Biological response to lake remediation by phosphate stripping: control ofCladophora

1. The North and South Basins of Windermere, Cumbrial have experienced a la rge increase in concentrations of nutrients, particularly phosphate, since 1945 when detailed measurements began. Over-winter concentrations have incr eased from 1 to 3 mg PO4-P m(-3) in the 1940s, up to 30 mg PO4-P m(-3) in t he South Basin of Windermere in the early 1990s where nutrient enrichment h as been most marked. A visible manifestation of this 'eutrophication' in re cent years has been the production of a large biomass by the green filament ous macroalga, Cladophora. 2. Since April 1992, tertiary chemical stripping of phosphate at the two se wage treatment plants on Windermere has reduced direct sources of phosphate to both basins. In the South Basin, over-winter concentrations of phosphat e have fallen to values similar to those in the early 1970s. 3. The biomass of Cladophora has declined markedly in response to the reduc ed phosphate availability. Significant relationships were found between the annual maximum biomass of Cladophora and two measures of phosphate availab ility: the over-winter concentration and, more strongly, the day of year wh en the concentration fell below 1 mg m(-3) 4. The annual biomass maxima of Cladophora since 1945, estimated from the r egressions, showed a gradual increased potential for biomass production aft er 1965 as phosphate concentrations increased, followed by a striking and r apid biological response to lake remediation by phosphate stripping.