Establishing light as a causal mechanism structuring stream communities in response to experimental manipulation of riparian buffer width

Abstract

Previous studies on the effects of logging on streams have suggested that light and water temperature were important variables structuring stream communities but, in many cases, these effects were confounded. We observed pronounced gradients in the flux of solar energy and water temperature in an earlier large-scale experiment in which we manipulated the width of riparian buffers along headwater streams. Associated with these abiotic changes were increases in periphyton biomass and primary consumer abundance. We present results from a study in streamside channels that was designed to isolate the effects of light on stream communities, while holding water tem- perature constant. Light treatments in the channel experiment simulated inputs of solar radiation created during the prior watershed-scale experiment. Results from the present study suggested that consumers limited periphyton biomass early in the study; however, a rainstorm midway through the experiment reduced periphyton biomass and insect consumer abundance. Following this disturbance, chlorophyll a biomass was 2 to 4 times higher in the full sunlight treatment compared to the 2 lowest light treatments. At the end of the study, primary consumer abundance, biomass, survival, and growth rate were positively related to light and periphyton resources. Therefore, we inferred biotic control of periphyton during the early part of the channel study, whereas light appeared to control periph- yton at the end of the study. Results from the large-scale and channel experiments suggested that light was the primary constraint on periphyton biomass accrual. Moreover, both experiments, espe- cially the channel study, showed that light indirectly influenced consumer performance as mediated by increased primary production.