only during daylight hours. The zooplankton thus
grazed 94.9% of the netplankton productivity (52.5% in daylight
and 42.7% at night). Thus, from a productivity viewpoint, they
were not quite in control of netplankton standing crops. In
31
-------
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s o
< CQ
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fact, they grazed 70.7$ of the standing crop (38.!% d-1 during
daylight and 32.6% d~l at night). When standing crops of phyto-
plankton are small, grazing 100% of daily productivity may be
very significant.
The cruise of 12 April 1975 presented an opportunity to
observe grazing at low temperatures (6.3ฐC). Copepods were the
dominant herbivores (88$ or 37342 of 42384 m~3). Nauplii, while
abundant, were characterized by a low filtering rate during day-
time and grazed only 1.4% of the netplankton C fixed (Table 11).
The copepodites of Diaptomus exhibited the highest filtering
rates and grazed 12.1$ of netplankton C fixed (7-8% during day-
time and 4.3% at night). Cyclopoid copepodites were the second
most important herbivore, consuming 11% of the netplankton C
fixed (5.6$ at night and 4.4$ during the daytime). Thus, the
crustacean community grazed 30.1$ of the netplankton carbon
fixed in April, and were characterized by a daytime grazing rate
of 6.6$ d"-1- and a nighttime rate of 5-0$ d~l. In contrast, this
same zooplankton community grazed 446$ of the nannoplankton car-
bon fixed (Table 10), and had a daytime grazing rate (17.8% d"1)
about equal to the nighttime rate (18,5$ d"1) on nannoplankton.
Obviously rates of carbon fixation for netplankton were low
relative to the standing crop of netplankton carbon. The cope-
podites of Cyclops grazed 146% of nannoplankton production
during the day and 109$ during darkness (Table 10). Chydorus
sphaericus grazed 28$ and 50.8$ of the primary production of
small cells, whereas the copepodites of Diaptomus took 15.3$ and
59-3$ of the primary production during daylight and darkness.
Obviously the nannoplankton was under heavy grazing pressure.
This pressure was predominately from small grazers, chiefly
copepodites of Cyclops and Diaptomus, and Chydorus sphaericus.
The netplankton was not grazed to such an intense degree.
These community foraging patterns thus characterize the classi-
cal synergistic effect of selective grazing, i.e., the enhance-
ment of netplankton production by herbivores selectively grazing
small cells (Tables 10-11).
One month later (5-9 May 1975) these same herbivores, in-
cluding immature calanoids, were responsible for most grazing
(Table 12). Nannoplankton was abundant (160.6 mg Cm~3) but
netplankton was not detected. The grazing rate on nannoplankton
was greatly reduced; only 1.6$ of the standing crop was grazed
during daylight and 2.2$ at night (Table 12). The percentage
of nannoplankton carbon fixed which was grazed was significant
(30.3$ during daylight and 42.3$ at night). Thus a total of
72.6$ of the primary productivity of the previous day was re-
moved by the herbivores; the standing crop could still double
in 4 days. Most of the grazing was done by immature copepods;
nauplii (13383 m~3) and adult Diaptomus (1637 m-3) accounted for
most (88%) of the productivity removed.
41
-------
By the end of May (28 May-2 June 1975), with warmer waters
(14C), the grazing responsibility had shifted from the copepods
to the cladocerans. Both the grazing rate and percentage of
nannoplankton primary productivity grazed had increased (Table
13). The herbivores grazed 512.9% of the primary production
during the day and 176.2% at night, for a total of 689.1%. The
grazing rate or percentage of the standing crop grazed was also
significant; 38.4% d"1 during daylight and 13.2 d"1 at night, or
51.6% d-1 of all available small phytoplankton. Eubosmina
coregoni and Bosmina longirostrls did most of the grazing (70.4%
followed distantly by copepodites of Cyclops). Pressures on
netplankton were much lower (Table 14).
Diversity of herbivores was greatest on 5 August 1975.
Twelve species of cladocerans accounted for most (77%) of the
grazing. The grazing rate on nannoplankton (relative volume
water filtered or percentage of available food consumed) in-
creased from the June level (Table 15). Of the available stand-
Ing crop of nannoplankton, 10.9% d"1 and 28.8% d"1 were removed
during daylight and darkness, for a total grazing rate of 39-7%
d"1. More significantly, 782% of the nannoplankton productivity
was grazed during daylight, versus 2116% at night. Thus, on 5
August, when the netplankton was present in large quantities
(74 mg Cm~3)3 the grazing pressure upon nannoplankton was in-
tense. We again emphasize that this selective pressure may have
a synergistic effect in stimulating production of netplankton,
since its presence as food is relatively low. In sharp con-
trast to the grazing upon nannoplankton, only 7-8 and 5-4% d
of netplankton standing crop was harvested (Table 16) for a
total grazing rate of 13.2% d"1. Only 0.34% and 383.9% of the
netplankton carbon fixed was grazed (Grazing Impact) during day
and night, respectively (Table 16).
By 19 October 1975, grazing as a control upon algal growth
had become Insignificant. The diversity of grazers remained
high, but the mean community filtering rate on nannoplankton
had dropped to 0.05 ml an"1 hr-1 (Table 17), from a high of
0.23 on 12 April (Table 10). Only Holopedium continued to fil-
ter at a high rate (0.21). Only 0.1% d-1 and 0.7% d-1 of the
standing crop of nannoplankton was harvested during daytime and
nighttime, respectively. In a similar respect, only 2.6% and
3.6% of the productivity of nannoplankton was grazed. In com-
parison, 0.1% and 0.7%, respectively, of the netplankton was
grazed during daylight and darkness. Thus, in October 19755
with large populations of grazers, the nannoplankton had
escaped from control by grazing. Obviously the late fall and
winter period represent a time of low productivity and little
zooplankton foraging. Nannoplankton populations have time to
recover from intense summer grazing pressures, but under condi-
tions of cold temperatures, reduced light, and slow growth
rates.
42
-------
Summary of Grazing by Taxonomic Grouping
The impact by order of the major crustacean grazers (Clado-
cera, Calanoida, and Cyclopoida) and unidentified immatures
(nauplii) is most obvious if grazing by order is ranked for
each of six intensive studies (Table 18). Considered are the
percentage of nanno- and netplankton net productivity which was
grazed each 24 hours. In addition, where one species or imma-
ture stage was dominant, its grazing impact is noted in paren-
theses.
During October 1974, total grazing on nannoplankton ex-
ceeded production, with 232$ (158.4 + 73.7%) of the daily pro-
duction grazed. The copepodites of calanoid copepods accounted
for most of this; the major grazing on nannoplankton occurred
during daylight hours. Netplankton production exceeded grazing.
Approximately 95% of the netplankton productivity was grazed,
again chiefly by calanoid copepodites.
Cyclopoid copepodites were dominant grazers in both April
and May 1975. In April, cyclopoid copepodites grazed 255$ of
the nannoplankton productivity. A cladoceran, Chydorus
sphaericus, was second in importance, removing 80$. NeTtplankton
was lightly grazed in April, with only 28.7% removed, mostly by
calanoid copepodites. Calanoid copepodites are typically larger
than those of the cyclopoids and are possibly better at handling
the larger netplankton. Little grazing occurred during May;
the large netplankton diatoms were not present. Nauplii removed
most (43%) of the 92% of the nannoplankton productivity grazed.
Thus, during April and May, the grazing impact fell upon the
nannoplankton, and small crustaceans (copepodites and nauplii)
were implicated. Any grazing of netplankton which occurred
(28.7% of productivity removed in April) was done by calanoid
copepodites. Adult calanoids were not significantly involved.
During June and August 1975 (Table 18), the cladocerans
were the important grazers. This was expected, since these
crustaceans dominated or thrived during the warmer months. Dur-
ing June, Eubosmina coregoni, the larger of the two Bosmina
species, took 339% of the nannoplankton productivity of the
total of 629% grazed. Thus, the nannoplankton were again
greatly overgrazed in June. In contrast, only 28.2% of the
netplankton was controlled by the zooplankton. Most of the
grazing on netplankton was done by cyclopoid copepodites and oc-
curred during darkness.
Cladocerans removed 2107% of the total 2839% of nannoplank-
ton productivity grazed during August 1975. Holopedium gibberum
(520%) was especially important, grazing 362% of the previous
day's productivity each night, as was Eubosmina coregoni^ (592%),
which was also especially active at night(459%) Together
Holopedium and Eubosmina combined to graze 152% of the netplank-
43
-------
TABLE 18. PERCENTAGE OF NET PRIMARY PRODUCTIVITY GRAZED WITHIN
SIZE GROUPS AND BY DAY AND NIGHT
Date Rank, Order and Dominant Species Daytime Nl#ittlne
Nanno Net Nanno Net
October 1971 1.
2.
3.
April 1975 1.
2.
3.
5-9 May 1975 1.
2.
3.
1.
1 June 1975 1.
2.
3.
5 August 1975 1.
2.
3.
H.
19 October 1975 1.
2.
3.
