EXPERIMENTAL MICROCOSM STUDY OF THE EFFECTS OF PHOSPHORUS REDUCTION ON PLANKTON COMMUNITY STRUCTURE

The effects of total phosphorus reduction on a lake community were inv estigated in a microcosm study involving 15 1000-L tanks containing na tural plankton assemblages. A 28-day replicated gradient-design tank s tudy was conducted, which included five treatments with three replicat es each. (i-iii) three treatment levels of ferric sulfate, a phosphoru s inactivant, (iv) one treatment of 6 M sulfuric acid, and (v) a no-ad dition treatment. The acid treatment served as the control for the fer ric sulfate treatments because ferric sulfate reduces pH. Total phosph orus, total nitrogen, and chlorophyll a concentrations and light atten uation at 1 m were significantly lower in all ferric sulfate treatment s than in the acid treatment. Effects on the phytoplankton community i ncluded lower total phytoplankton biomass and a shift from nearly 100% cyanophyte dominance toward dominance of diatoms and dinoflagellates. Effects on the zooplankton community consisted of a shift from cladoc erans and nonpredatory rotifers to calanoid copepods at all ferric sul fate levels and lower total zooplankton biomass in the tanks with high ferric sulfate levels. These results suggest that the effects of eutr ophication on plankton communities are reversible and predictable on t he basis of the opposite predictions of enrichment.