Phosphorus limitation in Daphnia: Evidence from a long term study of three hypereutrophic Dutch lakes
Limnol. Oceanogr., 44(6), 1999, 1557-1564 | DOI: 10.4319/lo.19126.96.36.1997
ABSTRACT: The Loosdrecht lakes comprise three shallow, interconnected hypereutrophic lakes in The Netherlands. A lake restoration project conducted during the 1980s resulted in reduced phosphorus loading. However, no changes in phytoplankton abundance or species composition were noted, although seston carbon : phosphorus (C : P) ratios increased. Filamentous cyanobacteria and detritus continued to dominate the seston. Moreover, planktivorous fish were very abundant. Relationships between zooplankton abundance and seston abundance and stoichiometry (C: P and carbon : nitrogen ratios) were analyzed using data collected during 9 yr of intensive study of the three lakes. Analysis based on annual means shows surprisingly strong and consistent inverse relationships 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 abundance 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 ratio, especially during the 2 yr following a biomanipulation project in this lake. Analysis of seston C: P ratios and Daphnia abundance in individual samples provides further evidence that high seston C: P ratios constrained Daphnia abundance in all three lakes. In contrast to Daphnia, the abundances of zooplankters with low P requirements, including Bosmina spp. and cyclopoid copepods, show little relationship to seston C: P ratios. These results suggests that the abundance of Daphnia in the Loosdrecht lakes was determined by the variable P content of their diets and consistently strong fish predation.