The relationship between species diversity and ecosystem functioning is a central topic in ecology today. Classical approaches to studying ecosystem responses to nutrient enrichment have considered linear food chains. To what extent ecosystem structure, that is, the network of species interactions, affects such responses is currently unknown. This severely limits our ability to predict which species or functional groups will benefit or suffer from nutrient enrichment and to understand the underlying mechanisms. Here our approach takes ecosystem complexity into account by considering functional diversity at each trophic level. We conducted a mesocosm experiment to test the effects of nutrient enrichment in a lake ecosystem. We developed a model of intermediate complexity, which separates trophic levels into functional groups according to size and diet. This model successfully predicted the experimental results, whereas linear food-chain models did not. Our model shows the importance of functional diversity and indirect interactions in the response of ecosystems to perturbations, and indicates that new approaches are needed for the management of freshwater ecosystems subject to eutrophication.

Summary1. Dispersal intensity is a key process for the persistence of prey-predator metacommunities. Consequently, knowledge of the ecological mechanisms of dispersal is fundamental to understanding the dynamics of these communities. Dispersal is often considered to occur at a constant per capita rate; however, some experiments demonstrated that dispersal may be a function of local species density. 2. Here we use aquatic experimental microcosms under controlled conditions to explore intra-and interspecific density-dependent dispersal in two protists, a prey Tetrahymena pyriformis and its predator Dileptus sp. 3. We observed intraspecific density-dependent dispersal for the prey and interspecific density-dependent dispersal for both the prey and the predator. Decreased prey density lead to an increase in predator dispersal, while prey dispersal increased with predator density. 4. Additional experiments suggest that the prey is able to detect its predator through chemical cues and to modify its dispersal behaviour accordingly. 5. Density-dependent dispersal suggests that regional processes depend on local community dynamics. We discuss the potential consequences of density-dependent dispersal on metacommunity dynamics and stability.

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Toxin-producing phytoplankton species may compensate for competitive disadvantages by secreting chemicals that affect toxin-sensitive phytoplankton species. Heterotrophic bacteria, however, may, in turn, degrade the toxins produced by allelopathic phytoplankton, thus confounding allelopathic interactions between phytoplankton species. Moreover, recent theoretical studies suggest that incomplete mixing of the water column might also modify allelopathic interactions. Here we analyze a model where phytoplankton species, bacteria, nutrients, and a toxin are linked through material cycling. The model considers a toxin-producing and a toxin-sensitive phytoplankton species and two species of heterotrophic bacteria. The model is analyzed for two scenarios: a simple well-mixed aquatic ecosystem is contrasted with an aquatic ecosystem with low mixing intensity. The results show that (1) the winner of competition between toxin-producing phytoplankton and toxin-sensitive phytoplankton species may depend on the species that becomes dominant first, (2) heterotrophic bacteria able to degrade allelopathic toxins will facilitate dominance of toxin-sensitive phytoplankton species, (3) heterotrophic bacteria unable to degrade allelopathic toxins may to some extent counter the facilitating effect of toxin-decomposing bacteria, due to competition between the bacterial species, and (4) there is a major effect of mixing intensity on these species interactions: when turbulent mixing rates are low, toxin concentrations are less diluted whereas degradation activities of heterotrophic bacteria are more localized. As a result, this model study predicts that weak mixing, especially when combined with the presence of bacteria unable to degrade allelopathic compounds, will favor the development of allelopathic phytoplankton populations.