The epilithic microbial and micro-invertebrate communities under conditions of light and shade were studied from April to November 1978. During a period of increasing light intensity from May to June, the level of chlorophyll a (an algal biomass indicator) and numbers of Bacillariophyta in the shade were considerably higher than the level and numbers in the light. This was considered to be evidence supporting the hypothesis that the midsummer decline in algal populations was due to light inhibition. Numbers of bacteria and carbohydrate, total organic carbon (TOC) and total organic nitrogen (TON) concentrations were significantly correlated with algal biomass in the light whereas only numbers of bacteria and carbohydrate were correlated with algal biomass in the shade. The potential causal relationships between these parameters are discussed. The population dynamics of the micro-Invertebrate populations are discussed in relation to the quantity and quality (C:N ratio of epilithon) of their food supply. Lastly, the relevance of these findings to oil sands development is discussed, where the alteration of the river's light regime by the removal of riparian vegetation could result in decreased production while silt additions during the months of May to July could result in increased productivity.