Dissolved nitrogen and organic carbon fluxes from urban stormwater runoff and rainfall into harmful algal bloom-prone waters: seasonal variability and insights into dissolved organic matter composition

Constraining the delivery window of labile nitrogen (N) and dissolved organic matter (DOM) loadings from rainfall and stormwater runoff is critical as these inputs can drive harmful algal blooms (HABs) and declines in water quality. Here, we quantified monthly and seasonal fluxes of dissolved N species and dissolved organic carbon (DOC) in rainfall and stormwater runoff to determine the timing of reactive DOM and N delivery to Tampa Bay, a highly urbanized estuary of national significance. Reactive DOM fractions were identified via fluorescence excitation-emission matrix-parallel factor analysis (EEM-PARAFAC). Rainfall and stormwater runoff samples were collected during forty storm events encompassing two wet and dry seasons from four urban residential sites which convey stormwater runoff directly into Tampa Bay. Overall, dissolved organic N (DON) was the dominant N form in runoff across all sites and seasons (up to 83%), and up to 57% of total N in rainfall samples. Fluxes of DON and DOC in runoff were the greatest in August – September, which can coincide with HABs in the bay. Rainfall was a source of labile proteinaceous DOM during the wet season only. Labile microbial humic- and protein-like DOM was observed only in the wet season in runoff and driven by increased imperviousness. Dry season DOM was recalcitrant humic and fulvic-like and driven by antecedent dry days. These results highlight the importance of nutrient flux monitoring and the utility of EEM-PARAFAC in identifying critical windows of labile DOM and N delivery which can be used for focused nutrient management efforts.