This dissertation is comprised of three chapters, each dealing with different, but related aspects of the interaction between planktonic herbivores and algae in lakes.

In Chapter 1, I derived predictions about how the magnitude of Daphnia effects on total algal biomass should vary across a gradient of enrichment using two, simple predator-prey models. These predictions were then compared with data from a survey of field experiments in temperate lakes. Algal responses to Daphnia manipulation were quantified as an Algal Response Factor (ARF), defined as total algal biomass in the low-Daphnia treatment divided by total algal biomass in the high-Daphnia treatment. Total phosphorus concentration (TP) was used as an index of algal carrying capacity, ranging from 10-460 ug/l over the 22 experiments surveyed. The Algal Response Factor ranged from 1-40 and was a positive, linear function of TP (ARF = -0.14 + 0.08(TP), rư = 0.81; logARF = -0.81 + 0.83(logTP), rư = 0.75). Thus, algal carrying capacity, as quantified by TP, explains much of the variation in Daphnia effects on total algal biomass across lakes. These results supported the prediction of the simpler model, a two-species, Lotka-Volterra model of pure exploitation. Incorporating the additional complexity of inedible algae into this model did little to improve its predictive power. In addition, a survey of Daphnia effects on the proportionate biomass of inedible algae provided no evidence that Daphnia grazing typically favors dominance by inedible algae in eutrophic lakes.

In Chapter 2, I examined the effect of Daphnia on the distribution and sedimentation of nitrogen and phosphorus in a eutrophic lake. Daphnia manipulation in large enclosures, and whole-lake observations before and after a fish kill, showed that intense Daphnia grazing produces large elevations in particulate N:P ratios during clear-water periods. The direction of this effect was consistent with expected taxonomic variation among zooplankton in the N:P of excretion, and may help to explain Daphnia's suppression of filamentous cyanobacteria (Chapter 3). N and P sedimentation (mg m-2 d-1) was reduced during Daphnia-induced, clear-water periods, despite increases in particle sinking velocities. In addition, there was an unexpected difference between seston N:P and the N:P of settled particles during clear water, which resulted in a differential increase in average sinking velocity calculated for PP relative to PN. Daphnia effects on N:P later in the season appeared to be opposite to those during clear-water. My data suggest that Daphnia grazing can reduce carbonate precipitation (whitings) by controlling algal biomass. Whitings were accompanied by large increases in sedimentary loss rates for TP and elevated TN:TP in the euphotic zone. Thus, Daphnia grazing may maintain relatively low TN:TP during the summer in eutrophic, hard-water lakes. Herbivory can potentially affect the speed and direction of plant succession by favoring the development of a community dominated by grazing-resistant species. In Chapter 3, this idea was tested by examining the effects Daphnia on phytoplankton succession. Daphnia manipulation in large enclosures, and whole-lake observations before and after a fish kill, showed that intense grazing promoted the transition from edible, spring-bloom species to similarly edible, cryptophyte flagellates. In contrast, Daphnia grazing retarded further succession to grazing-resistant, filamentous bluegreens. Thus, the effects of herbivory on algal succession were not predictable from the relative susceptibilities of these algal species to grazing mortality. These results underscore the importance of indirect effects in the herbivore-plant interactions of planktonic communities.