Gastropod grazers affect periphyton nutrient stoichiometry by changing benthic algal taxonomy and through differential nutrient uptake

Benthic grazers can affect the nutrient stoichiometry of periphyton through different pathways: 1) grazers can reduce periphyton biomass, thereby increasing the relative amount of nutrients for the remaining periphyton; 2) grazers can change benthic algal community composition toward grazer-resistant taxi that might differ in their C:N:P stoichiometry from less resistant benthic algae; 3) grazers can differentially recycle or incorporate nutrients, thereby changing periphyton nutrient content; and 4) mechanical removal of high-C detritus by grazers can decrease C:nutrient ratios. Nutrient-mediated grazer effects on periphyton nutrient stoichiometry depend on the relative availability of nutrients in the system in relation to the nutrients made available through grazing. We tested the relative importance of the different pathways under differing nutrient conditions in a 3-wk laboratory experiment in which grazer presence and dissolved N and P concentrations were manipulated. The effects of the gastropod grazer Viviparus viviparus (L.) on periphyton nutrient stoichiometry, algal taxonomy and biomass, and dissolved nutrients were determined with a 2 X 4 factorial design. Factors were grazers (+/-) and nutrient addition (ambient, +N, +P, and +NP). Grazers decreased periphyton C:P and N:P in the +N and +P treatments. Grazers mainly affected periphyton nutrient stoichiometry by changing benthic algal taxonomic composition because the percentage of mucilage-producing algae Chaetophora spp. was much higher in grazed than in ungrazed treatments. Mucilage has high C and N content, but low P content. Grazer stoichometry indicated that grazers might have retained more P in the +N and +P treatment than in the other nutrient treatments. We conclude that nutrient addition interacted with grazer effects on periphyton nutrient stoichiometry and that grazer effects on periphyton nutrient stoichiometry can be explained only by considering the combined effects of nutrient availability, differential nutrient recycling by grazers, and grazer-induced changes in algal taxonomy.