CARBON CYCLE OF AN URBAN WATERSHED: EXPORTS, FORMS, AND METABOLISM

Urbanization and climate change impact these ecosystems by altering hydrology, water temperature, nutrient loads, and stream metabolism rates. Aquatic carbon cycling is known to influence ecosystem processes, food webs, and greenhouse gas emissions, however the influence of urbanization on carbon sources, export, and CO₂ fluxes is not well understood. Using a combination of discrete samples, continuous water-quality sensors, and lab experiments, we quantified sources, metabolism, and export of carbon in four urban watersheds, and evaluated drivers of theses processes linked to climate and land use change. Despite high levels of impervious cover (17-40%), DOC export from four watersheds in the Baltimore-Washington D.C. metropolitan was within the range of published values from undisturbed areas (9 to 23 kg ha^-1yr^-1), and optical properties of organic matter resembled terrestrial soil and leaf leachates. DIC export varied from 19 to 59 kg ha^-1yr^-1 with stream respiration contributing between 3.4 and 13.4 of mean daily inorganic C inputs, and watershed sources (soil respiration + weathering) contributing the remaining majority. All tributaries were heterotrophic on the majority of days, but showed significant peaks in autotrophy during spring and summer. Net ecosystem production (NEP) varied with season, light availability, temperature, and flow conditions. Varying light availability across streams of different sizes and riparian shading likely drove Timing and magnitude of peaks in NEP. CO₂ was consistently super-saturated with respect to the atmosphere on all sampling dates, but contributed less than 10% of inorganic carbon export. Based on the patterns and processes observed, we will describe a conceptual framework for carbon production and export from watersheds which can be used to predict the timing of peak in-stream productivity and downstream export of labile organic matter.