Ecological stoichiometry of trophic interactions in the benthos of boreal lakes

The causes and consequences of elemental imbalances between trophic levels in oligotrophic lakes of the Canadian Shield are examined with the research presented here. Elemental imbalances, differences in the elemental composition between consumer and food, appear to commonly occur in littoral zones of boreal lakes based on extensive variability found in the elemental composition of epilithon (organic matter attached to rock surfaces) from the Experimental Lakes Area (ELA), Ontario, Canada. A correlative analysis tested preliminary hypotheses and showed significant relationships between epilithic carbon (C):nutrient ratios and the relative supplies of light, dissolved inorganic carbon (DIC) and nutrients in the water column. These relationships indicate that strong nutrient deficiency present in ELA epilithon is related to the high light, low nutrient conditions found in littoral zones of these boreal lakes.; Factors controlling epilithic elemental composition were further examined with experiments conducted at the ELA. Artificial carbon enhancement did not affect epilithic elemental composition in a low DIC lake. However, phosphorus (P) enrichment reduced epilithic C:P ratios in a lake with extremely low P concentrations. Shading had no significant effect on particulate chemistry of epilithon. 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. The hypothesis that benthic grazing affects the C:P stoichiometry of epilithon was also tested at the ELA. Grazers strongly affected epilithic chemistry and benthic algal densities. Together, these experimental results indicate that the elemental composition of epilithon is affected by a number of factors that affect benthic algal growth processes and the prevalence of different detrital types.; One consequence of large elemental imbalances is strongly constrained growth in benthic consumers. Mayfly growth rates were found negatively affected by high C:P ratios in food. A mass-balance model showed that mayfly growth should be limited by food P content at moderately low C:P ratios. In combination, results presented here indicate that stoichiometric constraints strongly and commonly affect benthic food webs in boreal lakes on the Canadian Shield.