Light-induced changes of plankton growth and stoichiometry: Experiments with natural phytoplankton communities

Abstract

Both low and high rates of light supply can restrict herbivore growth rates by limiting either the quantity (photosynthetically fixed carbon) or the nutritional quality (nutrient content per fixed carbon) of the herbivores' food. The "light-nutrient hypothesis," therefore, predicts that, if phosphorus (P) supply is sufficiently low, production of herbivorous zooplankton should be unimodally related to light intensity. We manipulated the light regime of six different algal communities in a field experiment and investigated the effect of these manipulations on Daphnia growth. The algal communities came from six lakes having different total phosphorus concentrations, ranging from oligotrophic to eutrophic. Seston carbon (C) and seston carbon-to-phosphorus ratios in communities from oligotrophic and mesotrophic lakes increased with higher light availability. Across all lakes, the strength of these responses was related to algal diversity. More diverse algal communities showed a stronger increase than less diverse communities in both their carbon biomass and their C:P ratio with increasing light. Furthermore, in oligotrophic and mesotrophic treatments, Daphnia growth was highest at intermediate light intensities. In contrast, seston parameters and Daphnia growth were only weakly related to light supply in communities from eutrophic lakes. © 2008, by the American Society of Limnology and Oceanography, Inc.