Carbon cycle of an urban watershed: exports, sources, and metabolism

Rivers transport and transform significant quantities of carbon to coastal zones globally. Urbanization and climate change impact the transport and transformation of carbon by altering hydrology, water temperatures, and in-stream metabolism rates. Changes in exports, sources, and metabolism of carbon influence ecosystem processes, food webs, and greenhouse gases. We characterized exports, sources, and metabolism of carbon in four urban watersheds using a combination of discrete stream chemistry measurements and continuous water-quality sensors. Over three years, watershed DOC exports in the Baltimore-Washington D.C. metropolitan area ranged from 9 to 23 kg ha(-1) year(-1). DIC exports ranged from 19 to 59 kg ha(-1) year(-1). Daily contributions from in-stream metabolism varied between -65 and 90 % of DIC export depending on stream size and streamflow conditions. Negative contributions from metabolism occurred on days when streams were autotrophic. All streams were heterotrophic during 60 to 87 % of each year, but showed significant peaks in autotrophy during spring and summer. Differences in the timing and magnitude of peaks in springtime net ecosystem productivity were likely driven by varying light availability across streams of different sizes and riparian shading. CO2 was consistently over-saturated with respect to the atmosphere on all sampling dates and was 0.25-2.9 mg C L-1. Exports, sources, and metabolism of DOC and DIC showed strong predictable patterns across streamflow. Thus, we present a new conceptual model for predicting carbon transport and transformation across changing streamflow and light availability (with impacts on sources and fluxes of DOC, DIC, and CO2). Overall, our results and conceptual model suggest that urbanization accelerates the transition of streams from transporters to transformers of carbon across streamflow, with implications for timing and magnitude of CO2 fluxes, river alkalinization, and oxygen demand in downstream waters.