Longitudinal patterns in carbon and nutrient export from urban watersheds with contrasting headwater management

Stormwater management in urban areas presents challenges and opportunities to enhance water quality while simultaneously protecting property and infrastructure. Through several generations, stormwater management practices have evolved from 'gray infrastructure' such as pipes and ditches designed to quickly transport water away from the landscape, to more 'green infrastructure' projects meant to allow for biological processing and retention of urban runoff. Implementation of these practices has replaced traditional stream burial with bioretention cells, wetlands, and ponds. We hypothesize that these contrasting green versus gray strategies for headwater management may have significant consequences for the delivery and processing of dissolved carbon, nitrogen, and phosphorous. To address this hypothesis, we compared two paired urbanized watersheds with different stormwater management by measuring the longitudinal export of DOC, DIC, TDN, PO4+, and major anions, and characterizing dissolved organic matter using Fluorescence Index (FI) and Spectral Slope. Both watersheds were located in the Baltimore Ecosystem Study Long Term Ecological Research (LTER) site. Dead Run is an urbanized catchment with prevalent stream burial and minimal stormwater management which was implemented after initial development. Red Run is a similarly sized watershed with more recent development and comprehensive stormwater management (wetlands, ponds, bioretention cells, sand filters) and 100m wide stream buffer areas. In each of these contrasting watersheds, we chose two headwater streams which drain SWM features and one stream that terminates at a storm drain. We measured longitudinal changes in export by conducting a synoptic survey of both watersheds in which flow and water chemistry were measured every 500m in the main stem and approximately every 250m in the selected tributaries. Within watersheds, we found differences in the C, N and P loads from SWM and non-SWM streams. In Red Run, DOC and DIC were higher in SWM streams, while TDN was higher in non-SWM streams. In contrast, Dead Run SWM streams had lower DIC and PO4+ concentrations than non-SWM, but DOC and TDN were comparable for this single sampling date. Overall, Red Run had lower C export with 4.6 Kg day-1 of DOC and 83 Kg day-1 of DIC, than Dead Run which exported 28.7 Kg day-1 of TOC and 174.2 Kg day-1 of DIC. The instantaneous TDN load was very similar with 5.5 Kg day-1 in Red Run and 5.4 Kg day-1 in Dead Run. Dead Run had an overall longitudinal increase in DOC and decline in DIC concentrations from the headwaters to the mainstem. Red Run showed an overall longitudinal increase in both DIC and DOC concentrations with distance downstream. Future work will investigate biogeochemical processing rates within these contrasting watersheds to explain the longitudinal patterns along stream networks. This work will connect how headwater management strategies alter downstream transport and transformation of carbon and nutrients across the urban watershed continuum.