High nutrient availability leads to weaker top‐down control of stream periphyton: Compensatory feeding in Ancylus fluviatilis

Benthic algal biomass and distribution in freshwater ecosystems are determined by both nutrient availability (bottom‐up control) and grazing activity by herbivores (top‐down control). Fluctuations in algal nutrient ratios may cause grazers to optimise their food intake through behavioural strategies in order to maintain a constant soft body stoichiometry. Such linkages between nutrient availability and grazing control of algal biomass are as yet poorly understood. In this study, we tested whether the stream‐dwelling freshwater gastropod Ancylus fluviatilis would increase its food consumption rate with decreasing periphyton nutrient content, a behaviour known as compensatory feeding. We performed a fully factorial microcosm experiment in which two levels of periphyton phosphorus content (low versus high) were crossed with grazer presence/absence in 12 circular flumes. After 1 week of grazing, food consumption rates were measured by determining the periphyton difference between grazed and ungrazed flumes, and the periphyton biomass variability in every flume was described with a coefficient of variation. The food consumption rate of A. fluviatilis was significantly higher in the low phosphorus compared to the high phosphorus treatment, supporting the compensatory feeding hypothesis. As a result, in the presence of grazers, periphyton biomass was significantly lower under low phosphorus availability, while biomass was not affected by nutrient enrichment in the grazer‐free flumes. Despite the strong difference in periphyton phosphorus content, A. fluviatilis soft body stoichiometry did not differ between the two nutrient treatments, suggesting strong stoichiometric homeostasis. Furthermore, the distribution of algal biomass was significantly more heterogeneous in the grazed phosphorus‐poor than in the phosphorus‐rich periphyton. Our findings suggest that nutrient enrichment may lead to a weaker top‐down control of algal biomass in stream ecosystems and to reduced spatial heterogeneity of periphyton abundance.