Patterns of abundance and dominance of the phytoplankton of Rostherne Mere, England : Evidence from an 18-year data set

Rostherne Mere is a small but relatively deep, turbid and highly eutrophic lake. In recent years, the productivity and dynamics of its phytoplankton have been particularly sensitive to the interaction between light income and thermal stability. Analysis reveals that interannual differences in the size and duration of phytoplankton crops, as well as in the predominating species, are not random but that there is a reproducible coherence with weather-generated external forcing. Severe light-limitation restricts phytoplankton development outside the stratified period, though delayed stratification in spring may promote relatively large diatom crops. In summer, Microcystis will usually dominate provided its recruitment period coincides with high water clarity. Otherwise Ceratium generally dominates. Examples of extremely stable stratification leading to Scenedesmus dominance and of persistent episodes of summer mixing favouring Oscillatoria dominance are found to agree well with previous matrix models.