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The paper summarizes the results of a ten-year (1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991) zooplankton research on the Lake Loosdrecht, a highly eutrophic lake. The main cause of the lake's eutrophication and deteriorating water quality was supply up to mid 1984 of water from the River Vecht. This supply was replaced by dephosphorized water from the Amsterdam-Rhine Canal in 1984. The effects of this and other restoration measures on the lake's ecosystem were studied. Despite a reduction in the external P-load from ca. 1.0 g P me2 y-l to ca. 0.35 g rnp2 y-l now, the filamentous prokaryotes, including cyanobacteria and Prochlorothrix, continue to dominate the phytoplankton.Among the crustacean plankton Bosmina spp, Chydorus sp. and three species of cyclopoid copepods and their nauplii are quite common. Though there was no major change in the composition of abundant species, Daphnia cucullata, which is the only daphnid in these lakes, became virtually extinct since 1989. Among about 20 genera and 40 species of rotifers the important ones are: Anuraeopsis j?ssa, Keratella cochlearis, Filinia longiseta and PoZyarthra. The rotifers usually peak in mid-summer following the crustacean peak in spring. The mean annual densities of crustaceans decreased during 1988-1991. Whereas seston (< 150 pm) mean mass in the lake increased since 1983 by 20-60%, zooplankton (> 150 pm) mass decreased by 15-35x.The grazing by crustacean community, which was attributable mainly to Bosmina, had mean rates between 10 and 25% d-'. Between 42 and 47% of the food ingested was assimilated. In spring and early summer when both rotifers and crustaceans have their maximal densities the clearance rates of the rotifers were much higher. Based on C/P ratios, the zooplankton (> 150 pm) mass contained 2.5 times more phosphorus than seston (< 150 pm) mass so that the zooplankton comprised 12.5 y0 of the total-P in total particulate matter in the open water, compared with only 4.5% of the total particulate C. The mean excretion rates of P by zooplankton varied narrowly between 1.5 and 1.8 pg P l-' d -', which equalled between 14 and 28% d-' of the P needed for phytoplankton production. The lack of response to restoration measures cannot be ascribed to one single factor. Apparently, the external P-loading is still not low enough and internal P-loading, though low, may be still high enough to sustain high seston levels. Intensive predation by bream is perhaps more important than food quality (high concentrations of filamentous cyanobacteria) in depressing the development of large-bodied zooplankton grazers, e.g. Daphnia. This may also contribute to resistance of the lake's ecosystem to respond to rehabilitation measures. 70

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1. This study uses descriptive data to examine the shift in dominance in the rotifer community in summer from Keratella cochlearis to Anuraeopsis fissa in a shallow eutrophic lake. Population density and egg ratio were estimated from May to September for these small loricate rotifers, as well as the soft‐bodied Filinia longiseta and Polyarthra spp., to compare rates of population 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 °C, and species‐specific responses to food and predation. Population variables suggest that the decline of K. cochlearis was the result of a food shortage which caused a decrease in reproduction and increased mortality. 3. High population densities of A. fissa and Polyarthra spp. were associated 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 correlated fluctuations with the abundance of diatoms. 4. Short periods of higher rates of increase of A. fissa conferred advantage 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. Furthermore, death rates of loricate and soft‐bodied rotifers were similar and high, which suggests that food shortage similarly increased death rates, or that predation may be sometimes substantial for soft‐bodied and loricate species, 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 to exploit resources seems more important. Perhaps temperature is also a factor, but the specifics remain unclear and await more experimental work.

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