The phytoplankton of an enriched, soft‐water lake subject to intermittent hydraulic flushing (Grasmere, English Lake District)

Abstract

SUMMARY. 1. The abundance and species composition of the phytoplankton of Grasmere were monitored over a 12‐year period following the commissioning of a small sewage treatment works, upstream of the lake.2. Although Grasmere has been subject to increased phosphorus‐loading and has quickly developed many features associated with eutrophication, the composition of its plankton has retained the characteristics of a mesotrophic, soft‐water lake: a vernal diatom maximum, generally dominated by Asterionella, is followed by summer growths of nanoplanktonic species, of various colonial Chlorophyceae, before a substantial return to Asterionella‐dominance in the autumn. In cooler summers there have been episodes of renewed diatom growth. followed by truncated recapitulations of the essential summer sequence. Anabaena solitaria was relatively common in drier summers when dissolved inorganic nitrogen fell to low concentrations, although many of the ‘larger’ algal species associated with stratified eutrophic lakes (Aphanizomenon, Microcystis, Ceratium spp.) have failed to become abundant in Grasmere.3. Mean biomass levels (as indicated by chlorophyll concentration) have increased in response to the enrichment, though they do not conform well with most chlorophyll/phosphorus regression‐models.4. The key factor resisting a more complete transition to a typical eutrophic plankton appears to be the efficiency of episodic flushing of the lake during periods of high fluvial discharge sustained by heavy rainfall over the extensive, mountainous catchment. The probabilities that these limnological conditions might fail to deter the development of ‘nuisance’ blooms are briefly assessed.