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

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. In pelagic ecosystems, primary production is determined by the supply of light and dissolved mineral nutrients. In lakes, the nutrient that limits primary production is often phosphorus (P) (Vollenweider 1976). The loose coupling between algal nutrient uptake and photosynthesis allows for highly flexible C : P ratios of algal biomass. It is therefore common that the P content of algal biomass relative to carbon (C) fixed by photosynthesis decreases with increasing light input (Sterner et al. 1997; Diehl et al. 2005; Berger et al. 2006). In contrast, the elemental composition of herbivorous zooplankton is largely homeostatically regulated (Andersen and Hessen 1991; Main et al. 1997; Elser et al. 2000). Zooplankton with high specific growth rates, such as Daphnia, tend to have a high body phosphorus content and, therefore, a C : P ratio that is considerably lower than that of phytoplankton (Elser et al. 1996; Main et al. 1997; Weider et al. 2004). If this mismatch in the elemental composition between autotrophs and herbivores becomes sufficiently strong, herbivore growth could become limited by the nutrient rather than by the carbon content of their food. For example, Daphnia growth has been reported to be limited by P at molar seston C : P ratios .300 (Hessen 1992; Urabe and Watanabe 1992; Urabe et al. 2002a), whereas only weak P limitation and stronger energy limitation is usually observed at molar seston C : P ratios ,300 (DeMott and Tessier 2002; DeMott et al. 2004). The degree of mismatch in the elemental composition between autotrophs and herbivores has implications for the efficiency with which biomass and energy are transferred up the food chain, summarized in the ‘‘light–nutrient hypothesis’’ (Sterner et al. 1997). In short, although increased light supply usually promotes phytoplankton growth, the resulting increase in primary production can only be fully transferred to the herbivore level if herbivore growth is predominantly carbon (energy) limited. In contrast, if herbivore growth is predominantly nutrient limited, increased light supply could actually decrease herbivore production because any light-induced increase in food quantity could be offset by a disproportional decrease in the food’s nutrient content (Andersen et al. 2004; Diehl 2007). The latter phenomenon has been termed the ‘‘paradox of energy enrichment’’ (Loladze et al. 2000) and has been observed in several laboratory experiments with Daphnia and monocultures of chlorophytes (Urabe and Sterner 1996; Sterner et al. 1998; Urabe et al. 2002a). Still, to date, only very few attempts have been made to investigate experimentally how widely applicable the light– nutrient hypothesis (LNH) is to the description of natural phytoplankton–zooplankton interactions with diverse algal communities. To our knowledge only one field experiment to date (Urabe et al. 2002b) has investigated the response of the plankton community of an oligotrophic lake to the factorial manipulation of light (shading) and nutrients (P enrichment) in field mesocosms. These manipulations had major effects on seston C : P stoichiometry and on zooplankton production and growth over the 4-week experiment, consistent with the predictions of the LNH. However, the short-term response of an algal community to an experimental manipulation can be constrained by its 1 Corresponding author ([email protected]. de). Acknowledgments We thank Sebastian Diehl, William DeMott, and two anonymous reviewers for comments that improved this manuscript; Angelika Wild and Achim Weigert for technical support; and Christian Matauschek, Johanna Vilsmaier, and Ferdinand M. Neuberger for help during the field experiment. This study was supported by funding from Deutsche Forschungsgemeinschaft (STI 180/2-1). Limnol. Oceanogr., 53(2), 2008, 513–522 E 2008, by the American Society of Limnology and Oceanography, Inc.