1. Grasmere is a small lake (area: 0.64 km2; mean depth 7.7 m) in a catchment with high rainfall (typically 2–3 m annually), which subjects the lake to intermittent episodes of rapid flushing.
2. An overview of the trends in water quality and of phytoplankton of Grasmere is presented, covering almost 40 years of observations following the construction and commissioning of a waste-water treatment plant to serve a nearby village.
3. There was a threefold increase in annual areal phosphorus loading during the 25 years following commissioning. Moving the original outfall of the sewage works from the River Rothay, just above its inflow into Grasmere, to a point in the lake, 7.5 m below the surface and, normally, below the summer thermocline, mitigated direct biomass response to epilimnetic enrichment. Maximum concentrations of chlorophyll a increased about twofold, but the annual mean concentrations altered little; interannual variability seems to relate more to flushing than to changing fertility. Cyanobacterial blooms occurred during years of relative summer drought.
4. Since 1996, nutrient loadings have been alleviated by better wastewater management and tertiary treatment of effluents discharged during dry weather. Recent chlorophyll a maxima have declined with the reduced supportive capacity of phosphorus availability.
5. An updated annotated list of phytoplankton species known to have occurred in Grasmere is provided. The incidence and abundance of species supposed to indicate eutrophication probably reflect the enrichment that has occurred, while others, mostly associated with oligo-mesotrophic lakes, seem to have declined. Inherently slow-growing species (of Ceratium, Microcystis) have been unable to establish a significant presence in the lake. A group of algae in Grasmere that indicate nutrient-poor, acidic habitats are suggested to originate in the catchment and flourish briefly in the wake of flushing events.
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