Species-specific algal responses to zooplankton: Experimental and field observations in three nutrient-limited lakes

A series of 4-day manipulations of zooplankton biomass and nutrient availability was performed in enclosures in three lakes to determine species-specific algal responses to herbivory and nutrient enrichment. Algal performance in enclosures was compared to the relationships between weekly algal growth rates and the zooplankton in situ. When in situ growth rates were significant functions of zooplankton biomass, the responses were generally consistent with responses in the enclosure experiments. The importance of both nutrients and zooplankton in mediating algal growth was demonstrated by numerous observations: strong algal community response to enrichment, unimodal or positive responses of certain algal taxa to zooplankton biomass, differences in degree of nutrient limitation among the algal response types, lack of nutrient limitation of non-grazed algal taxa and a preponderance of taxa with no net response to increasing zooplankton biomass. Variation in the zooplankton community may be the largest source of variability in nutrient supply rate during summer in stratified lakes, and causes substational variability in the algae. Algae responded more strongly to changes in zooplankton composition than to changes in zooplankton biomass. We conclude that, due to the close coupling of phytoplankton and zooplankton communities in these nutrient-limited lakes, major compositional changes in the zooplankton have greater effects on the algae than do changes in biomass of grazers already present.