Herbivory and irradiance shape periphytic architecture in a Swiss alpine stream

The effects of herbivory and light on epilithic periphyton from Minor Creek (2,200 m above sea level), a Swiss alpine stream, were examined by manipul ating light intensity and the densities of two abundant grazers in streamsi de channels. We hypothesized that the high light intensities in Minor Creek (exceeding 2,000 mu mol m(-2) s(-1) on the stream bed) would cause photoin hibiton and thus regulate algal composition and biomass accrual. after 18 d , full sunlight had negative effects on all measures of periphytic abundanc e in streamside channels. A five-level light gradient experiment in Minor C reek corroborated these findings: full sunlight suppressed periphytic accum ulation. To test the interactive effects of grazing and light, we used two herbivoro us insects with distinctly different feeding structures. Ecdyonurus sp. (He ptageniidae) possesses brushing mouthparts; Drusus lateralis (Limnephilidae ) has scraping mandibles. In accordance with a conceptual model that predic ts algal physiognomy and herbivore feeding made regulate algal susceptibili ty to grazing, we hypothesized that Ecdyonurus would primarily reduce physi ognomies composing the periphytic overstory (e.g., stalked and filamentous algae), whereas Drusus would have greater access to understory physiognomie s (e.g., prostrate forms). In addition, we hypothesized that tall physiogno mies would be less abundant under full sunlight due to the exposure of the periphytic mat to photoinhibiting irradiance. Thus, we expected Ecdyonurus to have a smaller effect on periphyton at high light intensities. Light in channels did not directly modify grazing effects; however, the dat a suggest light intensity altered periphytic susceptibility to grazing. Cha nnels receiving full sunlight had less periphyton, yet grazer weights did n ot differ between light treatments despite the fact that weight gain for ea ch grazer was negatively correlated with conspecific biomass, implicating c ompetition for limited periphytic food. Algal responses were most pronounce d under gazing by Ecdyonurus, which was associated with decreases in the ab undance of six of the seven algal physiognomies. Contrary to model predicti ons, Ecdyonurus reduced low-lying as well as overstory physiognomies. Drusu s, by contrast, was associated with reductions in only four physiognomies, Results from this study suggest that understanding periphytic community res ponses to grazing will require more sophisticated models of periphytic stru cture.