The influence of light and nutrients on interactions between a tadpole grazer and periphyton in two coastal streams

In small streams, shading from the riparian canopy can limit production by stream algae. Removal of the riparian canopy due to forest disturbance can alter light and nutrient regimes in small streams, which in turn may influence grazer-periphyton interactions within the stream community. To investigate the influence of forest disturbance on grazer-periphyton interactions, I conducted an experiment to test for the effects of light, nutrients, and grazing by tadpoles of the tailed frog (Ascaphus truei) on periphyton standing crop. Light, nutrients, and tadpole density were manipulated in replicated, in situ, flow-through enclosures in two streams using a complete block, fully-factorial design to test for effects on periphyton biomass measured as ash-free dry weight (AFDW), chlorophyll a, and individual tadpole relative growth rates. The experiment was conducted over a six-week period from August to September 1997. Light exerted a strong, positive effect on periphyton production resulting in a two to seven-fold increase in chlorophyll a abundance and a 30-40% increase in AFDW over the shaded treatment. The response of periphyton to light differed strikingly between the two study streams, with Dipper Creek responding more strongly than Klondike Creek. Nutrients had no significant effect on chlorophyll a, and a positive effect on periphyton biomass on cobbles but not on tiles. Tadpole grazing significantly decreased periphyton biomass across all densities at both streams. Tadpoles showed an absolute rate of increase of 13-17% in body weight over the sixweek period. Thirty percent of the variation in growth rates was attributable to differences between the two streams. Mean growth rate of tadpoles was 45% higher at Klondike Creek than at Dipper Creek. Light exerted a significant positive effect on tadpole growth resulting in a 14% increase over the shaded treatment. Tadpole density exerted a significantly negative effect on tadpole growth rate and accounted for a greater percentage (38%) of the overall variation than did light. Nutrient addition did not significantly influence tadpole growth rate. Periphyton production was found to be under simultaneous control by light and tadpole grazing. The strong, positive effect of light on periphyton production and tadpole relative growth rate indicates a tight trophic coupling between grazers and their algal food resource, and a potential positive direct effect of light on grazers. The decrease in individual tadpole growth rates with increasing tadpole density indicates that tadpoles are subject to intraspecific densitydependent interactions, probably resulting from food-limitation. This study demonstrates that several factors may act in unison to control periphyton production and grazer growth rate, and that the relative importance of these factors may vary significantly between nearby streams.Forestry, Faculty ofGraduat