CHANGES IN PHYTOPLANKTON SIZE-CLASS ABUNDANCE AND SPECIES COMPOSITIONCOINCIDING WITH CHANGES IN WATER CHEMISTRY AND ZOOPLANKTON COMMUNITY STRUCTURE OF LAKE MICHIGAN, 1983 TO 1992

Phytoplankton and zooplankton were collected at offshore sites of Lake Michigan during 37 cruises in the spring and summer from 1983 to 1992 . For the period, 39 common phytoplankton species accounted for 96.0% of the total abundance and 85.6% of the biomass. Over the 10-year stud y, the lake-wide average spring and summer phytoplankton biomass in th e pelagic waters ranged from 0.27 to 1.2 g/m(3) (mean. +/- S.E. = 0.54 +/- 0.03 g/m3) and phytoplankton abundance ranged from 5,132 to 39,78 0 cells/mL (mean +/- S.E. = 18,291 +/- 822 cells/mL). Mesotrophic diat oms accounted for 47.2% of the total phytoplankton biomass. The lack o f a trend in the ratio of mesotrophic to eutrophic diatom indicator sp ecies suggested that no change in trophic status of the pelagic region occurred during the 1983 to 92 period; that is, the water quality of the offshore of Lake Michigan did not change. A year-to-year shift in dominance from one mesotrophic diatom species to another was evident. Over the 10-year period, flagellates accounted for 52.2% of the summer phytoplankton biomass. The data support the concept of a shift in sum mer species composition away from blue-green algae dominance to flagel lates prior to 1982. However after 1987 the relative importance of the > 70 mu m size class increased to over 21% of the phytoplankton abund ance in the spring and over 55% in the summer. In particular, the colo nial blue-greens, Anacystis and Aphanothece, became substantially more prevalent during the summer, while the flagellates, Chromulina and Oc hromonas, decreased in abundance. The changes in the relative abundanc e of phytoplankton size classes and species composition were examined to determine if they correlated with either the top-down mediated chan ges in the zooplankton community during the 1980s and 1990s or with an y bottom-up variability in nutrient chemistry. Canonical correspondenc e analysis (CCA) suggested a significant portion of the variability of the spring and summer phytoplankton size classes from 1983-1992 (88.0 % and 99.2%, respectively) was explained by environmental axis I. Prim ary determinants of relative abundance of phytoplankton size class inc luded nutrients (silica, total phosphorus, and N:P ratio) and abundanc e of some species of zooplankton. Specifically, Diaptomus minutus, D. ashlandi, D. sicilis, and Daphnia galeata mendotae were inversely corr elated with the 0 to 10 mu m size class, which we interpret as a grazi ng effect on phytoplankton size structure, and positively correlated w ith an increase in abundance of the > 70 mu m size class of phytoplank ton. CCA suggested that top-down and bottom-up effects were affecting phytoplankton size composition and abundance simultaneously.