Stoichiometric Linkages Between Grazing Minnows and Periphyton Nutrient Content

Grazers influence periphyton elemental composition via direct consumptive effects and indirect bioturbation or nutrient recycling effects, but the influence of these two factors can vary depending on water column nutrients. We synthesize the results of two stream mesocosm experiments designed to examine interacting effects of nutrient enrichment and grazing by central stonerollers on stream periphyton biomass and elemental composition. In the 1^st experiment we compared direct grazer effects across three PO₄-P concentrations (8, 20, and 100 µg/L). Stonerollers decreased sediment and detritus and increased the proportion of algal biomass in periphyton, resulting in stronger elemental responses to experimental P enrichment on grazed substrates. In the 2^nd experiment we compared fish-mediated nutrient recycling effects in streams with low (11) and high (177) surface-water dissolved N:P molar ratios. In high N:P streams, downstream changes in periphyton nutrient ratios were greater in streams with than without fish, and chl. a significantly increased downstream of enclosures with fish. These results demonstrate that stonerollers influence periphyton elemental composition through several grazer-induced pathways, but highlight the influence of water column nutrients in these interactions. Additional studies that isolate stoichiometric links between fish and benthic resources and examine additional interacting factors are needed.