MEANS, ALL MONTHS
Calanolda
(Calanold cops)
Cladocera
(Holopedium glbberum)
Cyclopoida
TOTALS
Cyclopoida
(Cyclopold cops)
Calanolda
(Calanold cops)
Cladocera
(Cnvdorus sphaericus )
TOTALS
Nauplll (mixed)
Cyclopoida
(Cyclopold cops)
Calanolda
Cladocera
TOTALS
Cladocera
Eubosmlna coregpni
Cyclopoida
Cyclopoid cops
Calanoida
TOTALS
Cladocera
(Holopedium glbberum)
(Eubosmlna coregonl)
Cyclopoida
(Cyclopoid cops)
Mixed nauplii
Calanoida
(Calanoid cops)
TOTALS
Cladocera
Cyclopoida
Calanoida
TOTALS
1 . Cladocera
2. Cyclopoida
3. Calanolda
TOTAL OF MEANS
91.1
(61.3)
50.5
(36.6)
13.8
158.1
156.7
(116.1)
32.8
(15.3)
28.1
(28.1)
217.6
13.1
2.7
13.9
29.7
179.6
(286.1)
21.9
(23.9)
8.1
512.9
573.9
(158.5)
(132.9)
16.1
(29.fi)
81.8
26.8
731.9
1.0
0.7
0.8
2.5
188.9
10.9
29.5
259.3
31.1 39.3 32.5
(13.1) (22.6) (32.0)
15-9 20.7 7.1
(10.1)
1.9 13.7 2.9
52.2 73.7 12.8
5.6 117.6 6.0
- (109.1)
9.3 59.8 5.1
(59.3) -
0.9 50.8 1.8
(50.8)
15.8 228.2 12.9
29.7
23.3
(16.1)
9.1
62.1
17-3 87.5 21.1
(52.6) (12.5)
13.3 32.6
- (28.2)
15.0 3.7
17.3 115-8 57.1
0.3 1533-5 296.6
- (362.) (79.8)
- (159.) (72.5)
0 188.0 11.1
0 (120.)
0 219.0 16.8
0 166.8 26.1
- (107.1) (11.9)
0.3 2107.3 383.9
2.1
1.2
0.1
3-7
5.7 282.1 51.1
1.8 59-5 H.3
6.8 18.1 11.3
11.3 390.3 80.0
-------
ton productivity. The cyclopoids (especially their copepodites),
mixed nauplii, and calanoids were also important. August 1975
was characterized by large herbivorous zooplankton populations
which removed more algae each day than was produced. Thus,
standing crops were rapidly dropping in Lake Huron at that time,
due chiefly to biological control.
By October 1975, only small populations of nannoplankton
remained. These were little grazed (6.2% of productivity). The
nannoplankton thus entered the winter months at very low popu-
lation levels, due principally to the effect of the grazers
earlier In the summer, but without grazing stress under winter
conditions.
Mass Ingestion Rates
Mass ingestion rates provide an easily understood index to
the grazing effectiveness of various herbivores, since they
indicate the amount of algae ingested per unit body weight
(mgCd~l- mgC~^). The decimal fraction is thus identical to the
commonly expressed percentage of body weight per day (% wt.
d"-*-). For visual simplicity, the mass ingestion rates of clad-
ocerans, calanoids, and cyclopoids have been coded with symbols
as forms of circles, triangles, and squares, respectively (Fig-
ures 7-8). This coding enables the reader to discern the order
whose members have normally high or low rates. The species
codes are listed In Section 5-1.
Mass ingestion rates on nannoplankton (Table 19) In
Southern Lake Huron were high from April through August, but
fell abruptly In October. Typically, copepod nauplii and clado-
cerans were among the most efficient grazers on nannoplankton
(Figure 7)- In contrast, the adult calanoids were the least
efficient, particularly copepodites and DIaptomus adults. At a
maximum, nauplii ingested 0.5^ mgC of algae per day per mgC of
body tissue, or 5.4% of their weight. In August, In contrast,
during October, nauplii ingested only .007% of their body weight
In nannoplankton each day (Figure 7). One might presume that
high mass ingestion rates on nannoplankton from May through
August signified an abundant supply of food. Most likely during
August and September 1975 this resource was overgrazed, leading
to shortages in October. In October 197^, we had direct evi-
dence for the overgrazing of nannoplankton.
If the nannoplankton are characteristic of oligotrophic
lakes, it should be of interest to observe which species of
zooplankton are most effective in cropping them. Previously
it has been suggested that the calanoid copepods were the first
herbivorous residents of the Great Lakes (McNaught, 1975).
Thus, they should be good at grazing nannoplankton at low con-
centrations. But this does not imply that all stages consti-
-------
TABLE 19. MASS INGESTION RATES ON NANNOPLANKTON
Species
Yearly
Mean
Yearly
Range
Fast Ingestors (Herbivores)
Polyphemus pediculus
Mesocyclops edax (A)'
Nauplii(all copepods)
Chydorus sphaericus
Bosmina longirostris
Tropocyclops prasinus (A)
Cyclops bicuspidatus (A)
Daphnla galeata (A)
MEAN FAST ING.
Slow Ingestors (Omnivores)
Holopedium gibberum (A)
Cyclopoid copepodites
Eubpsmina coregoni
Cyclops '"vernal! s" (A)
CTmnocalanus macrurus (A)
Diaptomus spp~TA)
Holopedium gibberum (J)
Calanoid copepodites
MEAN SLOW ING.
Non-ingestors (Predators)
Leptodora kindtii
0.5
0.1x10
4.6 - 2
.007- 5
0.33
0.04
0.1
0.03
0.07
09xlO-2
4
3.2
3.7
2.7
3.5
2.0
0.8xlO~2
0.7
0.6
0.6
0.5
0.3
0.3
0.2
0.1
0.03
0.06
0.03
0.01
0.2
0.03
- 1.4xlO-2
- 1.0
- 0.98
- 1.3
_
- 0.6
- 0.5
- 0.5
-i
*22.7xlO~2 mgCd-'mgC-1 is equivalent to 22.7% of body weight
d-1 .
-------
tute the largest biomass, and we have seen that calanoid copepo-
dites are the slowest of the slow ingestors (Figure 7). Calan-
oids are important with regard to total grazing in the lake (see
Grazing by Taxonomic Grouping) simply because of their high
numbers. It is the nauplii of both calanoid and cyclopoid cope-
pods which are the most effective as well as the most common
fast ingestors upon nannoplankton. The efficiency of this stage
may reflect the evolution of the copepods in oligotrophic en-
vironments where nannoplankton predominates.
Great Lakes phytoplankton communities can be characterized
by a predominance of small nannoplankton, which we have shown
to be greatly overgrazed (Tables, 10, 12, 13, 15, 17). It is
not surprising that the small specialized nauplii or earliest
immatures of copepods, which evolved in oligotrophic Great
Lakes, take large amounts of nannoplankton. What was surprising
is that cladocerans like Polyphemus, Chydorus, Bosmina longiros-
trls, and Daphnia galeata take 0.07 to 22.7% of their body
weight in nannoplankton each day (Figure 7)- Thus the warm
water cladocerans, which have become a dominant fraction of
Great Lakes communities in recent geological time, are the
herbivores which overgraze the nannoplankton resource (see
Grazing by Taxonomic Grouping), possibly synergizing, through
concomitant excretion of algal nutrients, the production of net-
plankton.
In general, zooplankton crustaceans graze lightly upon the
netplankton. Mass ingestion rates (yearly means) range from 0.1
to 1.3$ of body weight per day (Figure 8). The mean yearly in-
gestion rate of 0.3% d~l is only 10% of the mean yearly inges-
tion rate of netplankton (3.0$ d~l). Only Chydorus and Tropo-
cyclops ingested significant amounts (Table 20). Indeed
Chydorus may be a netplankton specialist, having been observed
as a sessile sailor floating upon colonies of blue-green algae
like Gloeotrlchla (Birge, 1922). Most forms, including the
abundant nauplii and copepodites, take less than 0.6% of their
body weight in netplankton each day. The rare Polyphemus, long
suspected of being a crustacean predator, but in reality an ef-
ficient specialist of nannoplankton, took almost no netplankton
(0.1% wt. d-1) (Figure 8).
Assimilation of Nannoplankton Foods
Nannoplankton (algae < 22 ym diameter) are clearly the food
resource most heavily grazed by the crustacean zooplankton in
Southern Lake Huron (Summary Table 18). These small algae are
also readily digested and assimilated, as shown for the dominant
members of the community on October, 1975 (Table 21). Eight
species and two Immature developmental stages (copepodites of
calanoids and cyclopoids) were investigated. A sharp difference
in relative assimilation of nannoplankton is noted. Most (6
47
-------
0.0001
,-1
Figure 7- Mass ingestlon rates (mgC plant day x mgC~x animai)
for dominant herbivores feeding on nannoplankton.
-------
I.Oh
"0
O
o
"5
E
c
o
x.
T3
2?
"en
0>
o>
c
o
c.
o
0.0001
0.01 h
0.001 h
Figure 8. Mass ingestion rates (mgC
for dominant herbivores feec
)lant Uil-y " mec -1 animal
-ng on netplankton. mal
-------
species and cyclopoid cops) assimilated over 75% of algal carbon
ingested (mean 93.2%, range 78.8 to 100%). These efficient as-
similators were sharply divided from those crustaceans who in-
gested moderate amounts of nannoplankton (Tables 10, 12, 13, 15,
17) but assimilated less than 30% (mean 11.7%, range 0.5 to
26.
High assimilators on nannoplankton Included Diaptomus
minutus, the Important calanoid during October 1975, and the
cyclopoids Cyclops bicuspldatus, cyclopoid copepodites, and
Tropoeyelops prasinus. Among the cladocerans, Eubosmina coregoni,
Daphnla retrocurva, and Bosmina longirostris were characterized
by high relative assimilation. On the other hand, Daphnla
galeata and Holopedium gibberurn were inefficient at assimilating
nannoplankton, whereas the calanoid copepodites assimilated al-
most nothing (Table 21).
