Grazer–periphyton interactions are shaped, in part, by indirect effects of nutrient regeneration. They are an important model system with which to test predictions of ecological stoichiometry and the Growth Rate Hypothesis. We conducted a laboratory experiment to test how nutrient enrichment and grazer identity interact to regulate the nutrient content and stoichiometry of both periphyton and consumers. We considered a situation in which P concentration in the water column was high, in contrast to previous experiments in which P was the limiting nutrient. We added N and P, alone and in combination, to the water in experimental aquaria that contained periphyton communities on clay tiles and grazers (1 of 3 snail species) or no grazers (ungrazed control). Benthic algae incorporated nutrients in close proportion to their availability in each nutrient treatment. Algal biomass increased significantly with N P enrichment, but not with N or P enrichment alone. Grazers had no effect on periphyton C:N ratios and positive effects on periphyton C:P and N:P ratios. P content of grazers (% dry mass) increased and C:P and N:P molar ratios of grazers decreased in response to N enrichment of the water. Grazer P content increased in response to N enrichment, probably because of increased grazer growth rates. We hypothesize that the addition of N under N-limiting conditions led to increased P uptake or retention by grazers because of high growth rates and RNA production, consistent with the Growth Rate Hypothesis.
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