Effects of Caddisfly Grazers on the Elemental Composition of Epilithon in a Boreal Lake

Abstract

We examined how the intensity of benthic grazing affects the C:P stoichiometry of epilithon in an oligotrophic lake at the Experimental Lakes Area in northwestern Ontario, Canada. Caddisflies were enclosed over natural epilithon to compare epilithic C:P ratios along a grazer gradient (0, 1, 4, and 9 individuals/700 cm 2 enclosure) after 10 and 20 d. We also examined mechanisms that potentially explain changes in epilithic C:P ratios at different grazer levels by estimating algal biomass and taxonomic composition. After 10 d, no differences in epilithic C:P ratios were found among grazer levels. However, after 20 d, epilithic C:P ratios (∼500) at high grazer levels (9/enclosure) were significantly lower than C:P ratios (∼1200) in grazer-free enclosures. We also found that algal biomass was significantly reduced under intense grazing. No significant effects of grazer level were detected on epilithic C:chlorophyll ratios, the % contribution of algal C to epilithic organic C, or the taxonomic compos...