Clearly the majority of the crustacean community is well
equipped to graze small nannoplankton. Filtration systems have
evolved that are efficient in the removal of small cells. In a
similar fashion, these herbivores have evolved enzyme systems
that efficiently digest these foods.
Assimilation of Netplankton Foods
Netplankton (algae > 22 ym diameter) are assimilated to a
lesser degree than the nannoplankton, just as they are less in-
tensely grazed (Table 22). Again a clear difference is noted,
with a small group of efficient assimilators (primarily cope-
podites) on netplankton, and a large group (adults of 8 species
and nauplii) which assimilates significantly less. The net-
plankton specialists assimilated over 60% of algal carbon in-
gested (mean 74%, range 60 to 83.3%). Their most dominant mem-
bers were calanoid and cyclopoid copepodites which are extremely
abundant in Lake Huron. Thus, two principal categories of im-
mature copepods are primary feeders on netplankton. Likewise,
Eubosmina and Cyclops bicuspldatus had high assimilation effi-
ciencies on netplankton.
More species had low assimilation efficiencies. Eight
species of cladocerans and copepods, plus the very abundant
copepod nauplii, assimilated less than 40% of the tagged foods
ingested (mean 19.3%, range 3.4 to 37-5$). Nauplii assimilated
only l8.2%; of all copepods, they are by far the most common
plankters of Lake Huron. Thus, we see that copepods are not
efficient at utilizing netplankton until they become copepodites
and then lose this proclivity when they become adults (see
Diaptomus, Epischura, Mesocyclops, Tropocyclops).
Biological Control Through Grazing
The control of populations of nuisance algae through en-
50
-------
TABLE 20. MASS INGESTION RATES ON NETPLANKTON
Species
Fast Ingestors (Herbivores
Chydorus sphaericus
Trop_ocyclops prasinus (A
MEAN FAST ING.
Slow Ingestors (Omnivores)
Nauplii (all copepods)
Cyclops bicuspidatus (A)
Daphnia galeata (A)
Bosmina longlrostris
Mesocyclops edax (A)
Daphnia retrocurva
Cyclopoid copepodites
Eubosmina coregoni
Cyclops vernalis (A)
Leptodora kindtii
Calanoid copepodites
Limnocalanus macrurus
Polyphemus pediculus
Diaptomus spp . (A)
MEAN SLOW ING.
MEAN ALL CRUSTACEANS
)
1
) 0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Yearly
Mean
.3xlO-2*
.9
.1
.6
.3
.3
.3
.3
.2
.2
.2
.2
.2
.2
.2
.1
.1
.24
.3
Yearly
Range
0.
0.
0.
0.
0.
0.
0.
0.
0.
7 _
7 -
1 -
1 -
-
-
06-
1 -
-
-
06-
02-
06-
2
1
0
0
0
0
0
0
0
.8xlO-2
.5xlO~2
.8
.4
.4
.3
.3
.3
.3
*1.3xlO~2 mgCd-'mgC-1 is
d-1.
equivalent to 1.3$
of
body
weight
-------
1
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53
-------
gineering methods (nutrient diversion) is expensive. One of the
principal objectives of this study was to discover the degree of
biological control in Southern Lake Huron exerted by each species
of zooplankton. This was expressed as the species grazing impact
(see Section 5.1, Eq. Ill), and was discussed extensively and
summarized in Tables 10-17. If the species grazing impacts are
summed for all species, we have the community grazing impact
which can be expressed as either the percentage of nanno- or
netplankton primary productivity (mgCm~3 d"-^-) grazed by the zoo-
plankton (mgCm~3 d"1).
ฃ algal carbon grazed
IV) Community Grazing Impact=Z Species G.I.=Z algal carbon fixed
For each cruise on which selective grazing was determined, we
synthesized as part of the analyses an estimate of community
Grazing Impact (G.I.). These community G.I. for the period
October 1974-October 1975 are presented in Figure 9.
Nannoplankton and netplankton populations In Southern Lake
Huron were under the control of grazers from mid-May through
mid-October (Figure 9). Unfortunately, limnologists do not
have ready access to Southern Lake Huron during February and
March when phytoplankton populations are beginning to grow more
rapidly, leading to the pulse of diatoms in the spring. In
April 1975, the phytoplankton community was dominated by the
large diatoms Tabellaria fenestrata, Fragilaria crotonensis,
and Synedra fIliformis, and the nannoplanktons Cryptomonas
ovata and Rhodomonas minuta (pers. comm., E. Stoermer). These
large netplankton diatoms are not readily grazed, and the com-
munity grazing impact (C.G.I.) was 0.3 for netplankton. In
early May, neither the nannoplankton (0.2) nor the netplankton
(1.0) were growing much more rapidly than they were grazed.
There was essentially an equilibrium between productivity and
grazing (Figure 9)- From early June through early August, the
phytoplankton community was under the control of the grazers.
The C.G.I, ranged from 6.2 to 25-3 for nannoplankton, reflect-
ing the extreme grazing pressures and tight control over the
growth of these small algae. Likewise, but to a much lesser
degree, the sometimes noxious netplankton were under control
during this time, with the C.G.I, ranging from 1.5 to 3.4 (Fig-
ure 9). Thus, the phytoplankton communities of Lake Huron are
under the control of grazers throughout the growing season.
This apparently healthy state must be maintained.
-------
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CL
UJ
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or
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55
-------
5.3 EXPERIMENTAL MANIPULATION OF POOD CHARACTERISTICS
Hypothesis and Significance
Copepods and cladocerans found In Southern Lake Huron have
evolved in freshwater environments for over 12,000 years. The
early day Laurentian Great Lakes were extremely oligotrophic,
with phytoplankton populations dominated by small centric dia-
toms like Cyclotella, still prevalent in Lake Superior (Schelske
et al. , 1972). Calanoid copepods, which were the earliest In-
habitants of these environments, may have fed upon such small
diatoms, although both their feeding habits and food populations
have evolved since (McNaught, 1975). Thus, the likelihood of
size-selective grazing by extant forms is great. In more recent
times, especially the last 40 years (McNaught and Buzzard,
1973)j the cladocerans, which are typically warm-water inhabi-
tants, have become abundant in the shoreward regions of Lake
Huron and may constitute up to 85% of inshore populations. We
have pictured them as generalists with regard to diet (they eat
more blue-greens) and particularly with regard to size-selective
feeding (they consume a broader range of sizes). Thus, the
cladocerans are marked generalists, especially when contrasted
with the highly specialized calanoids.
The purpose of these experiments was to define the mechan-
isms of selective feeding. Most likely the underlying factors
are both quantifiable and interrelated. Size-selective feeding
alone is too simple an explanation to explain the niche struc-
ture of the pelagic zooplankton; taste and shape of foods and
other factors must be considered.
Understanding selective feeding is important in describing
the behavior of herbivores at the organismic level, but is im-
perative to define their combined impact on phytoplankton popu-
lations at the ecosystems level. Selective feeding also implies
that certain potential food resources are ignored. Under-
utilized foods may grow unchecked into noxious nuisance blooms.
Such systems selection on algal populations is likely based
both on selective grazing (inhibition) and nutrient reminerali-
zation (stimulation) of algae by zooplankton.
Selective Cropping of Paired Resources
Zooplankton graze at varying rates on different food re-
sources. The initial analysis of these experiments consisted
of calculating zooplankton filtering rates (ml an~^-hr~^) upon
paired algal resources, wherein one of the 6 pairs was a
sheathed blue-green and the other an unsheathed green. The
sheathed blue-greens Anacystis nidulans and Gloeocapsa sp. were
paired with the unsheathed greens Scenedesmus quadricauda,
Ankistrodesmus falcatus, and Pediastrum sp. Since the experi-
56
-------
ments were done In replicate, the results were amenable to an
unpaired t-test; some samples yielded more subreplicates than
others. Results showed whether zooplankton filtering rates
were significantly higher on one algal species than another.
However, these results did not indicate whether one food was
selected or another rejected. This second analysis was accom-
plished using an electivity Index. Those decisive characteris-
tics (size and sheath) were further examined with a 2x2 fac-
torial .
Most crustacean zooplankters were not highly selective
feeders when presented with paired resources made up of 5 common
algae. This generalization is based on the lack of selection of
food by 5 dominant species (adult stage) and 2 immature stages
(nauplii and cyclopoid copepodites) common enough to analyze
without low numbers a priori indicating insignificance. Three
common adult cyclopoids (Cyclops bicuspidatus, C_. vernalis, and
Tropoeyelops prasinus) were not selective upon any of the 6
paired foods (Table 23). Likewise, the early immature copepod
nauplii and the cladoceran, Eubosmina coregoni, were not selec-
tive. Thus, the predominant conclusion is that food selectivity
Is not a characteristic of these Important herbivores; they took
both sheathed and unsheathed blue-greens and greens of a variety
of sizes. However, two dominant forms (adult Diaptomus sicllis
and cyclopoid copepodites) were highly selective and deserve
further analysis.
