Effects of enrichment on simple aquatic food webs

Simple models, based on Lotka-Volterra types of interactions between predat or and prey, predict that enrichment will have a destabilizing effect on po pulations and that equilibrium population densities will change at the top trophic level and every second level below. We experimentally tested these predictions in three aquatic food web configurations subjected to either hi gh or low nutrient additions. The results were structured by viewing the sy stems as either food chains or webs and showed that trophic level biomass i ncreased with enrichment, which contradicts food chain theory. However, wit hin each trophic level, food web configuration affected the extent to which different functional groups responded to enrichment. By dividing trophic l evels into functional groups, based on vulnerability to consumption, we wer e able to identify significant effects that were obscured when systems were viewed as food chains. The results support the prediction that invulnerabl e prey may stabilize trophic-level dynamics by replacing other, more vulner able prey. Furthermore, the vulnerable prey, such as Daphnia and edible alg ae, responded as predicted by the paradox of enrichment hypothesis; that is , variability in population density increased with enrichment. Hence, by de scribing ecosystems as a matrix of food web interactions, and by recognizin g the interplay between interspecific competition and predation, a more com plete description of the ecosystem function was obtained compared to when s pecies were placed into distinct trophic levels.