Simulating the effects of biomanipulation on the food web of Lake Ringsjon

A dynamic, process-oriented, deterministic and phosphorus-based model was d eveloped to simulate the food web dynamics of Lake Ringsjon, in particular the long-term effects of biomanipulation in terms of reduction of omnivorou s fish. The model contains 14 state variables, each with a differential equ ation describing sources and sinks of phosphorus. The state variables encom pass piscivorous and omnivorous fish, zooplankton, phytoplankton, sediment and lake water. The model simulates densities of fish and phytoplankton ade quately, both before and after biomanipulation, although the actual lake ph ytoplankton density varied more year-to-year compared to the model predicti ons. According to the model, a biomanipulation will cause an increase in zo oplankton biomass. This prediction contradicts available field data from th e lake which do not indicate any significant change in zooplankton biomass resulting from the performed biomanipulation. This discrepancy may partly b e attributed to structural uncertainties in the model, related to the size structure of predators on zooplankton, i.e. the omnivorous fish community. The simulations suggest that phosphorus was routed along the pelagic food c hain to a larger extent after omnivorous fish were removed, whereas the amo unt of phosphorus routed via the sediment and benthivorous fish decreased f ollowing fish removal. Accordingly, translocation of phosphorus from sedime nt to water by benthivorous fish is predicted to be substantially reduced b y biomanipulation, resulting in an overall reduction in the release of new phosphorus to phytoplankton. Irrespective of simulated fishing effort (redu ction of less than or equal to 0.5% d(-1) for two years), the model predict s that P-release from the sediment and the external load will remain suffic iently high to force the system back to its previous state within a decade. Thus, recurrent biomanipulations and/or combined abatement strategies may be necessary to maintain low phytoplankton density. Known structural model uncertainties may however affect the robustness of such detailed prediction s about the system resilience.