First we will analyze the original filtering rates of
Diaptomus and the cyclopoid copepodites, since each showed dif-
ferent ingestion of the paired resources. Diaptomus selected
the small sheathed Gloeocapsa over the large plain Pediastrum,
(Table 23) with respective filtering rates of 0.08? vs. 0.043
ml an~lhr~l. SImlliarly, the cyclopoid copepodites selected
Gloeocapsa over Scenedesmus (filtering rates of 0.057 vs. 0.010
ml an-lhr-1. Thus, the small sheathed blue-green was always
selected. In contrast, the greens were always selected when
paired with the large sheathed blue-green Anacystis. Diaptomus
selected the spined Scenedesmus over Anacystis (filtering rates
of 0.021 vs. 0.009 ml an~-Lhr~^-) as well as Pediastrum (filtering
rates of 0.092 vs. 0.015 ml an-1hr-l). Thus, the large sheathed
Anacystis was never selected. Now we can determine whether
these different Ingestion rates were due to active selection or
simply the Ignorance of specific foods.
Electivity Matrix of Resource Characteristics
For a better understanding of the functioning and later
modeling of an ecosystem, it is desirable to identify those
resource characteristics selected by herbivorous zooplankton.
Electivities upon the 5 phytoplankton species presented in 6
pairs showed generally a selection for green algae when paired
57
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with a large blue-green and for a very small blue-green when
paired with a variety of greens (Table 23). Thus, neither the
greens nor the blue-greens were selected consistently, nor were
the smaller species always preferred. Since the five resources
had other characteristics than just taxon and size, such proper
ties deserve further attention.
Both blue-greens (Gloeocapsa and Anacystis) have a sheath
or gelatinous matrix. The species of Scenedesmus (quadricauda)
was selected for its unusually long spines. Ankistrodesmus ,
usually found in fasicles of long crescent-shaped cells, was
considered unspined and clearly unsheathed. Pediastrum, a
large colony of cells, was considered to have such short lobes
that it was emarginate or unspined. Thus, experimentally de-
termined electivities upon these various resources can be
placed in a matrix where size (large vs. small) can be paired
with the presence of spines or the presence or absence of a
sheath (Table 24). Ivlev's (1955) electivity index(E) was
employed, where:
E = (R-P)/(R+P) and wherein
R = proportion of resources taken
P = proportion of total resources available
The resource characteristics matrix (Tables 24-25a) is
useful in determining the degree to which size, sheath, or
spines influence electivity. By summing the rows we obtain the
mean electivity by size, and by summing the columns, the elec-
tivity for spined and sheathed cells. These electivities were
then ranked (Tables 2^.-25b). Diaptomus sicllls, a large
omnivorous calanoid copepod, selected large plain green algae
(Pediastrum) in proportions greater than offered (E = 0.20).
' sicilis took small Ankistrodesmus and the small sheathed
.
blue-green (Gloeocapsa') in nearly equivalent numbers to those
present (E = -0.006 and -0.05)- This numerous large herbivore
rejected small spined greens (Scenedesmus , E = -0.2) and large
sheathed blue-greens (Anacystis^ E = -0.4). Selection (E > 0)
can be differentiated from ingestion of what is available (E =
0) and ignorance or lack of utilization (E < 0). But the dif-
ference between ignoring and actively rejecting a resource is
not evident. Clearly Diaptomus (adults) reacted to food size
as well as the presence or absence of sheaths and spines in
feeding selectively.
The resource characteristics matrix (Table 25a) for
cyclopoid copepodites was similar to that for Diaptomus . The
highest electivity occurred when a large plain cell (Pediastrum)
was offered with a small sheathed cell (Gloeocapsa) . In ranked
order of preference, these numerous immature copepods selected
59
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61
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large plain and small plain cells and ignored the spined and
sheathed cells to differing degrees (Table 23b). Large sheathed
cells were taken less frequently than small sheathed cells, and
spined cells were taken the least. In fact, by number they con-
stituted 50% of the food offered, but only 15.1% of the ingested
amount when paired with Gloepcapsa. This analysis had one fault
in that size and sheathness were not examined independently.
Effect of Sheath and Size Upon Ingestion
A 2x2 factorial (parametric) was employed to discover
whether the effect of the sheath or relative size was indepen-
dently the most important factor influencing selection of food.
Filtering rates for Diaptomus and cyclopoid copepodites on
paired resources, previously shown as being significantly differ-
ent (Table 24), were cast into a 2x2 factorial to test the ef-
fect of size versus that of sheath. The presence or absence of
a sheath was the overwhelming factor of importance determining
whether a resource was ingested or ignored, both in the case
of Diaptomus (***, P < .001) and cyclopoid copepodites (***,
P < .001); it is noteworthy that the interaction was not signif-
icant (Table 26). Thus, in modeling food electivity, we can
rank the sheath and associated ramifications of taste and inhi-
bitory substances combined with it, above size as a matter of
critical importance. Two herbivores which were highly selective
ignored sheathed forms and selected large plain resources. From
the opposing viewpoint of resource fitness, it is best to be
large and sheathed, and this is precisely the direction in
which blue-green algae which form blooms, like Anacystis, have
evolved.
Utilization of Electivities in Modeling Grazing
Modelers should utilize electivities on specific phyto-
plankters in modeling zooplankton grazing. In addition to the
filtration rates characterizing specific herbivores, electivity
terms may be used to estimate selectivity on individual re-
sources. Electivity by fishes on zooplankton was employed to
estimate selective importance of size.and simulate fish preda-
tion (McNaught and Scavia, 1976). These first estimates of
electivity for zooplankton grazing on common phytoplankton
should be immediately applicable to modeling grazing in Lake
Huron and other unproductive lakes where these phytoplankters
occur.
Purpose of Experiments with Beads
The size range of food particles and their selective har-
vest by herbivores remains of critical importance to under-
standing the eutrophication of aquatic ecosystems; lake aging
is usually accompanied by successional changes in algal popula-
62
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tions, with larger forms assuming dominance. Experimentally one
can provide a uniform food source which varies only by size and
not in shape or taste. Polystrene beads were selected because
they are spherical in shape and uniform in chemical composition.
Such beads were added in small numbers to natural populations of
algae; the beads were of a known size-distribution and later
were readily identified in the guts of preserved zooplankton
(see Section 4.3, Methods).
Results of Size Selection Experiments with Beads
Copepod nauplii
Copepod nauplii were the most abundant grazers in Southern
Lake Huron. When presented with a natural population of phyto-
plankton spiked with beads, which were most numerous in abun-
dance at a diameter of 3 ym (Figure 10, top), they selected
beads approximately 1 ym in diameter (Figure 10, middle). These
small beads were relatively rare in the bead population, as
evidenced by an Ivlev electivity (E) of greater than 6 (Figure
10, bottom). Beads of a diameter of 2 ym were taken in equiva-
lent numbers to those offered (E = 20). Thus, nauplii, the
smallest of the herbivores, were highly size-selective feeders
when presented a population of small, spherical, uniform beads
which were rare in a zooplankton of natural phytoplankton.
The beads were in effect a non-radioactive tag.
Copepod copepodites
Copepod copepodites (DC) are the second most abundant
group of grazers (Section 5.4; Appendix). When presented beads
under the same experimental conditions as above, they selected
beads of approximately 0.5 ym diameter (Figure 10, middle).
Thus, the two most abundant herbivores in Southern Lake Huron
are highly size-selective; the Ivlev electivity index for cope-
podites selecting such small 0.5 ym particles was 5-5 (Figure
10, bottom), but dropped to 0 for particles of 2 ym diameter,
indicating no selection. Thus, copepodites relied upon food
particles less than 2 ym diameter, most likely small algae and
bacteria.
Diaptomus sicilis (adults)
Adult calanoid copepods are most likely omnivorous, since
they consume an average of 4% of their body weight per day
(Section 5.1). It has been our hypothesis that adult calanoids
are adapted to feeding on small phytoplankters characteristic
of the waters in which they live. D. sicilis took beads very
small (< 1 y) which was surprising given its large size (1.2 to
1.9 mm), but certainly not its natural spectrum of small foods.
Ollgotropbic lakes commonly contain populations of nannoplankton
less than 20 ym diameter (McNaught, 1975). The adults also feed
on the peak frequency of food items (3 ym). This is evidence
that "tracking" (Richman e_t_ al. 1977) of the predominant size
64
-------
c
o
en
o
Q.
E
So
Q>
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ct:
0
6
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Available
-DA
0' "r-^.
0
10
Diameter (jit)
Figure 10. Selective ingestion of beads by Diaptomus sicIlls
adults (DA), copepodites (DC), and nauplii (N)". Composition
of gut (middle) relative to bead population available (upper);
selectivity expressed as Ivlev's index (lower).
-------
particle may have played a part in their feeding behavior (Fig-
ure 10, middle).
Cyclops bicuspidatus (adults)
Adult cyclopoid copepods are also omnivorous and consume
an average of 5% of their body weight per day (Section 5.1).
When Cyclops adults were presented with a natural population of
phytoplankton spiked with beads, they took the smallest beads of
0.5 y diameter (Figure 11, middle), with maximum electivity (E =
1) at that diameter (Figure 11, bottom).
Thus, size selection for small foods (0-5 ym), in a food
spectrum that is exceptionally broad (0-125 ym), was character-
istic of these dominant herbivores in this oligotrophic eco-
system. This conclusion was reached using an experimental food
(beads) which varied in size but not in shape or taste. These
experiments were conducted in the field with animals acclimated
to their natural measure of the size preference of these species
and life stages.
