THE SIZE STRUCTURE OF PLANKTON DURING A DEEP BLOOM IN A STRATIFIED RESERVOIR

In this paper we describe the main features of planktonic size structure in a stratified reservoir during a summer, deep bloom due to the dinoflagellate Ceratium hirundinella (O. F. Muller) Bergh. As one might expect from the eutrophic character of the ecosystem, the complete size-biomass spectrum is dominated by large phytoplankton and small zooplankton. The relation between numerical abundance (N, organisms per ml) and body size (V, mum3) for the overall size spectrum is described by the model log N = 7.25-0.99 99 log V (r2 = 0.93). However, the existence of tendencies in the points around the regression line suggest limiting linear scaling to the size range of unicells (log N = 6.34-0.51 log V, r2 = 0.85). The size structure shows the less negative slope at the level of the thermocline and when the bloom of C. hirundinella reaches its maximum. In the epilimnion, important taxonomic changes have no effect on the size structure, which appears to be a conservative character of the planktonic community at this level.