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

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 biomass was lower on sheltered substrata than on exposed substrata. As algal biomass became low on sheltered substrata, snails tended to be more abundant on exposed substrata, and biomass values in the two current regimes became more similar. Differences in algal biomass and snail abundance between the two substrata were greater in streams illuminated with high irradiance than for the two lower irradiances. Algal assemblages with low irradiance contained adnate cell forms early in succession. At intermediate irradiance, succession in both current regimes moved towards adnate cells, but this occurred more rapidly on the more heavily grazed sheltered substrata. At high irradiance, parts of the algal assemblages in both current regimes remained ungrazed, producing a mix of successional stages on the substrata. Similar interactions may occur in coastal Oregon (USA) streams, because Juga is the competitive dominant herbivore with few predators, and its distribution corresponds mainly to irradiance (as one of the major determinants of algal abundance), current regime, and substratum size.