Historic Changes in Lake Michigan Zooplankton Community Structure: The 1960s Revisited with Implications for Top-Down Control

Recent changes in the Lake Michigan ecosystem provide a benchmark against which to reevaluate historic data. During the 1960s, the alewife (Alosa pseudoharengus) population exploded and then crashed. Offshore zoo-plankton data for the summers of 1954, 1966, and 1968 provided evidence that variations in alewife abundance had a major effect on zooplankton community structure. Based on these observations, other researchers have hypothesized that increased and decreased phytoplankton abundances during the 1960s as recorded at the Chicago water filtration plant were due to top-down effects rather than to phosphorus loading. This argument is reevaluated using two approaches. First, from the relationship between interannual variability in alewife and zooplankton species abundance during the summers of 1954, 1966, 1968, 1977, 1982, and 1984–87, I conclude that the effects of alewife predation on zooplankton community structure during the 1960s are less clear then originally proposed. Second, from estimates of Daphnia spp. grazing rates, considerations of the source of the long-term phytoplankton data used to support the top-down argument, regional differences in phytoplankton, zooplankton and alewife abundance trends, and historic water clarity observations, I conclude that existing data are insufficient to support the top-down argument that long-term trends in phytoplankton abundance were primarily affected by fluctuations in alewife abundance.