Daphnia Food Limitation in Three Hypereutrophic Dutch Lakes: Evidence for Exclusion of Large-Bodied Species by Interfering Filaments of Cyanobacteria

The Loosdrecht Lakes comprise three shallow, hypereutrophic lakes in The Netherlands. Research conducted over the past two decades suggests that absence of large-bodied Daphnia from these lakes can be explained by size-specific effects of both poor food quality and predatory mortality. The phytoplankton is dominated by filamentous cyanobacteria, which interfere with the feeding of large-bodied Daphnia. Moreover, dense populations of planktivorous fish are expected to feed selectively on larger prey. More recent research, however, suggests that the growth of the small native Daphnia species, Daphnia cucullata, is limited by low dietary phosphorus (P). In this study, we compared the growth and feeding rates of four Daphnia taxa representing a gradient in body size. In each of 10 growth experiments, native D. cucullata feeding in natural seston exhibited improved growth in response to additions of phosphate and the P-rich cyanobacterium Synechococcus. The two largest species, Daphnia magna and Daphnia galeata, however, failed to grow in natural seston, even when the seston was supplemented with phosphate, Synechococcus, or Scenedesmus. A D. galeata x cucullata hybrid exhibited weak growth in natural seston and no response to the supplements. Feeding experiments with polystyrene beads show that feeding inhibition in natural seston increased markedly with increased Daphnia size. Our results verify that Daphnia differing in size face different food quality constraints. The growth of native D. cucullata (adult size 0.6-0.8 mm) is reduced by P deficiency, whereas larger Daphnia appear to be excluded by interfering filaments of cyanobacteria.