Consumption patterns and excretion in aquatic food websenrichment

Sammanfattning: This thesis links biogeochemical cycling to classical ecology. Predictions derived from food chain theory, based on Lotka-Volterra type of interactions, were tested in aquatic environments with the main hypothesis that consumers affect lower levels of organization by both predation and excretion of limiting nutrients. From tank experiments it was concluded that predation may affect prey populations in agreement with food chain theory, but that species replacement within the prey community confounds predictions based on trophic level dynamics. By describing ecosystems as a matrix of food web interactions, and by recognizing the interplay between competition and predation, a more complete description of the ecosystem function was obtained compared to when species were placed into distinct trophic levels. Results from experimental studies showed that the effects of fish on nutrient cycling could provide a complement to zooplanktivory in explaining algae dynamics. It was argued that the concept of carrying capacity, as used in food chain theory, prevents an adequate understanding of cascading trophic interactions. Hence, producer growth rate is determined by the availability of nutrients, which is partly set by the recycling by consumers, rather than the total amount of nutrients present in a system. Determination of the relative importance of phosphorus (P) excretion by fish in comparison to other P sources was performed for a hypertrophic lake. The results showed that excretion by fish may be of the same magnitude as the external load, but it did only explain a minor part of the internal load. In contrast, model simulations of the P dynamics of a hypertrophic lake showed that excretion of P by fish could be a significant part of the total P supply to phytoplankton growth, mostly due to translocation of P from the sediment to the water by benthic feeding fish. The simulations suggested that P was routed along the pelagic food chain to a larger extent if omnivorous fish biomass was reduced (biomanipulation), whereas the amount of phosphorus routed via the sediment and benthivorous fish decreased after biomanipulation. However, field studies in a hypertrophic lake showed that the overlap in resource utilization within the fish community was considerable, resulting in compensatory responses when the fish community was manipulated. Such responses may offset the anticipated effects of biomanipulation, a method proposed for the restoration of eutrophicated lakes.