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 annually laminated sediments of Lake 227, Experimental Lakes Area, northwestern Ontario. Constrained cluster analysis of both total fossil Cladocera and littoral chydorid communities clearly distinguished between pre- and post-eutrophication communities and further differentiated between years of high and low nitrogen:phosphorus fertilization ratios. However, there were few chydorid species extirpations resulting from the manipulation. Total chydorid accumulation rates and indices of species diversity, richness, and equitability were relatively constant over the last century and were not affected by fertilization. Among pelagic Cladocera, Bosmina longirostris abundance declined >60% after initial fertilization. Although harsh chemical conditions (pH >10) may have contributed to reduced abundance of pelagic Cladocera, Bosmina populations were also naturally variable prior to manipulation. Changes in Bosmina morphology (mucrone, antennule, and carapace length) and cladoceran size ratios (Daphnia/[Daphnia + Bosmina]) suggested that zooplanktivory 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 errors in determining bulk sediment accumulation rates.