United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development
EPA-600/S3-83-074 Nov. 1983
Project Summary
Changes in the Zooplankton
Community in Response to
Reductions in Nutrient Loading to
Saginaw Bay (Lake Huron)
Donald C. McNaught, David Griesmer,and Reed Larson
Saginaw Bay (Lake Huron) receives
tributary waters from the industrial
heartland of Michigan. For 20 years.
University scientists have examined its
water chemistry, physical character-
istics including currents, and the
community composition of its biota.
This large amount of background
information has permitted interpreta-
tion of the EPA*s efforts to improve
water quality in the system by diverting
nutrients as phosphorus and nitrogen.
This Project Summary was developed
by EPA's Environmental Research
Laboratory, Duluth. MN, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
The U.S. Environmental Protection
Agency, in an attempt to reduce pollution
and maintain the biological integrity of
Saginaw Bay (Lake Huron), reduced
fluvial inputs of phosphorus by 50%
between 1975 and 1978. By 1980 this
action had resulted in a 24% decrease in
available orthophosphate for phyto-
plankton growth. This report discusses
changes in the zooplankton community of
the Bay resulting from these reduced
inputs.
Methods
Planktonic crustaceans (cladocerans,
copepods) and rotifers were collected
during survey cruises on Saginaw Bay,
Lake Huron. Approximately 37 stations
were sampled at 5 m intervals (1, 5, 10,
15m and bottom), for a total of about 60
samples per cruise. Cruises of 5 days
duration occurred each month from April
through November 1980.
Samples to be counted for rotifers were
collected with an electric pump. Approx-
imately 100 i. of water was pumped
through a plankton funnel fitted with 54
jt/m nitex nylon screening. Rotifers were
relaxed with carbonated water and
preserved in 5 percent buffered formalin.
The crustaceans and rotifers were
counted by University of Minnesota
technicians, hired and trained by the
principal investigator.
Traditional subsampling methods were
used. Each concentrated zooplankton
sample was adjusted to a constant
volume. The sample was then mixed and
an aliquot withdrawn with a Hensen-
Stemple pipette. The subsample of 10 ml
was counted in a gridded dish at 30 to
100x under a Leitz dissecting microscope.
This method sufficed for all crustaceans.
Conclusions
The zooplankton community of Saginaw
Bay relies on algal and detrital foods. A
reduction in phosphorus loading of 50%
resulted in significant changes in this
community. The crustacean zooplankton
were moderately reduced in abundance.
Total crustaceans fell from a yearly mean
of 155700/m3 in 1974 to 97800/m3 in
1980. The percentage composition of the
eutrophic indicator Bosmina longirostris
remained, however, relatively constant at
38%. A slight indication that populations
of the oligotrophic indicator Diaptomus
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sicilis were increasing in the Bay was
presented.
The rotiferan zooplankton responded to
nutrient diversion very dramatically. Total
rotifers decreased significantly between
1974 and 1980, experiencing approxi-
mately a 3-fold reduction. Likewise,
predatory rotifers decreased significantly,
an indication that one of the apex groups
of organisms had responded as predicted
to nutrient limitation, in a similar fashion
than an apex fish predator would be
encouraged by a decrease in toxic
substances.
Variations in zooplankton crops in
Saginaw Bay within and between years
were attributed to variations in the flow of
the Saginaw River. Care was taken to
account for this fact in attributing a
significant improvement in the Bay's
water quality to nutrient diversion.
Donald McNaught, David Griesmer, and Reed Larson are with the University of
Minnesota, Minneapolis, MN 55455
William L. Richardson is the EPA Project Officer (see below).
The complete report, entitled "Changes in the Zooplankton Community in
Response to Reductions in Nutrient Loading to Saginaw Bay (Lake Huron),"
(Order No. PB 83-252 643; Cost: $10.00. subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
EPA Large Lakes Research Station
Environmental Research Laboratory—Duluth
U.S. Environmental Protection Agency
9311 Groh Road
Grosselle,Mt48138
•ftUS GOVERNMENT PRINTING OFFICE 1983-659-017/7220
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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