Stoichiometric effects of warming on herbivore growth: Experimental test with plankters

Recent studies have shown that putative warming increases water temperatures in many lakes. A higher temperature would directly affect the growth rates of lake organisms; however, the growth responses of lake herbivores to increased temperatures may be complex. Under limited nutrient supplies, for example, increased temperature may accelerate the growth of phytoplankton but consequently decrease their nutrient content, which may in turn reduce herbivore growth. Thus, temperature-driven declines in food nutrient content may offset the positive effects of warming on herbivore growth depending on the trophic condition of habitats. We examined this hypothesis by laboratory experiments that simultaneously manipulated temperature and stoichiometric interactions between green algae (Scenedesmus) and a planktonic herbivore (Daphnia) under various phosphorous conditions. The following experiments were performed with Scenedesmus and Daphnia at the same temperatures (12, 16, 20 and 24°C). In the first experiment, Scenedesmus were grown in semi-batch cultures at four different phosphorus supply rates with a fixed dilution rate (i.e., algal loss rate). In the second experiment, algae were grown at four different dilution rates with a fixed phosphorus supply level. Then, the steady-state algae were harvested daily and fed directly to juvenile Daphnia to measure their specific growth rates in both experiments. Increased temperature accelerated Daphnia growth in both experiments. However, the effect of temperature decreased with decreasing phosphorus supply rate and dilution rates, although algae were sufficiently abundant. This was because the algal phosphorus:carbon ratio decreased with increasing temperature, especially when phosphorus availability for algae was small due to low phosphorus supply or dilution rates. The results clearly showed for the first time that direct stimulation of herbivore growth by warming can be offset by poor food stoichiometry. We suggest that in lakes with low algal growth rates due to low nutrient inputs or in lakes with low algal loss rates due to a shortage of algal grazers, warming may indirectly slow herbivore growth despite increased algal biomass.