STERNER, Robert W.; FORMAN, Rebecca; HENDRIXSON, Heather; HOOD, James; ZIMMER, Kyle; Univ. of Minnesota; Univ. of Minnesota; Univ. of Minnesota; Univ. of Minnesota; Univ. of Minnesota: Stoichiometric Patterns at the Small and the Large Scale

We undertook several interconnected studies under the auspices of the IRCEB project. We reared Daphnia pulex in conditions of: high and low food quantity, high and low food P content, and four temperatures. Across all treatments, nucleic acids (%RNA and RNA:DNA) were positively associated with growth. Animal body P was not. In contrast, at high food quantity animal P content was associated with growth with a strong temperature dependence. At high food quantity and above 5 C, a Boltzmann term successfully normalized growth rates to produce a single stoichiometry-growth rate relationship. These results contribute to the development of a single macroecological growth function that includes both temperature and stoichiometry, and they help define conditions under which the Growth Rate Hypothesis holds. Scaling up to environments with simple spatial structure, we examined vertical migration of Daphnia in temperature-stratified tubes as a function of stoichiometry. Food quality did influence vertical migration behavior. Animals feeding on low quality foods are less likely to migrate into warmer upper layers. Finally, at large scales, we performed a regionally-based survey, sampling 100+ lakes and ponds of the upper Midwest. Combining these with previous results from Elser et al, we developed a new model for C:N:P in freshwaters. Rather than a single ratio with variation, there are consistent shifts in seston nutrient ratios with changed productivity. An exponential function of C and N vs. P fits the data. These lake and plankton studies are a microcapsule of the entire project and illustrate how ecological stoichiometry can be used to address research problems that span multiple scales.