Effects of light and nutrients on the net accumulation and elemental composition of epilithon in boreal lakes

1. Two experiments in the Experimental Lakes Area (ELA) in north-western Ontario, Canada examined the effects of light and two key elements on the net accumulation and elemental composition of epilithon. In Lake (L) 224, benthic algae were grown under different light intensity and phosphorus supply, while in L302S we provided three levels of two different carbon sources (bicarbonate and glucose) to algae colonizing nutrient-diffusing substrata. After 1 month of accumulation, we sampled biofilms for chlorophyll (chl), carbon (C), phosphorus (P) and algal C. 2. Increased C supply did not significantly affect algal growth (C or chl) or elemental composition (C/P ratios) in L302S. However, P enrichment increased chl and algal C, dramatically reduced the C/P ratio of epilithon, and did not affect total organic C in L224. Phosphorus enrichment also increased the proportion of algal material in the total particulate organic matter and altered the taxonomic composition of algae in L224 biofilms. Shading had no significant effect on the C/P ratio and total organic C in epilithon from the L224 experiment. 3. Our results demonstrate that P supply affects the elemental composition of organic matter that collects on rock substrata. It thus appears that low availability of P relative to C and light drives the formation and retention of high C/P organic matter on rock surfaces in oligotrophic boreal lakes.