Export and Metabolism of Carbon in Urban Watersheds: Climate Implications

Abstract:

Rivers export and transform globally-significant quantities of carbon from watersheds to coastal ecosystems. Urbanization and climate change influence these fluxes by altering the hydrologic regime, water temperature, and anthropogenic sources of organic matter. Here, we quantify export and metabolism of carbon in highly urbanized, coastal watersheds, and evaluate the importance of physical drivers linked to climate and land use. Using a combination of discrete-samples, continuous water quality sensors, lab experiments, and modeling, we quantified rates of DOC, DIC, and CO₂ export as well as changes in DOC quality and in-stream metabolism in four highly developed watersheds of the Chesapeake Bay over three years. Annual DOC and DIC exports from the four watersheds varied from 9 to 23 and 19 to 59 Kg ha^-1yr^-1 respectively. The range of daily CO₂ concentrations was 0.01 to 2.6mg L^-1, equivalent to between 0.37 and 53% of daily DOC export across all streams and dates. All sites were net-heterotrophic for the majority of the year (NEP<0), with strong peaks in gross primary production and net autotrophy (NEP >0) during spring and early summer. There was a significant (P<0.05) relationship between gross primary production and ecosystem respiration, suggesting that autotrophic respiration is an important fraction of ER, and labile algal biomass may influence oxygen demand downstream. Our results suggest that urban watersheds can export significant amounts of DOC, DIC, and CO₂ to coastal zones. The influence of urbanization on coastal water quality and greenhouse gases may be exacerbated by climate change as temperatures and storm frequency continue to rise.