In this dissertation I investigated the nutrition of a carnivorous cladoceran, BYTHOTREPHES CEDERSTROEMI, using both an energetics and a nutrient cycling perspective, and the nutrition of the related herbivorous cladoceran, DAPHNIA PULICARIA(for which energetics are well-understood), from a nutrient cycling perspective. First, radioisotopes were used to measure the bioenergetic parameters ingestion and assimilation for BYTHOTREPHES CEDERSTROEMI fed DAPHNIA PULICARIA, helmeted D. GALEATA MENDOTAE, or unhelmeted D. GALEATA MENDOTAE. B. CEDERSTROEMI were found to ingest 50-60% of a DAPHNIA prey and to assimilate 60-65% of the ingested prey tissue. The helmet of D. GALEATA MENDOTAE did not interfere with the feeding process. These feeding efficiencies, in combination with recent diet analyses and fish behavior studies, imply that the invasion of B. CEDERSTROEMI into Lake Michigan should result in a two-fold decrease in energy transfer to large fish.

Next, the relative carbon:nitrogen:phosphorus (C:N:P) ratios of BYTHOTREPHES CEDERSTROEMI, BOSMINA LONGIROSTRIS, calanoid copepods, cyclopoid copepods and copepod nauplii were determined. Copepod nauplii differed sharply in elemental composition from adults, having the lowest C:P values of any taxon tested. The phosphorus levels found in B. CEDERSTROEMI indicate that cladocera and copepod nauplii should be the highest quality food, consistent with the results of previous diet choice studies.

Finally, changes in DAPHNIA PULICARIA nutritional status were evaluated in response to manipulations of the N:P loading ratio. The regression of dry weight to ash weight ratio versus body length differed for nitrogen and phosphorus-limited DAPHNIA and served as a useful proxy for nutritional status of D. PULICARIA. Mineral limitation was found to depress fecundity, growth and survivorship of D. PULICARIA, but phytoplankton and bacterial growth rates indicated that feedbacks present in more complicated systems may be mediating this limitation and need to be quantified further. This study demonstrates that both energetics and nutrient dynamic approaches can be useful for answering specific questions about individual organisms and for making predictions about how these organisms will interact with other trophic levels.