Watershed-Scale Analysis of Changes in Dissolved Organic Matter Quality and Nitrogen/Phosphorus Concentrations across an Urbanization Gradient during Baseflow and Stormflow

Dissolved organic matter (DOM) is a pivotal variable in aquatic ecosystems that can influence multiple factors, including pH, light penetration, stream metabolism, contaminant (e.g. metals and nutrients) mobility, and water-treatment efficiency. DOM also serves as an important carbon linkage from terrestrial to marine systems via freshwaters. With the projected increase in intense storm events in the Northeastern US, better understanding of land-water transfer of DOM and DOM fate within drainage networks under various land use and hydrologic conditions is critical for water quality management. Past studies on headwaters during baseflow have shown that urbanization is leading to DOM compositional changes. Urban DOM sources can include leaky septic/sewer systems, treated wastewater effluent, and stormwater runoff. Additional research is needed, however, extending from headwaters to the mainstem during storm events to facilitate more comprehensive spatiotemporal analyses. For two years, we sampled a range of stream orders along an urbanized land use gradient in the Farmington River watershed, which provides drinking water to Hartford, CT (USA). We investigated the effects of urbanization on changes in DOM quality and nitrogen/phosphorus levels during both baseflow and stormflow. Throughout the catchment, we deployed ten multiparameter sondes continuously measuring water temperature, specific conductivity, pH, dissolved oxygen, turbidity, and fluorescent DOM with most sites co-located at U.S. Geological Survey (USGS) streamgages. We combined grab sampling with refrigerated autosampler use to increase sample frequency and capture higher flows. Our urban vs. forested DOM analyses (via fluorescence spectroscopy and ultrahigh resolution mass spectrometry) show that urban-derived DOM is lower in molecular weight, enriched in protein-like fluorescence, and comprised of more sulfur- and nitrogen-containing compounds. With little known about the ecological consequences of such urban DOM changes, we also sampled DOM quality across Connecticut alongside the USGS Northeast Stream Quality Assessment Summer 2016 program to monitor a suite of biological and water quality parameters. Concurrent sampling enabled the evaluation of ecological trends with DOM to inform future watershed management.