Seasonal variation in the epilithic algal communities from four lakes of different trophic state

Abstract

The epilithon of three soft-water lakes in the English Lake District (oligotrophic Wastwater, mesotrophic Coniston Water and eutrophic Esthwaite Water) and of Malham Tarn, a mesotrophic hard-water lake in the Yorkshire Dales, was sampled monthly from January 1997 until December 1998. Samples were analysed for chl-a content and algal community composition with special reference to the diatoms. At the oligotrophic Wastwater the maximum chl-a value occurred in early autumn, but at the other sites spring maxima were observed. The epilithon in all lakes was numerically dominated by diatoms. Achnanthes minutissima was particularly abundant at all the soft-water sites where it grew throughout the year. At Wastwater, Brachysira spp. were also characteristic of the epilithic algal community. The epilithon at Coniston Water contained larger cells of Fragilaria spp. (in 1997) and Gomphonema spp. (in 1998). At Esthwaite Water the community changed markedly between the monthly samples and seemed highly influenced by grazing. Here diatoms were a less important component of the epilithic biomass and filamentous Chlorophyceae were often dominant. The epilithic algal community at Malham Tarn was characterised by a spring bloom of Diatoma spp. and higher numbers of Cymbella spp., but in the summer filamentous green algae contributed most to the biovolume. Analysis of similarity (SIMI) between monthly samples of the epilithic algal communities within each lake over the two year period using the coefficient of variation of the SIMI values, demonstrated increased seasonal variability with increasing productivity.