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Grazing and production by zooplankton in lakes of the Cedar River watershed

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  • Zooplankton grazing rates and life history stage densities for secondary production estimates were determined in three lakes in the Cedar River watershed, Washington. Eight copepod species, seven cladoceran and nine rotifer species were identified in the three lakes. No one species of cladoceran or copepod occurred in all the lakes, but the rotifers are common throughout. Copepods in the lakes showed no clear diet migration, while two cladocerans did migrate. Copepods were most important numerically in the lakes. Some principal members were Diaptomus ashZandi which reached a seasonal maximum adult density of 11/2 (mean of water column) in Lake Sammamish and LimnocaZanus in Chester Morse which reached a maximum of 0.7/2. Two Diaptomus species are Important in Findley Lake. Densities averaged about 10 times greater in Sammamish than in the two oligotrophic lakes. Data on population dynamics from the egg ratio technique are given for the common cladocerans in the three lakes. Grazing of phytoplankton by zooplankton stimulated photosynthetic carbon uptake in the light in excess of 200 percent over ungrazed populations. Phytoplankton loss rates from grazing ranged from 0.002 to 0.268 pg chi a/ animal day. Feeding was found to be maximum at low concentrations of algae. These rates account for from 2-247 percent of the average growing season productivity in the lakes. Net plankton (>500 were seldom grazed and since that size fraction and algal mass in general increased with mean productivity and nutrient content in the lakes such decreasing efficiency in food transfer is seen as a major effect of eutrophication.
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