Response of Stream Macroinvertebrate Community to Canopy-opening Manipulations

Stream light availability is an important factor influencing aquatic food webs. In forested headwaters, stream algal production is often highly light-limited, so an increase in light enhances benthic algal growth, which in turn increases food availability for primary consumers in the stream. In forested headwater streams, light availability is almost entirely mediated by the canopy structure of stream-side vegetation. Over the last century, many streamside forests in the Pacific Northwest were heavily harvested, leading to dense regenerating stands along streams today. Under current conditions, the dense closed canopies, allow for limited primary production, and we hypothesize that low standing stocks of benthic primary producers under these closed canopy forests lead to a low abundance of invertebrates that feed on stream algae. We investigated the reach-scale responses of benthic periphyton, stream macroinvertebrates, and prey consumption by trout to a localized release from light limitation in a paired-reach study design. We expected that increases in light availability would promote elevated algal production causing the invertebrate community to shift toward scraper dominance, and predicted that this change in community structure is detectable in the diets of trout. In contrast to our expectations, we found that the presence of a canopy gap had little influence on the invertebrate community, and this lack of change was not being masked by increased consumption of grazing invertebrates in summer trout diets.