Grazing minnows increase benthic autotrophy and enhance the response of periphyton elemental composition to experimental phosphorus additions

Excessive nutrient inputs and grazers can influence biomass and elemental composition of primary producers in freshwater ecosystems. How interactions between nutrient enrichment and grazing fish alter benthic habitats through effects on periphyton autotrophy, biomass, and elemental composition has been studied rarely. We compared the effects of grazing by central stonerollers (Campostoma anomalum) on autotrophic and total periphyton biomass, sediment mass, and C, N, and P stoichiometry of periphyton in 12 flow-through stream mesocosms randomly assigned to 1 of 3 different PO4-P concentrations (control: 8 mu g/L, low:20 mu g/L, high:100 mu g/L). Fish grazing suppressed periphyton ash-free dry mass (AFDM) and sediment accumulation, regardless of P treatment. However, grazing also increased the proportion of algal biomass in the periphyton, evidenced by a reduction in benthic C:chlorophyll a on grazed substrates. The response of periphyton stoichiometry to experimental P enrichment was stronger on grazed substrates because central stonerollers maintained a higher proportion of algae in the periphyton matrix. Grazing enhanced the response of P standing stocks to enrichment, reduced C:P and C:N in high-P streams, and increased N:P in control and low-P streams. Shifts from detritus-and sediment-bound nutrients to algal resources probably increase the palatability of benthic food resources and nutrient availability for other grazing organisms. Grazing fish may play a stronger role in benthic processes, such as nutrient cycling, than is currently recognized. Our results suggest that fish drive periphyton toward autotrophy, enhance sequestration of excess nutrients in periphyton and, thus, may relax stoichiometric constraints on fast growing organisms in stream communities.