Recent shifts in Daphnia community structure in southeastern Lake Michigan: A comparison of the inshore and offshore regions1,2

Before 1982, Daphnia retrocurva and Daphnia galeata mendotae were the dominant species of Daphnia in Lake Michigan. Between 1972 and 1981, Daphnia community structure in the offshore region shifted toward greater dominance of the larger D. galeata mendotae, with Daphnia pulicaria, another large species, dominating by 1982. This continued through summer 1984. Shifts in offshore Daphnia community structure appear to be related to a reduction in predation pressure by declining alewife (Alosa pseudoharengus) populations and a hypothesized increase in relative predation pressure by Mysis relicta. From 1972 to 1984, Daphnia community structure changed only slightly in the inshore region; D. retrocurva generally remained the summer and autumn dominant. However, summer Daphnia abundances decreased after 1980 as yellow perch (Perca flavescens) and bloater (Coregonus hoyi) increased. From an examination of historic data we conclude that inshore region Daphnia populations probably have changed little since the late 1880s. The 1982–1984 offshore Daphnia complex, however, differed markedly from the earliest (mid‐1950s) record of zooplankton community structure in this region of southeastern Lake Michigan.