5.4 HORIZONTAL DISTRIBUTIONS OF DOMINANT ORGANISMS
Description of Distribution of Dominant Species
Distributions of the dominant species of crustaceans are
presented; since a large amount of distributional data has been
collected and is available, only the distributions for the month
of maximum abundance for each life stage or species has been
presented. Detailed summaries are found in the Appendix (Sec-
tion A) .
In examining these distributions, the influences of the
physical limnology of Southern Lake Huron are evident. First,
surface waters enter Saginaw Bay from the northeast, circulate
in a counter clockwise fashion, and spill out Into the main lake
around the thumb (Pointe aux Barques). This dominant pattern of
circulation, in addition to the highly eutrophic nature of
Saginaw Bay, influences all of our distributions. Secondly,
the calanoids tend to be found In offshore waters, whereas the
cladocerans grow best In the warmer inshore areas. These dif-
ferences are evident and must be considered in utilizing these
groups as pollution indicators.
Calanoid copepods
The Calanoids are large, herbivorous or carnivorous cope-
pods common to cold, well oxygenated, and generally oligotrophic
waters. They commonly have been referred to as clean-water
indicators. Usually the nauplii and copepodites are herbivor-
ous (Section 5.1), while the adults become carnivorous. In the
Great Lakes, the adult calanoids of the genus Diaptomus are
66
-------
-^ 1
CD
or
0
0-
CA
_L
0
5
Diameter
10
Fieure 11.
Selective Ingestion of beads by Cyclops bicuspidatus
(adults). For description see Fig. 10.
67
-------
highly sensitive to predation. Thus, where predation, especial-
ly by the alewife Alosa pseudoharengus, is common, adult distri-
butions must be carefully interpreted. Since the nauplii, and
to some extent the copepodites, are free from predation, distri-
butions of these immatures may best indicate high quality waters.
Nauplii--
These small (40 ym) immatures of both calanoid and cyclo-
poid copepods were most abundant in the surface waters (0 to 5 m)
of the midlake region in June 1974. Here concentrations of
50j000 animals m~3 were common, with localized densities over
150,000 m~3 offshore from Goderich, Ontario (Figure 12). In
contrast to the open-water nursery grounds, the outer waters
of Saginaw Bay supported less than 40,000 m~3. This is indica-
tive of the low quality of these waters, especially since sta-
tions near Tawas, representative of the input of nauplii to
Saginaw Bay, had densities of over 100,000 m~3. it should be
noted that the above discussion applies to the nauplii of all
copepods. Nauplii are more abundant than copepodites and adults,
since significant early season mortalities occur.
Calanoid copepodites
Probably the best high quality environmental indicators,
the copepodites of Diaptomus, and to a small extent, Eplschura
and Scenecella, provided insight because they require clean
waters, yet are more likely to escape predation than their re-
spective adults. Again inshore waters leaving Saginaw Bay con-
tained less than 10,000 animals m~3 in October 1974. Whereas
the open waters had concentrations as high as 64,800 m~3} with
most locations represented by numbers greater than 40,000 m~3
(Figure 13) during October. As with the nauplii, an area of
low concentration was evident northeast of Port Huron, possibly
due to upwelling of deep waters in this area (Stations 60, 65).
Diaptomus minutus--
Adults (? and d*) were much less common than copepodites,
yet exhibited similar patterns of distribution (Figure 14).
Following the copepodite abundance in October, adults of D.
mlnutus were most common in November 19741 Inshore areas were
characterized by less than 2,000 m~3 in Saginaw Bay and along
the Michigan coast. Offshore areas and the Ontario coast were
represented by densities greater than 4,000 m~3j while a pocket
of maximum abundance was again evident off Goderich, Ontario
(Station 58) .
Diaptomus oregonens^s
Again adults (+ and cf) were most abundant during November
1974. Less than 750 m~3 were found in outer Saginaw Bay and
along the Michigan coast. The Ontario shores were character-
ized by densities greater than 1,000 m~3, while a pocket in
Southern Lake Huron (Stations 60, 64, 66) had numbers above
1,500 m-3 (Figure 15).
68
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Diaptomus sicilis--
This organism Is the original colonizer of the Great Lakes.
Today, Diaptomus sicilis is the most Important species in Lake
Superior, and most likely represents our best oligotrophic in-
dicator. In all areas, densities Increased from shore toward
open lake. Maximum numbers were found during November 197^ in
the midlake region, with an additional small pocket off Goderich,
Ontario (Figure 16). Frequent observation of high standing
crops near Goderich (Station 58) may point to the unique nature
of this area of the lake. Other investigators in the team have
spoken of high nutrient concentrations in this area (Stoermer,
1978), which may be related to increased primary production.
Cyclopoid copepods
The adult cyclopoids are predaceous and may reflect abun-
dances of prey organisms (immature copepods and cladocerans)
rather than clean waters. Thus, their distributions are pre-
sented with little or no inference at this time concerning their
use In applied water quality work.
Cyclopoid copepodites
In marked contrast to the calanoid copepodites, these im-
mature cyclopoids were most common In inshore waters in Novem-
ber, 197^. Densities as high as 35ป000 m~~3 were found in outer
Saginaw Bay, whereas the open water contained only 15-20,000 at
most stations (Figure 17). Thus, high densities probably re-
flect the high quantity of available food resources In more eu-
trophic waters. In contrast to the calanoids, these copepodites
are most abundant in the late fall.
Cyclops bicuspidatus
As this is by far the dominant Cyclopoid of Southern Lake
Huron, most of the above copepodites developed into C_. bicus-
pidatus . Yet with strong surface currents carrying copepodites
out of Saginaw Bay, maximum densities (6,000 m~3) of adults oc-
curred in inshore waters of Saginaw Bay in June, 197^. The
densities on the Tawas and Harbor Beach areas were similar,
indicating that adults may enter and leave the Bay with little
change in population size. A second pocket of abundance was
evident in the midlake area (Figure 18) and again above Goderich.
Tropocyclops prasinus--
This relatively rare cyclopoid was common along the Michi-
gan coast In the area of Harbor Beach (Figure 19).
Cladocera
All cladocerans appear to prefer warm, inshore waters.
They are generally characterized by high birth rates. These
high natal rates are maintained by heavy grazing on phytoplank-
ton which is characterized by high mass Ingestion rates (Section
5.1). Thus, distributions of the cladocera are vital to under-
standing Southern Lake Huron. They reflect, among other eco-
73
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system relationships, the areas of most intense grazing on
phytoplankton, and thus, areas that could escape from biological
control without such grazing.
Daphnia retrocurva
Daphnia was commonly the most important grazer (Section
5.1) in terms of mass ingestion rate (the amount of algae con-
sumed relative to its size). During August 1974, it was most
abundant in the inshore waters of outer Saginaw Bay (Figure 20).
It appears that its numbers decreased; 20,000 m~3 entered the
Bay off Tawas, but only 2 to 5,000 m~3 left the Bay in water-
masses that moved around the thumb of Michigan. These water-
masses may move offshore in Southern Lake Huron, carrying Daphnia
with them.
Eubosmina coregoni--
This relatively large bosminid (0.4 mm) was originally
thought to be an ollgotrophic form. In the context of current
literature, it is best characterized by its relatively high sus-
ceptibility to predation in comparison with its smaller congeni-
tor, Bosmina longirostris, which becomes dominant at high ale-
wife densities(McNaught and Scavia, 1976). Eubosmina coregoni
was most abundant inshore in outer Saginaw Bay in June 19747ft
occurred in colder waters (Figure 21) and earlier in the season
than B. longirostris.
Bosmina longirostris--
Eutrophic waters have been suggested where this classic
indicator occurs, although it is likely a better example of an
organism ideally adapted to avoid fish predation (McNaught and
Scavia, 1976). Abundances of 120,000 m~3 were recorded in the
outer waters of Saginaw Bay in June 1974 (Figure 22). As with
most crustaceans, their populations spilled over around Pointe
aux Barques into the main lake. Thus, the Bay is a major
spring producer of Bosmina, and its production in these waters
most likely influences the rest of the lake.
Holopedium gibberum--
Encased in a heavy gelatinous capsule, Holopedium floats
around the lake performing as one of the most effective grazers
controlling algal populations. It was slightly more abundant
offshore (large areas > 4,000 m~3) than immediately inshore
during August 1974 (Figure 23).
Chydorus sphaericus
As the smallest of the cladoceran grazers, Chydorus is un-
usually dense arid often requires an algal float upon which it
may rest and graze. It usually occurs concomitant with blue-
green blooms, arid it often exhibits high grazing rates while
resting on these bouyant algae (see netplankton grazing, Section
5.2). During June 1974, Chydorus was found only in Saginaw Bay
and the waters spilling around Pointe aux Barques into the
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Southern Main Lake (Figure 24). Certainly its distribution was
correlated with the production of netplankton in the Bay.
