CASCADING TROPHIC INTERACTIONS FROM FISH TO BACTERIA AND NUTRIENTS AFTER REDUCED SEWAGE LOADING - AN 18-YEAR STUDY OF A SHALLOW HYPERTROPHIC LAKE

The effects of major reductions in organic matter, total phosphorus (T P), and total nitrogen (TN) loading on the chemical environment, troph ic structure, and dynamics of the hypertrophic, shallow Lake Sobygard were followed for 18 years. After the reduction in organic matter load ing in 1976, the lake initially shifted from a summer clear-water stat e, most likely reflecting high grazing pressure by large Daphnia speci es, to a turbid state with extremely high summer mean chlorophyll a (u p to 1400 mu g L-1), high pH (up to 10.2), and low zooplankton grazing . Subsequently, a more variable stale with periodically high grazing r ates on phytoplankton and bacteria was established. Changes in zooplan kton abundance and grazing could be attributed to variations in cyprin id abundance due to a fish kill (probably as a consequence of oxygen d epletion) and pi-I-induced variations in fish recruitment and fry surv ival. The results suggest strong cascading effects of fish on the abun dance and size of zooplankton and phytoplankton and on phytoplankton p roduction. A comparatively weak cascading effect on ciliates and bacte rioplankton is suggested. Due to high internal loading, only minor cha nges were observed in lake-water TP after a reduction in external TP l oading of approximately 80% in 1982; net retention of TP was still neg ative 13 years after the loading reduction, despite a short hydraulic retention time of a few weeks. TN, however, decreased proportionally t o the TN-loading reduction in 1987, suggesting a fast N equilibration. Only minor improvement in the environmental state of the lake has bee n observed. We suggest that another decade will be required before the lake is in equilibrium with present external P loading.