RESPONSES TO EXPERIMENTAL FISH MANIPULATIONS IN A SHALLOW, HYPEREUTROPHIC LAKE - THE RELATIVE IMPORTANCE OF BENTHIC NUTRIENT RECYCLING AND TROPHIC CASCADE

Mesocosms were used in a shallow hypereutrophic lake, and three treatments were established in duplicate: (a) Fish - stocked with native fish at their natural relative densities, (b) Fish/Net - stocked with fish, but with a net blocking access to the sediments, and (c) No Fish. Total P and chlorophyll concentrations reached 140 and 170 mug l-1 in the Fish treatment. Total P was below 40 mug l-1 in both the Fish/Net and No Fish treatments. However, despite the nearly identical total P concentrations, chlorophyll averaged 40 mug l-1 in the No Fish treatment, where zooplankton biomass was high and the large cladoceran Moina was abundant, and 50 mug l-1 in the Fish/Net treatment, where zooplankton biomass was low and large herbivores were rare. The results showed that both benthic nutrient recycling and cascading trophic interactions were regulators of algal biomass, but that the former effect was by far the more important, accounting for roughly 60% of the chlorophyll increase associated with the presence of fish.