Planktonic Ratios: An Indication of Eutrophication
Introduction--
Calanoid copepods are the most abundant crustacean zoo-
plankton in the more oligotrophic waters of the Laurentian Great
Lakes. While adults of all species of Diaptomus are only moder-
ately abundant, their nauplii and copepodites constitute the
bulk of the plankton biomass in these unproductive and usually
unpolluted waters. Diaptomus sicilis, still the dominant organ-
ism in Lake Superior (Patalas 1969), is probably the original
colonizer of the Great Lakes. As the most oligotrophic form,
it is a useful indicator of high water quality. Trends in
changing species composition have been detected from scant his-
torical data available for crustacean populations in the Great
Lakes. In Lake Ontario, the calanoid copepods decreased between
1939 and 1972, with a concomitant increase in relative numbers
of the eutrophic cladoceran, Bosmina longirostris (McNaught and
Buzzard, 1973)- Two conclusions are warranted from this and
other similar observations: high ratios of cladocerans to
calanoid copepod densities are indicative of eutrophic waters,
while the crustacean Bosmina longirostris is the ultimate eu-
trophic organism now inhabiting the Great Lakes, with an ecology
in sharp contrast to the oligotrophic Diaptomus sicilis.
Use of Indicators--
To properly utilize an organism as an indicator of water
quality, its total ecology should be understood. Diaptomus
sicilis is here treated as an oligotrophic indicator. During
our Investigation of grazing in Southern Lake Huron (Sections
5.1, 5-2), we have discovered that it feeds primarily upon
small nannoplankton, themselves dominant In and indicative of
oligotrophic waters. Moreover, such calanoid populations are
characterized seasonally by an abundance of immature nauplii and
copepodites which feed almost entirely upon nannoplankton.
Adult calanoids are subject to heavy fish predation, so that a
lack of adults may be a sign of predation and not gross pollu-
tion. The Immatures, however, are relatively immune to preda-
tion and serve as excellent indicators of unproductive waters
of high quality.
At the other extreme, the cladocerans prefer warmer waters
and exhibit higher growth rates. We have recently found that
they consume the larger algae (greens and diatoms, but not
necessarily blue-greens), thus they have access to a larger
resource base in eutrophic waters. Being small at the age of
maturity, they reproduce before the minimum size at which ale-
wife predation becomes important (McNaught 1975). Since Bosmina
longirostris Is relatively predation-free, like the calanoid
copepodites previously mentioned, its densities should reflect
83
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the greater abundance of algal foods and the warmer temperatures
of shallow, more eutrophic waters. No species or group Is a
perfect indicator, but we feel strongly that immature calanoids
and Bosmina longirostris, two extreme forms at opposite ends of
the trophic spectrum, are among the most useful indicators of
pollution.
Evidence of Perturbed Areas--
During the summer of 197^, populations of Bosmina longir-
ostris predominated in outer Saginaw Bay (Seg. 5)5 the nearshore
waters of the Michigan coast (Seg. 4), and the offshore waters
of western Lake Huron (Seg. 6) adjacent to Saginaw Bay and the
forementioned coastal zone (Figure 25). Densities were lower
in these areas in the spring and fall than during summer. In
sharp contrast, B_. longirostris was found in low numbers in
northeastern offshore waters (Seg. 10), as well as in the most
southernly midlake watermass (Seg. 9). But most encouraging,
Bosmina was also rare along the Canadian coast (Segs. 7, 8),
indicative of either the lower temperature or the higher
quality of water in these areas contrasted to the Michigan
coast. In contrast, Eubosmina was most abundant in Segs. 5 and
6 which is indicative of its ability to survive faster offshore
(Figure 26).
Cladocerans were dominant In Southern Lake Huron during the
summer months of 197^, whereas early in the spring they were
common only in Saginaw Bay (Seg. 5) with densities as high as
77*000 m~3 (Figure 27). However, it is evident that the group
has a different distribution during summertime than Bosmina.
Total cladocerans were common along the eastern coastline (Segs.
7, 8), an area of the lake essentially void of Bosmina. Gener-
ally, densities of cladocerans suggest water quality problems
In all areas except that northern, open lake, deep water area
(Seg. 10) which never developed a significant population of
these eutrophic indicators. While not lending themselves to
identical conclusions, these two sets of data suggest water
quality problems in Saginaw Bay and along the Michigan coast,
as well as high quality water in the northern offshore water-
masses (Seg. 10).
The large, brackish-water calanoid, Eurytemora affinis, is
a recent invader of the Great Lakes ecosystem. Its distribu-
tion was examined with regard to areas of environmental stress,
since It may have a requirement for waters of high organic con-
tent. Likewise, since It was originally adapted to more saline
waters, it might be suspected of prefering polluted areas. Ob-
viously we do not know enough about the biology of Eurytemora
to carefully utilize it as an Indicator. It was most abundant
In oegs. 6 and 10 (Figure 28), as were total calanoid copepods
(Figure 4). This finding illustrates the need for basic infor-
mation on the nutritional, and reproductive, and growth aspects
of each zooplankton to be used as an indicator, especially when
85
-------
10
10
0
NO/M3
xlO4 5
16
14 41 II
\ ,
28
MEANffl-JiSDTDEV} Range
32
10
22
Sp S F
SEASON
10 r-
0
I0r-
796
T
_ I A i i
0
15 14 8
Figure 25. Mean abundance (with standard deviation and range)
of Bosmina longirostris in 7 segments of Southern Lake Huron
during the spring (Sp), summer (S), and Fall (F) of
86
-------
10
10
NO/M3
xlO4 5
0
28
MEANS-ฃSDT DEV > Range
32
J
10
22
, I rffl-. I
14
16
A
"Sp S F
SEASON
0
796
- I Xl _ I
10 i-
0
15 14 8
Figure 26. Mean abundance of Eubosmina coregoni in 7
segments of Southern Lake Huron during 1974.
87
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32 MEAN ffl-PSDTDEVV Range
10
0
14
16
NO/M3
xlO4 5
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28
22
10
Sp S F
SEASON
3 6 NS
10 r-
I0r-
796
m I 37 I
0
15 14 8
Figure 27- Mean abundance of total cladocerans in Southern
Lake Huron during
-------
MEAN-EJHiSDT DEV> Range
796
I I I
15 14 8
Figure 28. Mean abundance of the exotic invader Eurytemora
affinis in Southern Lake Huron during
-------
decisions regarding water quality have great social importance.
Presently Eurytemora does not meet the criteria for such an in-
dicator .
Evidence for Areas of High Water Quality
It is vital to management objectives to identify those
areas of Southern Lake Huron presently exhibiting high water
quality. The presence of an eutrophic indicator may point to
trouble, but the absence of the same organism may not indicate
oligotrophic conditions. Calanoid copepods, and especially
their superabundant nauplii and copepodites, are indicative of
water of good quality, as opposed to cladocerans and cyclopoid
copepods (Figure 29). Adult calanoids and their abundant cope-
podites were found in greatest numbers during the spring in
Segs. 9 and 10, suspected areas of high quality water (Figure
30). They were also abundant in nearshore waters (Segs. 4, 7,
8) during the summertime. At these same times, calancids were
rare in Saginaw Bay (Seg. 5). Such populations developed in
the spring, in shallow water, while these nearshore areas were
cold and contained fewer of the large, eutrophic types of phyto-
plankton. By July, however, calanoids were excluded from in-
shore (Figure 30). Thus, the distribution of calalnoid copepods,
as contrasted to cyclopoids (Figure 29) and cladocerans (Figure
27), suggests again that the highest quality waters in Southern
Lake Huron are in Sections 9 and 10.
Information on the ratio of calanoids to cyclopoids and
cladocerans (Gannon, 1977) provided us with a final overview of
water quality. Due to the construction of this index, normal
statistics may not apply, but comparative monthly means of this
ratio (Figure 31) suggested that the deep waters of the most
northerly area (Seg. 10) were of highest quality. The waters
of outer Saginaw Bay (Seg. 5) and the nearshore waters of both
the east and west coasts (Segs. 4, 7, 8) exhibited very similar-
ratios, which may reach a value of 2 during April and May when
cold water calanoids predominate, but usually did not exceed
unity.
In summary, all 5 indices suggested the same relative de-
gree of eutrophication In the 7 areas of Lake Huron analyzed.
Saginaw Bay was the most eutrophic, and Seg. 10 was the least
productive. Generally the western coast of Lake Huron (Seg. 4)
was more eutrophic than the eastern coast (Segs. 7, 8).
Evidence for Historical Trends
Comparable data collected during 1971 and 1974 by CCIW in
the main lake (apart from Georgian Bay) have been compared with
regard to variations between years. The two most diagnostic
groupings from our viewpoint, Bosmina longlrostris alone and
total calanoid copepodites and adults, have been used, and are
characterized by both the seasonal mean and the standard devia-
tion (Figure 32). Essentially, there was no evidence for in-
90
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10
NO/M3
xlO4 5
32
MEANffl-}iSDT DEV> Range
\.L> )
10
22
28
I0r-
0
Sp S F
SEASON
10 r-
7 9
X
tTTn. I Cffh I
0
Figure 29. Wean abundance of cyclopoid copepods in Southern
Lake Huron during
91
-------
10
10
NO/M3
xlO4 5
28
sal
MEAN-BHiSDT DEW > Range
32
22
10
14 16 II
Sp S F
SEASON
9 6
101-
15 14
Figure 30. Mean abundance of oligotrophic calanoid copepods
(predominantly copepodites) in Southern Lake Huron during
92
-------
51-
A MJ J JuA 0 N
Month
5r-
5r~
I I I
Figure 31. The ratio of calanoids to cyclopoids (adults and
cops) plus cladocerans for the months April through
October 197^ in Southern Lake Huron. The higher
values are indicative of relatively oligotrophic
conditions commonly found in segment 10.
