Particulate and Dissolved Organic Matter in Stormwater Runoff Influences Oxygen Demand in Urbanized Headwater Catchments

Increasing inputs of organic matter (OM) are driving declining dissolved oxygen (DO) concentrations in coastal ecosystems worldwide. The quantity, source, and composition of OM transported to coastal ecosystems via stormwater runoff have been altered by land use changes associated with urbanization and subsequent hydrologic flows that accompany urban stormwater management. To elucidate the role of stormwater in the decline of coastal DO, rain event sampling of biochemical oxygen demand (BOD) in samples collected from the outfall of stormwater ponds and wetlands, as well as samples of largely untreated runoff carried by stormwater ditches, was conducted across a range of urban and suburban development densities. Sampling also included measurements of particulate and dissolved carbon and nitrogen, carbon and nitrogen stable isotopes, and chlorophyll-a. Results suggest stormwater may be a significant source of labile OM to receiving waters, especially during the first flush of runoff, even though BOD concentrations vary both among and within sites in response to rain events. BOD variability was best predicted by particulate OM (POM) and chlorophyll-a, rather than the larger pool of dissolved OM. These findings demonstrate the importance of managing episodic stormwater discharge, especially POM, from urbanized areas to mitigate DO impairment in larger downstream systems.