Recent shifts in Daphnia community structure in southeastern Lake Michigan: A comparison of the inshore and offshore regions1T2

Before 1982, Daphnia retrocurva and Daphnia galeata mendotae were the dominant species of Daphnia in Lake Michigan. Between 1972 and 198 1, Daphnia community structure in the offshore region shifted toward greater dominance of the larger D. galeata mendotae, with Daphniapulicaria, 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 lMysis 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 jlavescens) 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 (mid1950s) record of zooplankton community structure in this region of southeastern Lake Michigan. Lake Michigan, like the other Laurentian Great Lakes, has been exposed to various disturbances over the last several decades (Beeton 1969), including eutrophication, habitat alteration, sea lamprey predation (Petromyzon marinus), and invasion of exotics such as the alewife (Alosa pseudoharengus) (Damann 1960; Wells and McLain 1972; Christie 1974). With no significant predation by the reduced lake trout (Salvelinus namaycush) populations, and with reduced competition from lowered populations of endemic planktivores, the planktivorous alewife rapidly dominated the Lake Michigan fish community (Christie 1974). Furthermore, zooplankton community structure apparently was altered as a consequence of eutrophication and the alewife population explosion (Brooks 1969; Wells 1970; McNaught 1975). During the late 1960s and 197Os, programs were instituted to slow eutrophication, utilize alewife populations, and restore the commercial fisheries. As part of the fisheries management program, the lakes were stocked with lake trout and other salmonids, species which thrive in clean, oligo1 Indiana & Michigan Electric Company and Michigan Sea Grant provided support for the study. 2 Great Lakes Research Division, University of Michigan, contribution 423. trophic waters (Smith 1972). Stocking has been highly successful; an economically valuable salmonid fishery has been produced and alewife populations have been more efficiently managed. Stewart et al. (198 1) suggested that if salmonids and lake trout became too abundant, they would begin to deplete their alewife forage base. This in turn might affect the zooplankton community. Recently, Daphnia populations in southeastern Lake Michigan have changed dramatically in different ways in the inshore and offshore regions, while alewife abundances declined in both regions. Concurrently, abundances of bloaters (Coregonus hoyi), primarily an open-lake, deep-water species, and yellow perch (PercaJZavescens), an inshore species, increased (Jude and Tesar 1985; Wells and Hatch unpubl.). Here we report changes in the Daphnia community between 1972 and 1984 and examine the long term relationship among Daphnia species assemblages and alewife, bloater, and yellow perch populations. We then re-examine the current understanding of long term (1887 to the late 1960s) changes in Lake Michigan zooplankton community structure. The NOAA Great Lakes Environmental Research Laboratory, Ann Arbor, provided ship time in 1982. Internal reviewers were