93
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creased production of zooplankton during this brief period of 3
years between 1971 and 1974. In fact, in all cases the 1974
means were less than those for 1971. Bosmlna evidently began
growing later in 1974 (effect of weather?), and the seasonal
mean was only 1.004 x that for 1971. Bosmina, however, charac-
teristically has a very patchy distribution, as evidenced by
large standard deviations. The calanoid copepods were less
abundant in 1974 than in 1971ป but in all cases the comparative-
ly narrow standard deviations overlapped, even though calanoid
standing crops in 1971 were 2.6 x more abundant than in 1974.
Certainly long-term historical trends have occurred over
the 12,000 year history of Lake Huron, but we are not aware of
data sets over a 50-100 year span to document such changes.
5.5 ESTIMATES OF COLUMN BIOMASS OF ZOOPLANKTON
Dry-Weights of Animals from Net Samples
The purpose of net sampling was to determine the horizontal
and vertical distribution of the zooplankton by species over
time. Precise estimates of dry-weight combined with densities
determined in the field provided a crude estimate of the hori-
zontal distribution of biomass by species. In contrast, the
acoustic sampling results provided a good estimate of biomass,
but cannot be used at the species level.
Q
Column dry-weight biomass (mg m ) was greatest In shallow
inshore waters during late April 1974 and range from a high of
0.8 to 5.1 mg m~2 in the mouth of Saglnaw Bay to 0.9 to 1.9 mg
m~2 off Goderich (Figure 33)- Midlake biomass was very low
(< 1 mg m~~2). This may be attributable to the low water temper-
atures (4.2 C) at this time. By 14-17 May 1974, column biomass
was still highest inshore, ranging from 2.8 to 5.8 mg m~^ in
outer Saginaw Bay to 2.4 to 3.8 mg m~2 off Goderich (Figure 34),
an increase of approximately 2 x over April.
By 4-8 June 1974, the column biomass of zooplankton had
increased to 5-50 mg m~2 jn the mouth of Saginaw Bay (Figure
35). Inshore waters of the Southern Lake were uniformly 10 mg
m~^ or greater; these Inshore standing crops were 5-fold greater
than those in the midlake region (2 mg m~2). Eight days later
(16-21 June 1974), the inshore waters were still characterized
by standing crops of 10 mg m , but the midlake waters now sup-
ported 10 to 25 mg m~~2} an(} the outer Saginaw Bay 10 to 36 mg
m-2 (pigure 36). Throughout Southern Lake Huron, column esti-
mates varied by a factor of 3 x (10 to 30 mg m~2) and total
zooplankton biomass was relatively evenly distributed (although
not by species ) .
95
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Inshore zooplankton biomass increased markedly by 17-20
July 1975. Inshore waters of outer Saginaw Bay previously
characterized by 10 to 30 mg m~2 in June had 20 to 100 mg m~2.
Biomass increased fourfold in inshore waters south of Goderich
(Stations 9, 10) from 20 to 40 mg m~2 (Figure 37). As evidenced
from an examination of counts (Appendix), deepwater biomass re-
mained the same or decreased slightly to 5 mg m~23 due to de-
creases in populations of copepod nauplii, Cyclops blcuspidatus,
and Chydorus sphaerlcus.
By 26-31 August 1975 (Figure 38), large populations of
zooplankton in the mouth of Saginaw Bay had crashed, falling
from a range of 20 to 100 mg m~2 to 10 to 45 mg m~2. Open water
values in midlake (Stations 53, 54, 55, 21, 23) were all below
10 mg m"2m However, the biomass In the extreme southern lake
(south of Station 60) had increased, as evidenced by the move-
ment offshore of the 20 mg m~2 isocline. Again a pocket of
high standing crop was noted at Station 9> south of Goderich.
This area is one where high phytoplankton productivity and
nutrient levels have often been observed (Stoermer, 1978).
Fall populations (8-12 October) of zooplankton exceeded 20
mg m~2 only in a narrow band of waters 16 to 24 km offshore in
the Southern Lake and in the deep waters of the mouth of Sagin-
aw Bay (Figure 39). Midlake biomass had dropped to 5 mg m~2 by
early October. By 10-14 November, the only waters to support
more than 20 mg m~2 were in the mouth of Saginaw Bay, off
Pointe aux Barques, and in the two areas of unusually high pro-
ductivity which are north (Station 57) and south (Station 9,
10) of Goderich, Canada (Figure 40).
Clearly, two areas of high zooplankton production existed
In Southern Lake Huron during 1974. Populations often exceeded
200 mg in"2 from 16 June to 14 October 1974 in the mouth of
Saginaw Bay. Likewise, limited Inshore populations exceeding
300 mg m~2 were common south of Goderich at that time. Cer-
tainly the Saginaw Bay populations were stimulated by abundant
phytoplankton and detritus, resulting from nutrient enrichment.
We have seen that this high secondary productivity may be car-
ried by currents around Pointe aux Barques and may circulate
further In the longshore currents. Similar observations are
evident from acoustical estimates (Section 5.5). The foodchain
dynamics off Goderich are based on hydrodynamic conditions
which are unclear. Certainly this is an area of turbid waters,
due to beach erosion and resuspension of detritus. The fre-
quency of upwelling in this area may bring up nutrients to en-
rich surface waters and stimulate primary productivity. In
any case, these inshore waters are often characterized by high
standing crops of zooplankton.
100
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Acoustic Estimates of Column Blomass
One objective during the Southern Lake Huron progi.
to make large scale estimates of particulate biomass, be
throughout the water column at each station, and over the
sampling end of 43 stations (see Maps, Figure 41-43).
Areal Estimates of Acoustical Biomass
Applied Great Lakes ecology requires the estimation of
secondary production with regard to the location, primary pro-
ductivity, and associated degree of nutrient enrichment of an
area. Acoustical estimates of particulate biomass may serve as
an integrated estimate of the standing stock of particulate
organic carbon (POC), and at least in shallow waters, of the
standing crop (in a column) of zooplankton within a prescribed
size-range. Routine surveillance studies of the Great Lakes
require that research vessels visit a prescribed series of
stations on a regular basis. The deployment of a specially
constructed particle counter has permitted us to make routine
particle surveys. Since resource management can best utilize
data collected over a broad geographic scale, we will present
monthly and seasonal summaries of column particle abundance
on an areal basis (mgC m~2).
Seasonal Summary of Acoustical Biomass
Acoustic estimates of total particle biomass for all
particle sizes exhibited two maxima in Southern Lake Huron,
one in early June and the other in August (Table 27). The
ready division of particles into size classes is one of the
major advantages of our custom-built sonar (McNaught et al.,
1975). For the purposes of this analysis of plankton, the
total particle spectrum was divided into three size classes,
with the smallest classes (0.8 to 2 mm and 2 to 3 mm) including
the zooplankton, and the largest class (3 to 5 mm) including
the remaining biomass. Acoustical returns converted to biomass,
and thus representative of the small size-class biomass (mgC
m~3), also show maxima in June (691.9 mgC m~3) and August (190.8
mgC m~3). From earlier detailed analysis of data from Lake
Ontario (McNaught, unpublished), we know that most of the parti-
cles in the appropriate range for zooplankton (.08 to 2 mm) are
indeed zooplankton if located within the upper 40 m of the water
column, but that other very significant echoes have been de-
tected from below 50 m, probably from aggregates of particulate
organic matter. Thus, while the mean acoustic biomass shows
seasonal patterns which, like the biomass estimates from net
sampling, are bimodal, these acoustic estimates are certainly
high if interpreted as representing only zooplankton. Acoustic
estimates (on a volume basis) have compared with biological net
sampling estimates from the same stations (Table 28) to provide
an idea of how often over estimation may have occurred (about
33$ of the time).
105
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June 1974 (0.8 to 2 mm particles)
Large densities of zooplankton and other particles were
frequently observed on sonar. Often they were associated with
longshore currents that were leaving Saginaw Bay around Pointe
aux Barques and flowing southeast past Harbor Beach. During
June 1974j the main concentrations of particulate biomass were
seen specifically in watermasses leaving Saginaw Bay and in the
deeper waters of Goderich (Figure 4l). Generally biomass levels
in Southern Lake Huron south of a line from Port Sanilac east to
the Canadian Shore were below 100 mgC m~2. However, in water-
masses leaving Saginaw Bay that exceeded 100 mgC m~^, consider-
ing that these are shallow waters average only 25 m depth, the
mean volume biomass can be estimated to exceed 4 mgC m~3. Most
of it was likely zooplankton. In the deep hole off Goderich,
the column biomass exceeded 100 mgC m~2, but the mean volume
biomass was only about 1.2 mgC m~3, since the mean depth is ap-
proximately 85 m. Most of the biomass off Goderich was in deep
water and was probably detrital in nature.
October 1974 (0.8 to 2 mm particles)
During October 1974 high particle densities were observed
in the mouth of Saginaw Bay (> 200 mgC m~2)} whereas in the main
lake densities above 100 mgC m~2 they were not observed south of
a line from Port Sanilac east to Canada. Generally the north-
erly stations in midlake and along the Canadian shore were not
sampled, due to changes in cruise scheduling, and this is indi-
cated on the map (Figure 42). Particle densities entering
Saginaw Bay in the longshore currents off East Tawas were below
100 mgC m~2? whereas those exiting Saginaw Bay around the thumb
were greater than 200 mgC m~2. One shallow station (Station 16)
reached 487 mgC m~^. Again, similar evidence indicated that
Saginaw Bay was a productive area generating zooplankton and
detritus and was then spilling this secondary productivity into
the shoreward waters of the main lake. However, with the inten-
sity of sampling available it is difficult to estimate to what
degree Saginaw Bay enriches the main lake with populations of
zooplankton and quantities of organic particulate matter. Cer-
tainly the earlier growth in the warmer waters of the Bay may
act to seed the open lake with adult cladocerans. Such aggre-
gations of zooplankton entering the open lake may also increase
the biological control on phytoplankton through grazing, if
those local phytoplankton populations are relatively nutrient-
limited .
