Effect of grazing and nutrient supply on periphyton biomass and nutrient stoichiometry in habitats of different productivity

The impact of grazing and nutrient supply on epilithic periphyton was inves tigated in factorial field experiments in four seasons at three Swedish sit es of different productivity and herbivore composition (Lake Limmaren, Lake Erken, and Vaddo, a low salinity coastal site). Nutrient supply was enhanc ed by a granulose fertilizer containing nitrogen (N) and phosphorus (P), an d grazer density was manipulated by exclusion cages. Algal biomass was incr eased by nutrient enrichment and reduced by grazer presence, but effects we re highly variable between sites and seasons. Generally, grazers had strong er effects on algal biomass than nutrient enrichment, but there was no over riding effect of either grazing or nutrients. This indicated a simultaneous top-down and bottom-up control of algal biomass, Taxonomic composition of the periphyton was more affected by grazer presence than by nutrients. Inte rnal nutrient ratios of the algae indicated N limitation at two of the site s. At all sites, the content of N and P in the periphytic assemblage was en hanced by the experimental nutrient enrichment, resulting in decreased C:N and C:P ratios. The presence of herbivores also increased periphytic nutrie nt content (decreased N:P and C:P ratios) in some experiments, suggesting a n increase in algal P due to excretion. The effect strength of grazers and nutrients on periphyton was affected by different abiotic characteristics s uch as light availability, nutrient concentrations, and temperature. Howeve r, single environmental characteristics were not sufficient to explain the relative importance of grazing and nutrients.