CLADOCERA COMMUNITY RESPONSE TO EXPERIMENTAL EUTROPHICATION IN LAKE-227 AS RECORDED IN LAMINATED SEDIMENTS

The response of pelagic zooplankton to experimental fertilization was compared with the fossil record of Cladocera obtained from the annuall y laminated sediments of Lake 227, Experimental Lakes Area, northweste rn Ontario. Constrained cluster analysis of both total fossil Cladocer a and littoral chydorid communities clearly distinguished between pre- and post-eutrophication communities and further differentiated betwee n years of high and low nitrogen:phosphorus fertilization ratios. Howe ver, there were few chydorid species extirpations resulting from the m anipulation. Total chydorid accumulation rates and indices of species diversity, richness, and equitability were relatively constant over th e last century and were not affected by fertilization. Among pelagic C ladocera, Bosmina longirostris abundance declined >60% after initial f ertilization. Although harsh chemical conditions (pH >10) may have con tributed to reduced abundance of pelagic Cladocera, Bosmina population s were also naturally variable prior to manipulation. Changes in Bosmi na morphology (mucrone, antennule, and carapace length) and cladoceran size ratios (Daphnia/[Daphnia + Bosmina]) suggested that zooplanktivo ry by fish and invertebrates exercised important control of herbivore populations. Fossil Bosmina concentration (remains .[g dry wt](-1) or remains .[g organic matter](-1)) were significantly correlated (r = 0. 66, P < 0.01, 1970-1989) to standing crop in the water column (animals . m(-2). Fossil accumulation rate (remains . cm(-2). yr(-1)) was not significantly correlated to Bosmina abundance, perhaps because of erro rs in determining bulk sediment accumulation rates.