November 1974 (0.8 to 2 mm particles)--
The areal biomass (mgC m~^) for November 1974 showed a high
standing crop in the mouth of Saginaw Bay (Figure 43) There
were clumps present which ranged in concentration from 22 to
594 mgC m . Thus, integrated column biomass was high in the
area of the mouth of Saginaw Bay, but was characterized by high
heterogeneity or spatial variability. Only a few stations were
visited in the main lake. In this area south of a line from
108
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Oscoda to Kincardine, the column biomass was high for deep sta-
tions and low for inshore .stations; most likely an indication
that it was strongly influenced by the deep detrital biomass and
not the zooplankton component. A secondary conclusion suggested
that Saginaw Bay provides a large biomass of zooplankton and
other large particles, which again are transported by water-
masses moving around Pointe aux Barques and then flowing in a
southeast direction along the shore of the thumb of Michigan.
This conclusion is in part based on a conversion of areal bio-
mass to unit volume biomass, obviously since the highest column
biomass occurred in the midlake region.
112
-------
REFERENCES
Birge, E.A. and C. Juday. 1922. The inland lakes of Wisconsin
The plankton. I. Its quantity and chemical composition.
Bull. Wise. Geol. Natl. Hist. Surv., 64:1-222.
Bogdan, K.G. and D.C. McNaught. 1975. Selective feeding by
Diaptomus and Daphnia. Verh. Int. Ver . Limnol., 19:2935-
2942.
Chisholm, S.W., R.G. Stress and P. A. Nobbs. 1975- Environmen-
tal and intrinsic control of filtering and feeding rates
in arctic Daphnia. J. Pish. Res. Bd . Can., 32:219-226.
Frost, B.W. 1975. A threshold feeding behavior in Calanus
pacificus . Limnol. Oceanogr., 20:263-266.
Frost, B.W. 1977- Feeding behavior of Calanus pacificus in
mixtures of food particles. Limnol. Oceanogr . , 22 : 472-91 .
Gannon, J.E. and R.S. Stemberger. 1977. Zooplankton (espe-
cially crustaceans and rotifers) as indicators of water
quality. Trans. Amer. Micros. Soc . , 97:16-35.
Haney, J.F. and D.J. Hall. 1975. Diel vertical migration and
filter-feeding activities of Daphnia. Arch. Hydrobiol.,
Ivlev, V.S. 1955- Experimental ecology and nutrition of fishes,
Yale Univ. Press (original edition by Pishchemizdat ) .
McMahon, J.W. and F.H. Rigler. 1963. Mechanisms regulating
the feeding rate of Daphnia magna S. Can. J. Zool., 41:
321-332.
McNaught, D.C. 1966. Depth control by plankton cladocerans
in Lake Michigan. Proc . 9th Conf. Great Lakes Res.
Univ. Michigan, Ann Arbor. pp 98-108.
McNaught, D.C. 1975- A hypothesis to explain the succession
from calanoids to cladocerans during eutrophication. Verh.
Int. Ver. Limnol., 19:724-731.
McNaught, D.C. and M. Buzzard. 1973- Changes in zooplankton
populations In Lake Ontario (1939-1972). Proc. 16th
Conf. Great Lakes Res. Univ. Michigan, Ann Arbor, pp 76-86.
113
-------
McNaught, D.C. and D. Scavia. 1976. Application of a model
of zooplankton composition to problems of fish introduc-
tions to the Great Lakes. In Mathematical Modeling of
Biochemical Processes'in Aquatic Ecosystems. Edited by
R.P. Canale. Ann Arbor Science Publishers, Ann Arbor,
Mich. pp. 281-305.
Nauwerck, A. 1959. Zur Bestimmung der Filtrierrate limnischer
Planktontiere. Arch. Hydrobiol. Suppl., 25:83-101.
Patalas, K. 1972. Crustacean zooplankton and eutrophication
of St. Lawrence Great Lakes. J. Fish. Res. Bd. Can.,
29:1451-1462.
Porter, K.G. 1973. Selective grazing and differential diges-
tion of algae by zooplankton. Nature, 244:179-180.
Rigler, F.H. 1961. The relation between concentration of food
and feeding rate of Daphnia magna Straus. Can. J. Zool.,
39:857-868.
Richman, S. 1966. The effect of phytoplankton concentration
on the feeding rate of Diaptomus oregonensis. Ver. Int.
Verh. Limnol., 16:392-398.
Richman, S., D.R. Heinle and R. Huff. 1977. Grazing by adult
estuarine calanoid copepods of the Chesapeake Bay. Mar.
Biol., 42:69-94.
Ryther, J. 1954. Inhibitory effects of phytoplankton upon
the feeding of Daphnia magna with reference to growth,
reproduction, and survival. Ecology, 35:522-533.
Saunders, G. 1969. Some aspects of feeding in zooplankton.
In Eutrophication: Causes, Consequences and Corrections.
Nat. Acad. Sci., Washington, D.C. pp. 536-573-
Schelske, C.L., L.E. Feldt, M.S. Santiago and E.F. Stoermer.
1972. Nutrient enrichment and its effects of phyto-
plankton production and species composition in Lake
Superior. Proc. 15th Conf. Great Lakes Res. Univ.
Michigan, Ann Arbor. pp. 149-165.
Stoermer, E.F. arid R.G. Kreis. 1980. Phytoplankton composition
and abundance in Southern Lake Huron. U.S. EPA Ecological
Research Series. U.S. Printing Office, Washington, D.C.
(in press).
Sorokin, J.I. 1968. The use of C in the study of nutrition
of aquatic animals. Mitt. Int. Verein. Limnol., 20:1-41.
114
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APPENDIX A
Lakewide Abundance of Zooplankton (mean density)
Numbers are corrected for net efficiency for each depth in-
terval; to obtain raw data divide by constants as follows:
(0-5, 1.6?; 0-10, 1.78; 0-20, 2.00; 0-40, 2.17).
115
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119
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APPENDIX B
Water Quality Sections, Mean Zooplankton Density
Numerical densities of zooplankton are necessary for many
purposes, including modeling of aquatic ecosystems. We have
included this summary of densities for Southern Lake Huron for
the convenience of those who need such data, although not in
the detail available in the complete data set and not corrected
for net efficiency.
120
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APPENDIX C
Graph of Lakewide Zooplankton Density
100
Cladocera
IM. Cyclopoida
AD.Cyclopoida
IM. Calanoid
AD.Calanoid
Nauplii
early mid
June July Aug Oct Nov
1974
Aug Oct
1975
Figure 44.
Percentage composition of crustacean zooplankton
in Southern Lake Huron in 1974-1975
(IM: Immature, AD: Adult).
128
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-80-069
RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Zooplankton Grazing and Population Dynamics Relative tc
Water Quality in Southern Lake Huron
5. REPORT DATE
JULY 1980 Issuing Date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Donald C. McNaught, Marlene Buzzard,
Michele Kennedy
8. PERFORMING ORGANIZATION REPORT NO.
David Griesmer,
9. PERFORMING ORGANIZATION NAME AND ADDRESS
State University of New York at Albany
Department of Biological Sciences
1^00 Washington Avenue
Albany, New York 12222
10. PROGRAM ELEMENT NO.
1BT69
11. CONTRACT/GRANT NO.
803178
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Duluth, Minnesota 5580k
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/03
1 SUPPLEMENTARY NOTES
Large Lakes Research Station, 9311 Groh Road, Grosse lie, Michigan I|8l38
16. ABSTRACT
Planktonic herbivores controlled the growth of phytoplankton in the open waters
of Southern Lake Huron during the late summer and fall; larger algae remained un-
touched during the spring diatom bloom. Over a growing season these crustaceans
grazed 0.3 to 41.9% of the standing crop of phytoplankton. Immature copepods
(nauplii and copepodites) were the most effective grazers. Selective grazing by
crustaceans centered on nannoplankton (<22 ym diameter"), whereby 92 to 2804% of the
daily productivity was grazed by large cladocerans and immature copepods. In contrast,
netplankton was harvested to a much lesser degree. Laboratory experiments reinforced
this picture of selective chopping. Presently in Lake Huron such selective grazing
accounts for an effective biological control over algal production; such control
will be lost if these phyto.plankton populations are allowed to shift to ones
dominated by large blue-gi?een algae. Populations of herbivores were also used
to characterize water quality. Generally outer Saginaw Bay and most inshore waters
were of lower water quality than offshore surface waters. These inshore areas also
had the highest productivity of zooplankton.
This report was submitted in fulfillment of Grant 803178 by the State University
of New York at Albany under the sponsorship of the U.S. Environmental Protection
Agency. This report covers the period 15 May 1974 through 14 May 1976.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Zooplankton, Water Quality
Lake Huron
06F
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report/
unnl assi fi
21 NO. OF PAGES
20. SECURITY CLASS (This page I
unclassified
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
129
GOVERNMENT PRINTING OFFICE
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