Periphyton removal related to phosphorus and grazer biomass level

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Summary1. The proliferation of nuisance periphyton in enriched streams may be dependent on the biomass of the grazing macroinvertebrates present. In the present study, the effectiveness of grazer size and biomass in controlling periphyton and the extent to which grazing effectiveness was affected by enrichment level were determined.2. Two sets of experiments with two caddisfly grazers were conducted in laboratory channels during spring and summer 1995 and 1996. The first set tested the combined effect of phosphorus enrichment and grazing, while the second set tested the effect of variable grazer biomass on periphyton biomass.3. Grazing reduced periphyton biomass in excess of 80%, compared to ungrazed controls. Grazers were equally effective in controlling filamentous green algae, Stigeoclonium, diatoms and small colonial greens. Near complete removal of periphyton biomass by grazing occurred at even at the lowest grazer biomass level (750 mg m−−2, i.e. approximately one‐third of natural levels).4. Grazing controlled periphyton biomass more than did enrichment with soluble reactive phosphorus (SRP).5. Grazing rates in the phosphorus‐grazing interaction experiments averaged about 6 mg chl a g invertebrate−−1 day−−1, which was similar to past work in these channels and elsewhere, while rates were about five‐fold higher in the variable grazer biomass experiments.6. Simulating effects of SRP and grazing with a calibrated model suggests that higher SRP levels would be necessary to exceed a nuisance periphyton biomass level if grazers were present. However, if grazer biomass was more than 1500 mg m−−2, a nuisance level would probably not be exceeded at any SRP.