PERIPHYTON RESPONSES TO HIGHER TROPHIC LEVELS AND LIGHT IN A SHADED STREAM

Presence or absence of a predatory fish Semotilus atromaculatus and a grazing snail Elimia clavaeformis were manipulated, while spatial variability in ambient light was related to variability in primary productivity. Indirect effects of fish on periphyton were insignificant, possibly because grazers that are vulnerable to fish predation were scarce during the experiment, or because fish preyed upon other predators of grazers. Snails had compensatory effects within the periphyton: they diminished biomass and productivity in the loosely attached layer, but stimulated productivity in the tightly attached layer. Snails appeared to mitigate shading of the tightly attached layer by the loosely attached layer. Benthic primary productivity was highly correlated with ambient light levels, accounting for 42% of the variability in total primary productivity. Light availability controls realized as well as potential production in this unproductive stream. -from Authors