Invertebrate predation in lake Michigan : regulation of Bosmina longirostris by Leptodora kindtii

The density of Bosmina longirostris (0. F. Mueller) declined loo-fold between 25 June and 20 August 1985 in Lake Michigan; a similar pattern of changing abundances was observed in 1986 but not in 1987 or 1988. Bosmina collected from periods before and during the decline in 1985 and 1986 were examined for size at maturity and clutch size. Population size-frequency distributions suggest that some animals began to mature at smaller sizes during the decline in 1985. In some cases, the average clutch size for Bosmina of similar body lengths was significantly reduced during the decline in 1985 and 1986. These data suggest that food limitation was a factor in the species decline in both years. Gut content analyses indicate that of seven potential predators, Leptodora kindtii (Focke) was probably most responsible for Bosmina mortality. There is ample evidence that Leptodora does not consume strictly fluid from its prey: juvenile Leptodoru ate mainly smaller prey that included Bosmina and the rotifer Conochilus; adults ate a broader size range of prey that included mainly Bosmina, Daphnia, and copepods. In 1987 and 1988, Bythotrephes cederstroemii Schoedler was abundant in Lake Michigan and was contemporaneous with collapse ofthe Leptodora population and concurrent increase of the Bosmina population in August and September. Predation by the large copepods Epischuru lucustris S. A. Forbes and Limnoculanus macrurus Sars did not appear to have any significant effect on the population dynamics of Bosmina. Lake Michigan has long been regarded as an ecosystem in which visual planktivory by fish exerts a dominant structuring influence. The exposition by Wells (1970) documenting changes in zooplankton populations in response to predation by alewife, Alosa pseudoharengus (Wilson), helped to establish size-selective predation as one of the paradigms of plankton community ecology. Wells observed that the successful invasion of alewife into Lake Michigan was associated with depressed densities of Daphnia, Leptodora, Limnocalanus, Epischura, and other large-bodied crustacean taxa and with increased densities of smaller taxa, particularly Bosmina longirostris. Although Wells (1970) argued convincingly that the large plankton were removed by selective planktivory, no rationale was advanced for the numerical increase in Bosmina. In retrospect, the taxon’s success may have stemmed from relaxed competition for Acknowledgments We thank James Bowers, Douglass Burdette, Robert Gensemer, Robert Dorazio, Glenn Warren, and the captain and crew of the RV Laurentian for assistance with field sampling. Robert Black and Patricia ChowFraser provided suggestions for the manuscript. This research was supported by NSF grants OCE 8415970, OCE 86-13880, and OCE 87-16187. food resources or from reduced planktivory by invertebrates. That planktivory played a role in Bosmina’s dynamics seems plausible but because several putative predators declined simultaneously the relative importance of each is indecipherable from his data. Recent changes of Lake Michigan’s zooplankton community, wrought by a new invading species, provide an opportunity to further understand factors controlling Bosmina dynamics there. In summers 1985 and 1986, Bosmina populations in offshore Lake Michigan experienced similar abundance cycles. Populations quickly reached peak densities in late June of 3 1,000 m-2 (1985) and early July of 70,000 me2 (1986), and then decreased steadily to densities of 600 m-2 (1985) and 9,000 rnw2 (1986) by midAugust. In 1987, however, major changes occurred in the composition of the offshore zooplankton community; they were attributed in part to predation by the invading cladoceran, Bythotrephes cederstroemii (Lehman 1988). That summer, and in 1988, Bosmina did not decline in August and September as it had in 1985 and 1986 during those months. In this report we use the population changes associated with this event to investigate the controls on Bosmina in Lake Michigan.