TROPHIC INTERACTIONS IN A SHALLOW LAKE FOLLOWING A REDUCTION IN NUTRIENT LOADING - A LONG-TERM STUDY

After the diversion of a nutrient-rich inflow, the eutrophic lake, Ald erfen Broad, initially showed reduced total phosphorus concentrations and phytoplankton populations, clear water and the establishment of su bmerged macrophytes. Internal P loading then increased, perhaps stimul ated by the senescence of submerged macrophytes and exacerbated by the lack of flushing. Cyanophytes appeared in the summer of two years. As a consequence of poor recruitment of roach (Rutilus rutilus (L.)), th e chief zooplanktivore, and a summerkill of the fish population, popul ations of large-bodied Cladocera (Daphnia hyalina/longispina and ultim ately D. magna) developed. In the long-term, these may have limited th e further development of phytoplankton populations and clear water and submerged macrophytes returned. During this latter period, internal P release has remained high (> 3 80 mug l-1), thereby indicating the sc ope for biomanipulation even in eutrophic conditions. However, isolati on of the lake has led to a decrease in water level (which through inc reased temperatures and lowered dissolved oxygen levels was probably r esponsible for the fish deaths) and further concentration of internal P load. Sediment is now being removed to reestablish greater water dep th.