In order to study relationships between seston food quality and zooplankton, especially Daphnia dynamics in freshwater pelagic ecosystems, three independent studies were conducted. The first study was performed to examine the relation between seston food quality and biomass transfer efficiency between primary producers and herbivores in Castle Lake. To study how much of the variation in Daphnia growth can be explained by two different food quality indices, Daphnia growth experiments were performed using a flow-through system in controlled environment. Finally, pelagic planktonic food web components and food quality indices were monitored in Lake Berryessa to examine the roles of seston food quality in pelagic planktonic food webs.

The first study showed Daphnia rosea and total macrozooplankton growths were dependent on primary productivity while the biomass transfer efficiencies, the ratio of secondary production to primary production, were dependent on phytoplankton species composition change. Life table experiments of D. rosea showed highly unsaturated fatty acids (HUFA) facilitated growth, reproduction, and survival of D. rosea, indicating HUFA is probably an important factors in determining seston food quality.

In the second study, algal essential fatty acid content alone could explain 71% of Daphnia magna growth while algal essential fatty acid and carbon to phosphorus ratio (C:P) together explained 72% of D. magna growth. These results clearly show that essential fatty acid content is important in algal food quality regardless of algal P limitation and also that phosphorus content in algae plays a significant role in algal food quality under P limitation.

The monitoring study at Lake Berryessa showed the essential fatty acid content in seston explained most of Daphnia c.f. pulex and macrozooplankton biomass change in spring time, and biomass transfer efficiency between phytoplankton and zooplankton. Comparison of potential growth of Daphnia pulex obtained with growth experiments and Daphnia c.f. pulex dynamics in the lake showed that Daphnia c.f. pulex dynamics was dependent on seston food quality in the spring as well as other seasonal factors in Lake Berryessa.