SPECIES-SPECIFIC PHYTOPLANKTON RESPONSES TO NUTRIENTS AND ZOOPLANKTONMANIPULATIONS IN ENCLOSURE EXPERIMENTS

1. In situ enclosure experiments were performed in the mesotrophic Ber mejales reservoir to evaluate the algal response to changes in the nut rient supply and in the zooplankton size structure and density in a 2 x 2 factorial design. The experiments were conducted during the spring bloom of nanoplanktonic diatoms in 1989. 2. Nutrient enrichment promo ted a great increase of phytoplankton biomass indicating a strong nutr ient limitation on phytoplankton growth. Total phytoplankton biomass w as significantly lower in the Daphnia-added enclosures at a given nutr ient level and strong direct an indirect effect of zooplankton on phyt oplankton community structure and nutrient availability were observed. 3. Most of the nanoplanktonic species were effectively grazed but spe cies with protective coverings and large size colonies were favoured b y grazers and small chlorococcales were unaffected probably because of their compensatory high growth rates. The decrease in total biomass i mposed by grazers is attributable mainly to the decrease of Cyclotella ocellata, the most abundant species. This taxon suffers two net effec ts of zooplankton: direct grazing and the indirect decrease of Si avai lability caused by the growth of C. ocellata which was promoted by P e xcretion by zooplankton. Indirect effects of grazers on Si availabilit y should, therefore, be taken into account in explaining phytoplankton succession and community structure. 4. In this experiment grazers aff ected considerably the nanoplanktonic community in Bermejales reservoi r. The extent which they were affected, however, depended not only on the algal size as a determinant of edibility but also greatly on the s pecific nutrient requirements and taxonomic features of the algal spec ies.