Abstract
Urban areas are increasingly adopting the use of ecologically-based technologies for stormwater management to mitigate the effects of impervious surface runoff on receiving water bodies. While stormwater control measures (SCMs) reduce runoff, their ability to influence ecosystem function in receiving streams is not well known. To understand the effect of SCMs on net ecosystem function in stream networks, we measured sediment denitrification in four streams across a gradient of urban and suburban residential development in Charlotte, NC. We evaluated the influence of SCM inputs on actual (DNF) and potential (DEA) denitrification activity in stream sediments at the SCM-stream confluence to quantify microbial processes and the environmental factors that control them. DNF was variable across sites, ranging from 0–6.60 mg-N·m−2·h−1 and highly correlated with in-stream nitrate (NO3-N) concentrations. Sites with a greater impervious area showed a pattern of significantly higher DEA rates upstream of the SCM compared to downstream, while sites with less imperviousness showed the opposite trend. We hypothesize that this is because of elevated concentrations of carbon and nitrogen provided by pond and wetland outflows, and stabilization of the benthic habitat by lower peak discharge. These results suggest that SCMs integrated into the watershed have the potential to create cascading positive effects on in-stream nutrient processing and thereby improve water quality; however, at higher levels of imperviousness, the capacity for SCMs to match the scale of the impacts of urbanization likely diminishes.
Highlights
Urbanization is a rapidly growing form of land use change throughout the world.Urban impervious surfaces and associated stormwater collection systems circumvent infiltration in upland and riparian zones, causing changes in urban stream flow regimes that lead to flashier hydrographs, increased peak flows, shorter rainfall–discharge lag times, and increased mobilization of chemical pollutants within the watershed [1,2,3]
We focus on denitrification as an indicator of the effect of stormwater control measures (SCMs) inputs on net ecosystem function in receiving streams because N pollution is a common concern in urban stream systems and denitrification is the only process that removes N from the system
We observed a significant increase in DNF rates at Suburban wetland (SL)-DS3 compared to SL-US during the fall (ANOVA, p = 0.046), concurrent with seasonal backwater effects from a downstream sedimentation pond
Summary
Urban impervious surfaces and associated stormwater collection systems circumvent infiltration in upland and riparian zones, causing changes in urban stream flow regimes that lead to flashier hydrographs, increased peak flows, shorter rainfall–discharge lag times, and increased mobilization of chemical pollutants within the watershed [1,2,3]. These hydrologic changes lead to changes in water quality, channel morphology, ecology, and a reduction in the resilience of the stream ecosystem to altered urban hydrologic and water quality regimes [4,5]. Low erosion rates and high stability of channel sediments facilitate the retention of organic C sources, microbial community growth and activity, and oxygen consumption via heterotrophic metabolism, all of which enhance denitrification [16,17]
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