BACTERIOPLANKTON INTERACTIONS WITH DAPHNIA AND ALGAE IN EXPERIMENTAL ENCLOSURES

Development of bacterioplankton was studied by manipulation of plankti vorous fish and/or nutrients in experimental enclosures in a fish pond . Grazing pressure exerted by large zooplankton (Daphnia galeata and D aphnia pulicaria) strongly influenced the counts and size distribution of bacterial populations. Morphometric analyses by scanning electron microscope revealed a shift in size distribution from larger mainly ro d-type bacteria under low grazing pressure towards smaller mainly cocc us-type under strong grazing pressure. The metabolic activity of bacte ria measured as glucose uptake was higher under strong grazing pressur e. After removal of large daphnids, the increase in bacterial density was probably the result of two additive factors: low grazing pressure and high level of dissolved organic matter (DOM) due to photosynthetic activity of more abundant algae. Composition of bacterial populations shifted toward larger, rod-type bacteria, and their metabolic efficie ncy measured by uptake, was lowered. The basic dimensionality of the s ystem and interactions between variables was describe by R-mode factor analysis. The manipulated enclosures were relate with factor score.