Grazer control of nutrient availability in the periphyton.

Benthic algal assemblages are regulated by both abiotic (e.g., nutrient) and biotic (e.g., grazing) constraint. The objective of this study was to determine how changes in these two factors affected the structure of an algal assemblage in an ephemeral stream. Coverslips were incubated for 21 days in enclosures containing one of three nutrient environments (ambient, phosphorus-enriched, or phosphorus and nitrogen enriched) and one of four densities of the snail Gonibasis (0, 40, 80, or 120 snails/m2) and examined directly to enumerate the algal assemblage. The effect of grazing on algal biomass was dependent on the nutrient environment. An overstory of diatoms was susceptible to removal by grazing and was not strongly affected by nutrient enrichment. An understory of Stigeoclonium was more resistant to grazing and responded strongly to nutrient enrichment only in the presence of grazers. Snail grazers may mediate nutrient availability to the understory indirectly by removing overlying cells or by direct excretion of nutrients. Multiple interactions occur between benthic herbivores and algae, and, as shown here, some of them are positive and involve modifications of the nutrient environment.