Selective feeding and differential nutrient excretion by aquatic invertebrates plays a substantial role in nutrient recycling. Grazing larvae of the caddisfly Glossosoma intermedium construct a portable case for protection that also serves as a good substrate for periphyton colonization because it is constantly fertilized by larval excreta. We tested whether case periphyton was nutrient enriched compared to streambed periphyton and whether selective feeding by caddisfly larvae on case periphyton facilitates P remineralization. We measured total N, total P, and N:P in stream water and streambed cobble periphyton in 3 western Wisconsin streams. We collected larvae and measured N and P content and N:P of case periphyton, G. intermedium body, and G. intermedium excretion products. Cobble periphyton N:P at 2 streams suggested P limitation (368 ± 109 and 50 ± 10), but case periphyton N:P at those streams did not (11 ± 3 and 11 ± 0.8, respectively). Neither cobble nor case periphyton N:Ps suggested P limitation at the 3rd stream. Measured excretion N:P was more similar to the excretion N:P predicted for caddisfly grazing on case periphyton than for caddisfly grazing on cobble periphyton. Our results suggest that larval excretion alleviated P limitation of case periphyton and that case periphyton may serve as an important dietary resource for the grazing caddisfly larvae. Feeding on this P-rich case periphyton promotes P remineralization in P-limited, lotic ecosystems.
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Vol. 33 • No. 4