Aquatic Food Web Responses to Patchy Shading Along Forested Headwater Streams

In small forested streams, changes in age and structure of riparian vegetation covering the stream have been shown to directly influence the amount of light reaching the stream benthos. Light has the potential to impact in-stream resources that support secondary production through constraints on primary productivity. The influence of landscape changes in riparian vegetation cover and their effect on in-stream light has been evaluated; however smaller, patchy changes of in-stream light have yet to be thoroughly explored. We worked to clarify the role of light as a bottom-up driver of in-stream food webs in the western Cascade Mountain range streams, in Oregon, through exploring how decreased light availability, via patchy shading, affected in-stream biota. With patchy decreases in the amount of light reaching the stream benthos we expected to see a reduction in the growth of in-stream autotrophs through the summer and subsequently a decrease in macroinvertebrates, cutthroat trout (Oncorhynchus clarkii clarkii) and salamanders as a result of decreasing resource availability in the manipulated reaches. We established three sets of paired stream reaches and experimentally manipulated light in one reach from each pair by adding patches of shade. We then evaluated how periphyton, invertebrates, fish, and total vertebrate predators responded in the manipulated reach relative to the unmanipulated reference reach in a before-after control-impact (BACI) study design. Our patchy shading manipulation significantly decreased light, causing a decrease in algal biomass and subsequently invertebrate, fish, and salamander biomass. Local decreases in light fluxes to forested headwater streams along a larger reach decreased biota throughout the aquatic food-web. This trend was consistent in streams bordered by both second-growth and old-growth forests. This research indicates that smaller changes in riparian forest structure which impact in-stream light, such as those that occur in forests develop processes, can impact in-stream biota.