Cumulative impacts of stream burial on network structure and functional connectivity in headwater stream systems

Abstract

Stream burial is common during urbanization, and disproportionately affects headwater streams. Burial undermines the physical, chemical, and spatial processes governing aquatic life, with consequences for water quality and biodiversity, both within headwaters and in downstream waters. Network changes associated with headwater burial have not been explored, limiting our understanding of changes in biotic composition with urbanization of these critical ecosystems. To address this need, I predicted stream burial across the Potomac River Basin (PRB) from impervious cover data and training observations from high-resolution aerial photography. Results across the PRB urban gradient reveal consistent burial patterns related to catchment area and topographic slope. I discuss these results in the context of physiographic constraints on stream location and urban development, including implications for management of aquatic resources. Second, I examined burial-related changes to headwater network structure and habitat connectivity, using a series of topological and distance measures, and a novel application of circuit-theoretical modeling to stream networks. Results show stream burial significantly affects both the number and size of remnant stream segments and their spatial orientation. Significant decreases in landscape connectivity were observed with burial, around ecologically important features such as confluences, and for urbanized headwater systems as a whole. Third, I used biological data to compare environmental and spatial controls on species turnover in fish and insect communities across headwater systems. Turnover was analyzed using generalized dissimilarity modeling, which accommodates variation in rates of species turnover along and between gradients, and two novel measures of resistance distance, which combine aspects of space and environment, specifically the spatial extent, orientation, and relative favorability of habitat across the landscape. Results show headwater species are more sensitive to environmental parameters, with less mobile species more sensitive to habitat fragmentation and required dispersal distances. Rapid compositional turnover occurred within short distances from the sampled reaches, suggesting headwater taxa disperse only short distances, with even small obstructions or habitat loss having potential to impact diversity within headwater systems. Knowledge gained from this research is critical for understanding the cumulative impact to stream networks, and for future decision-making allowing for urban development while protecting stream ecosystem function.