Effects of caddisfly grazers on the elemental composition of epilithon in a boreal lake

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 cm² 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 composition of algal communities after 10 or 20 d. In addition, only ∼2 to 3% of epilithic C was estimated to be in benthic algae throughout the experiment. These results indicate that differences in epilithic C:P ratios caused by increased grazing intensity cannot be explained by changes in prevalence of different organic C pools (detritus and algae) comprising the epilithon, changes in the nutrient content of algae, or by the taxonomic composition of the algal community. Factors affecting the nutrient content of detritus likely account for the lower epilithic C:P ratios found in highly grazed enclosures.