Herbivory and irradiance shape periphytic architecture in a Swiss alpine stream
Limnol. Oceanogr., 45(1), 2000, 64-75 | DOI: 10.4319/lo.2000.45.1.0064
ABSTRACT: 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 manipulating light intensity and the densities of two abundant grazers in stream-side channels. We hypothesized that the high light intensities in Minor Creek (exceeding 2,000 mmol m-2 s-1 on the stream bed) would cause photoinhibiton and thus regulate algal composition and biomass accrual. After 18 d, full sunlight had negative effects on all measures of periphytic abundance in streamside channels. A five-level light gradient experiment in Minor Creek corroborated these findings: full sunlight suppressed periphytic accumulation. To test the interactive effects of grazing and light, we used two herbivorous insects with distinctly different feeding structures. Ecdyonurus sp. (Heptageniidae) possesses brushing mouthparts; Drusus lateralis (Limnephilidae) has scraping mandibles. In accordance with a conceptual model that predicts algal physiognomy and herbivore feeding mode regulate algal susceptibility to grazing, we hypothesized that Ecdyonurus would primarily reduce physiognomies composing the periphytic overstory (e.g., stalked and filamentous algae), whereas Drusus would have greater access to understory physiognomies (e.g., prostrate forms). In addition, we hypothesized that tall physiognomies 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 data suggest light intensity altered periphytic susceptibility to grazing. Channels receiving full sunlight had less periphyton, yet grazer weights did not differ between light treatments despite the fact that weight gain for each grazer was negatively correlated with conspecific biomass, implicating competition for limited periphytic food. Algal responses were most pronounced under grazing by Ecdyonurus, which was associated with decreases in the abundance of six of the seven algal physiognomies. Contrary to model predictions, Ecdyonurus reduced low-lying as well as overstory physiognomies. Drusus, by contrast, was associated with reductions in only four physiognomies. Results from this study suggest that understanding periphytic community responses to grazing will require more sophisticated models of periphytic structure.