UK ECN Site. Esthwaite Water is a natural lake situated in a glacial valley and is generally agreed to be the most productive or eutrophic lake in the English Lake District. It lies approximately 65 m above sea level and has an area of 1 km2 and a maximum depth of 15.5 m. The average retention time is 90 days. The catchment area is 17.1 km 2 and the hills are composed geologically of Bannisdale slates and grits. The surrounding land is used chiefly for agricultural purposes and forestry. The lake is a grade 1 Site of Special Scientific Interest and has been a designated "Ramsar" site since November 1991. The diverse aquatic invertebrate fauna includes a number of species with restricted distributions in Britain, one of which is the flatworm, Bdellocephala punctata. The slender naiad, Najas flexilis, which is listed as Nationally Scarce, has been found in Esthwaite Tarn. Artificial enrichment of the lake occurs by input from the Hawkshead Sewage Treatment Works (which has operated a continuous programme of phosphate stripping since 1989) and originally from effluents from the fish farm which used to be situated towards the south of the lake. The lake undergoes summer stratification with oxygen depletion regularly below 7 m and sometimes as shallow as 5 m. The phytoplankton tends to be dominated by diatoms in spring and by cyanobacteria for much of the summer.