Effects of Hydraulic Refuge and Irradiance on Grazer-Periphyton Interactions in Laboratory Streams

Abstract

A 54-d experiment was conducted in laboratory streams to examine relationships between temporal and spatial patterns of algal development and grazing behavior of the snail Juga silicula in a patchy physical environment. Each stream contained nine channel-spanning blocks (22.5 × 22.5 × 4 cm) spaced 22.5 cm apart and was illuminated with a photon flux density of 375, 90 or 20 μ E m -2 s -2 . We monitored periphyton succession and the number of snails in fast current regimes on tops of blocks (exposed substrata) and in slower current regimes on surfaces recessed between blocks (sheltered substrata). At the end of the experiment, algal biomass in streams illuminated with high irradiance was approximately 80× and 8× greater than in streams with low and intermediate irradiance, respectively. Growth rates of Juga in streams with high and intermediate irradiances were similar and approximately 15× greater than rates at low irradiance. At all irradiances, snail densities were higher and algal bi...