INVERTEBRATE PREDATION IN LAKE-MICHIGAN - REGULATION OF BOSMINA-LONGIROSTRIS BY LEPTODORA-KINDTII

The density of Bosmina longirostris (O. F. Mueller) declined 100-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 Leptodora 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 of the Leptodora population and concurrent increase of the Bosmina population in August and September. Predation by the large copepods Epischura lacustris S. A. Forbes and Limnocalanus macrurus Sars did not appear to have any significant effect on the population dynamics of Bosmina.