Effects of Light and Nutrients on Grazer–Periphyton Interactions

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, we tested the effects of light, nutrients, and grazing by tadpoles of the tailed frog (Ascaphus truei) on periphyton standing crop and tadpole growth rates in 2 coastal streams. Periphyton biomass was found to be under simultaneous control by light and tadpole grazing. Light exerted a strong, positive effect on periphyton standing crop and tadpole relative growth rate, indicating a tight trophic coupling between grazers and their algal food resource. Tadpole growth rate decreased significantly with increasing tadpole density, showing that tadpoles are subject to intraspecific density-dependent interactions. The 2 streams differed in both abiotic limitation of periphyton biomass and tadpole growth. This study demonstrates that several factors may act together to control periphyton biomass and grazer growth rate, and that the relative importance of these factors may vary between nearby streams. Micheline A. Kim1 Department of Forest Sciences, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada John S. Richardson Wildlife Branch, British Columbia Environment, and Department of Forest Sciences, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada 1 Present address: AXYS Environmental Consulting Ltd., P.O. Box 2219 1045 Mills Rd. West, Sidney, BC, V8L 3S8, Canada [email protected] K I M A N D R I C H A R D S O N 498 Proc. Biology and Management of Species and Habitats at Risk, Kamloops, B.C., 15–19 Feb. 1999. rence, density, and biomass of tailed frog tadpoles by influencing the availability of the algal food supply. This study assesses the effects of forest disturbance on abiotic factors, periphyton biomass, and larval growth response, which may be a correlate of fitness for individual tadpoles. The major objectives of this study were to: 1. examine the influence of changes in abiotic factors (light and nutrients) on primary production by stream periphyton; 2. assess whether tadpole growth is food limited under different light and/or nutrient conditions; and 3. determine whether tadpole growth is influenced by density-dependent interactions.