Food limitation and seasonal population declines of rotifers.

Keratella cochlearis exhibited consistent seasonal abundance patterns during a four-year study in Little Rock Lake, Wisconsin, U.S.A. In each year, spring population peaks were followed by strong summer reductions. Concomitant with population declines, there were reductions in rotifer egg ratios. Keratella taurocephala abundance patterns were similar to those of K. cochlearis during 1984 and 1985, but not in 1986 and 1987, when spring peaks and summer declines were not apparent. However, summer declines in the egg ratio of K. taurocephala were observed during each year. The reduction in rotifer populations simultaneously with decreased egg ratios suggested that population declines were caused by food limitation. Food-addition experiments conducted in situ in small enclosures indicated that food was limiting for K. cochlearis when its populations were declining, but not during other periods of the year. Keratella taurocephala did not show a consistent response to food addition.