Currently two different ecological frameworks are used to explain multitrophic interactions: 1) Ecological Stoichiometry (ES) which explains consumer responses to different food qualities and consumer effects on prey nutrient composition and 2) Metabolic Theory of Ecology (MTE) which focuses on body mass and temperature as determinants of metabolic rates, such as growth and consumption rates which control population and community productivity. However, ES and MTE have rarely been applied in the context of Biodiversity and Ecosystem Functioning (BEF), or in combination so far. In the present study we combined both theories to investigate interactive effects of temperature, nutrient composition, and consumer body mass on trophic dynamics in a marine plankton food web, using two different zooplankton consumers (a small ciliate and a lager rotifer) and phytoplankton prey in a microcosm experiment. After an initial phytoplankton growth phase at 3 different temperatures (12°C, 18°C and 24°C) and 6 different nutrient regimes (altering nitrogen and phosphorus, N:P), consumers were added alone or in combination to determine consumer grazing rates and production. A simple mathematical model including parameters from ES and MTE was used to compare experimental results with theoretically expected dependencies of metabolic rates on temperature, consumer body mass and food stoichiometry.
Results/Conclusions
In the absence of consumers, phytoplankton biomass increased with temperature and with nutrient (mainly N) supply, but only at lower temperatures, while biomass was not affected by nutrients at 24°C. Algal C:nutrient ratios increased with temperature and with decreasing N and P supply. Consumer grazing rates and production generally increased with temperature as predicted by MTE, reflecting increased metabolism at higher temperature. Grazing rates also increased with decreasing algal food quality due to compensatory feeding, while consumer growth rates and final biovolume decreased due to poor food quality as predicted by ES. Nutrient effects on consumer biovolume increased with temperature, while nutrient effects on grazing rates decreased. In this context, temperature and food quality effects were much stronger for the smaller consumer, which exhibited higher grazing and growth rates compared to the larger one. Overall, our study demonstrated highly interactive effects of temperature, nutrient supply and consumer body mass. Our model was able to explain most of the observed patterns, supporting that the combination of ES and MTE is a powerful tool to make predictions about the effects of temperature and nutrient quality on herbivore dynamics, which is highly important in the context of global change.