Effects of Snail Grazing on Benthic Algal Community Structure in Different Nutrient Environments

Abstract

The effects of interaction between nutrient availability and grazing intensity on the structure of benthic algai assemblages were investigated in an ephemeral stream. Small (250 ml) enclosures containing substrata releasing either: 1) no nutrient, 2) 0.5 M KH₂ PO₄ or 3) 0.5 M NaNO₃ were exposed for 19 d to one of five densities of the snail Goniobasis ranging from 0 to 160 snails/m². Algal standing crop (total cell density and community biovolume) on coverslips inside each enclosure decreased with increased grazing pressure in control and nitrate-enriched environments, but increased at low grazing pressure in the phosphate-enriched environment. Responses among constituent populations differed for three reasons: 1) stalked and loosely attached diatoms were more susceptible to snail grazing than firmly attached, prostrate algae; 2) some species were stimulated by phosphorus enrichment and/or grazing while others were not; 3) algal species differed in their response to interactions between grazing and nutrient enrichment. Two alternative hypotheses may explain this interaction: 1) grazers stimulate the growth of understory species by removing overlying cells; 2) excretion of nitrogenous compounds by grazers enhances the effects of phosphorus enrichment alone. Although diversity peaked at intermediate grazer densities in unenriched and nitrate-enriched environments, no significant change was found in phosphate-enriched environments, where the community was dominated either by grazer-sensitive populations that increased faster than they were grazed, or by grazer-resistant algae.