Low-Head Dams Alter Stream Physicochemical Conditions And Leaf Litter Decomposition

Changes to land use in headwater catchments may impact stream invertebrate communities and ecosystem functions. Increasingly, dam removals are often part of stream restoration projects that focus on improving water quality and aquatic ecosystem connectivity. Anthropogenic barriers including mill dams alter stream habitat conditions and influence benthic communities and stream ecosystem functions. I conducted two field experiments to better understand the impacts of small impoundments on leaf decomposition, a key biologically-mediated ecosystem process. In the first experiment, I deployed leaf packs up- and down-stream of a relict, breached, and intact dam and removed them after 8 weeks. I calculated decomposition rates and quantified invertebrate community structure at each site. In a follow up study I conducted an experiment to document the effects of altered physicochemical conditions on drivers of decomposition. I tested whether macroinvertebrate shredding or microbial colonization was driving leaf breakdown rates in the presence of a small dam. The highest rate of leaf decomposition for both experiments was in the tailwaters of an intact dam. These data suggest that some intact low-head dams may improve habitat conditions for benthic macroinvertebrates while increased nutrient retention within the impoundment may increase biofilm accumulation and microbial decomposition.