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.