Two experiments in aquatic mesocosms were undertaken in 1991 and 1992 to assess the impact of fish on planktonic populations and their recovery. Five treatments (n=2) were applied in oligotrophic Wolf Lake (New York): (1) high fish biomass (30 kg/ha), (2) low fish biomass (10 kg/ha), (3) high removal of zooplankton, (4) low removal of zooplankton and (5) control. TP, chlorophyll a, zooplankton biomass, and zooplankton species richness decreased from high fish > low fish > control > low removal > high removal treatments. A phosphorus bioenergetic model revealed that fish excretion in the high fish treatments was responsible for the highest levels of TP observed (3.6 times higher than the control). Zooplankton excretion was not a major contributor to the phosphorus pool when fish were present. The main role of fish was to modify nutrient cycling. These results suggest that in oligotrophic systems, planktivorous fish are significant nutrient sources and that bottom-up forces structure the planktonic community.

Six treatments (n=2) were applied to oligotrophic Ranger Lake (Ontario) in a factorial design. Half of the mesocosms were enriched by adding nutrients to double natural lake concentrations. The treatments were: (1) oligotrophic high fish biomass (161 kg/ha), (2) oligotrophic low fish biomass (47 kg/ha), (3) oligotrophic control, (4) eutrophic high fish biomass, (5) eutrophic low fish biomass, (6) eutrophic control. The experiment was divided into three periods: (a) before addition of fish, (b) during fish presence, and (c) after fish were removed. These three periods were compared to evaluate fish impacts and zooplankton recovery. TP and ammonia increased in the fish mesocosms, indicating an effect of fish on nutrient levels. Planktonic communities with low levels of fish were more resistant to changes in density in both nutrient environments. However, the populations in the eutrophic mesocosms recovered faster from fish predation. The most omnivorous group of zooplankton had high levels of interaction and contributed to community recovery. Planktonic populations in the eutrophic mesocosms were more persistent over time. Oligotrophic systems were relatively more dynamic and had ample stability domains. Differential nutrient recycling rates for oligotrophic and eutrophic systems may be the cause underlying the observed recovery rates.