Phosphorus limitation in Daphnia: Evidence from a long term study of threehypereutrophic Dutch lakes

The Loosdrecht lakes comprise three shallow, interconnected hypereutrophic lakes in The Netherlands. A lake restoration project conducted during the 1 980s resulted in reduced phosphorus loading. However, no changes in phytopl ankton abundance or species composition were noted, although seston carbon: phosphorus (C:P) ratios increased. Filamentous cyanobacteria and detritus c ontinued to dominate the seston. Moreover, planktivorous fish were very abu ndant. Relationships between zooplankton abundance and seston abundance and stoichiometry (C:P and carbon:nitrogen ratios) were analyzed using data co llected during 9 yr of intensive study of the three lakes. Analysis based o n annual means shows surprisingly strong and consistent inverse relationshi ps between Daphnia abundance and the seston C:P ratio for two of the three lakes. In these two lakes (Loosdrecht and Vuntus Lakes), the annual mean ab undance of Daphnia cucullata ranged from 104 to 0.7 individuals L-1 over a range in mean seston C:P ratios from about 250 to 500 (molar). In the third lake, Breukeleveen, Daphnia abundance was higher for a given seston C:P ra tio, especially during the 2 yr following a biomanipulation project in this lake. Analysis of seston C:P ratios and Daphnia abundance in individual sa mples provides further evidence that high seston C:P ratios constrained Dap hnia abundance in all three lakes. In contrast to Daphnia, the abundances o f zooplankters with low P requirements, including Bosmina spp. and cyclopoi d copepods, show little relationship to seston C:P ratios. These results su ggests that the abundance of Daphnia in the Loosdrecht lakes was determined by the variable P content of their diets and consistently strong fish pred ation.