Does gravel size influence benthic macroinvertebrate density, biomass and feeding guild composition?

Background/Question/Methods

There has been an increase in river restoration projects on regulated California Central Valley rivers due to undesired anthropogenic effects (e.g. dams, diversions, agriculture, levees and urban development).  These effects interrupt and reduce the quantity and quality of bed sediment substrates necessary for anadromous salmonid spawning and also reduce essential ecosystem processes important to rearing juvenile salmonids. Aquatic macroinvertebrate production is an example of an ecosystem process that has been altered because assemblage richness and composition is generally influenced by substrate size. Aquatic macroinvertebrates play a pivotal role in river ecosystem functions including prey sources for juvenile anadromous salmonids. Gravel augmentation is a widely accepted technique for restoring anadromous salmonid spawning habitats throughout the Central Valley. We examined the effects of gravel augmentation on colonization and composition of aquatic macroinvertebrates by comparing density, biomass and community structure among three different substrate sizes (small, medium and large) in a gravel augmented area on the lower American River, a regulated Central Valley stream.

Results/Conclusions

Our observations show that large gravel produced significantly higher macroinvertebrate density than small and medium substrate which suggests that larger gravel may support increased forage production for juvenile salmonids. Large gravel has lower total surface area, but provides more stable substrate and interstitial spaces that are accessible to both macroinvertebrates and juvenile salmonids. Although larger sized gravel had a higher density of benthic macroinvertebrates, the biomass did not differ significantly between medium and large gravel. Therefore, because medium-sized gravel produces more optimal spawning habitat, it may ultimately be the most appropriate strategy for augmentation to simultaneously enhance habitat and prey productivity for multiple salmonid life stages.