Population dynamics of planktonic rotifers in Lake Loosdrecht, the Netherlands, in relation to their potential food and predators

1. This study uses descriptive data to examine the shift in dominance in th e rotifer community in summer from Keratella cochlearis to Anuraeopsis fiss a in a shallow eutrophic lake. Population density and egg ratio were estima ted from May to September for these small loricate rotifers, as well as the soft-bodied Filinia longiseta and Polyarthra spp., to compare rates of pop ulation increase, birth and death. 2. Keratella cochlearis was succeeded by A. fissa in late May, perhaps as a result of the shorter egg development times of the latter at temperatures > 15 degrees C, and species-specific responses to food and predation. Popul ation variables suggest that the decline of K. cochlearis was the result of a food shortage which caused a decrease in reproduction and increased mort ality. 3. High population densities of A. fissa and Polyarthra spp. were associate d with low egg ratios, birth rates and rates of increase within a species, suggesting intraspecific competition. The egg ratios of the two soft-bodied rotifers were strongly intercorrelated in the study period, although their diets supposedly differ. The egg ratios of A. fissa showed positively corr elated fluctuations with the abundance of diatoms. 4. Short periods of higher rates of increase of A. fissa conferred advantag e to this rotifer and manifested in its dominance. The mean birth rates of A. fissa and F. longiseta were twice as high as for K. cochlearis. Furtherm ore, death rates of loricate and soft-bodied rotifers were similar and high , which suggests that food shortage similarly increased death rates, or tha t predation may be sometimes substantial for soft-bodied and loricate speci es, or both. 5. Predation was probably not an important factor in steering the seasonal succession in the lake. A difference in abilities of the studied rotifers t o exploit resources seems more important. Perhaps temperature is also a fac tor, but the specifics remain unclear and await more experimental work.