Grazers and Nutrients Simultaneously Limit Lotic Primary Productivity

TLDR: Although nutrient effects on periphyton ash-free dry mass were statistically insignificant, nutrient additions significantly increased chlorophyll a, especially where snails were excluded, and Photosynthesis–irradiance data suggested that nutrient enrichment increased self-shading in the peripHYton.

Abstract: Nitrate and phosphate were added to two of four flow through channels in a second-order stream in eastern Tennessee to raise nutrient concentrations to 3–4 times background, while two of four sets of colonized ceramic tiles in each channel were raised above the substratum to exclude grazing snails (Elimia clavaeformis). Snail grazing maintained a thin layer of periphyton dominated by Stigeoclonium basal cells, regardless of nutrient regime. Although nutrient effects on periphyton ash-free dry mass were statistically insignificant, nutrient additions significantly increased chlorophyll a, especially where snails were excluded. Snail densities were 89% higher in nutrient-enriched channels. Photosynthesis–irradiance data suggested that nutrient enrichment increased self-shading in the periphyton. Areal-specific productivity was simultaneously limited by grazing and low nutrient concentrations: snail exclusion and nutrient enrichment both increased productivity > 2 times. The negative effect of snails on area...