REU: The N:P Stoichiometry of Nutrient Release by Daphnia

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Aquatic herbivores utilize a food base that can vary widely in its C:N:P ratio. The animals adjust their assimilation of N and P so that the element in shorter supply is assimilated more efficiently. The hypothesis of this proposal on the stoichiometry of grazing, assimilation and release of nutrient predicts that grazers make nutrient limitation patterns diverge, leading their algal prey away from co-limitation by N and P and toward limitation by either N or P. A Mass balance technique will allow ingestion, assimilation and re-release of N and P to be measured simultaneously for Daphnia faced with different concentrations of algae of differing C:N:P. Determinations of tissue C:N:P of Daphnia that have been reared on algae of different C:N:P will also be performed. This project will determine the recycling efficiencies of N and P and whether nutrient concentration ratios are related to supply ratios, as is commonly assumed. Also, it will explore fundamental consequences of food web dynamics in aquatic ecosystems while providing badly needed basic data on the N and P physiology of a major aquatic organism.