Diel Variation in the Abundance, Vertical Distribution, and Species Composition of Phytoplankton in a Eutrophic English Lake

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(1) Diel variation in the phytoplankton community was studied in a eutrophic lake, Esthwaite Water, during eight sampling periods between June 1977 and September 1978. The abundance of different species of alga in the phytoplankton was recorded, and the effects of environmental conditions (light flux, wind-induced water movements, dissolved oxygen and thermal stratification) on these were studied. (2) The seasonal variation of environmental factors, such as thermal and density stratification, largely controlled the size of the diel variations in biomass concentrations. Under strongly stratified conditions and with little turbulence the amplitude of the diel variations increased, and the influence of many factors was restricted to the upper water layers. (3) High daytime light flux influenced the distribution of the dominant motile algae. Sub-surface population maxima under high surface light flux were also observed for two buoyant species, A nabaenaflos-aquae and A. solitaria. (4) Loss or accumulation of phytoplankton at the sampling site by lateral transport was associated with wind-induced water movements. Wind-induced mixing of the epilimnion also led to a random distribution of non-motile algae and suppression of diel variation. Sinking of senescent and parasitized non-flagellate species was also recorded. (5) Two flagellates, Ceratium hirundinella and Cryptomonas ovata, showed diel vertical migration. (6) Pseudo-diel variations were recognized; these were associated with weather events (e.g. strong winds) with some degree of regularity. These, with other irregular changes related to weather, further complicate the interpretation of diel variations in the phytoplankton community.