Stoichiometry in Consumer-Resource Interactions Links Between Nutrient Dynamics and Population Demographics

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9421925 Sterner Consumers and their food resources, imbedded in a complex ecological web, do not have identical chemical composition. This, in the conversion of biomass from resources to consumers, considerable chemical rearrangement takes place. Stoichiometry, or mass balance, constrains consumer-resource ilteractions in important ways. The proposed research will extend upon the results so far obtained with an experimental system consisting of nutrient-limited algae and freshwater zooplankton. Results so far indicate that P-limited algae are much inferior to algae from other growth conditions as food for upper trophic levels. Herbivore growth correlates with algal P content. There are several major goals for this renewal project. For one, direct tests for mineral limitation in zooplankton growth will be performed using microencapsulated phosphorus. Tests will be performed under laboratory diets using field algae. Next, the research will compare the results so far limited to Daphnia to other species of zooplankton. Stoichiometry, in the form of the chemical content of different zooplankton consumers, provides a means to make a priori predictions about which species will be most affected by reduced P content in their food. If P content is indeed the important dietary factor in these studies, freshwater copepods, with a low P content in their tissues compared to Daphnia, should be less affected by low P food than Daphnia. A final major goal of this project will be to gather a large amount of data on the internal stoichiometry of the consumer, and develop that into a model of P balance in the animal analogous to bioenergetics models. Elemental contents of somatic tissue, amictic eggs, ephippia (resting eggs), and mots will be measured for several zooplankton species differing in N:P ratio. These results will be compared to current models of bioenergetics to see how differing chemical composition might influence the animal's partitioning of energy and other substances under different food cond itions. %%% This research represents a unique approach to consumer dynamics and food web structure. The chemical basis of resource distribution is poorly known. By focusing on how resource supply and composition within the food base affects the consumer populations, the PI will provide valuable information regarding species interactions in aquatic communities. This kind of information, however, provides an important framework for understanding chemical composition of food resources and nutrition, in general.