Influence of terrestrial inputs on continental shelf carbon dioxide

Abstract. The US South Atlantic Bight (SAB) is a low-latitude shallow continental shelf bordered landward by abundant salt marshes and rivers. Based on previously published data on sea surface partial pressure of carbon dioxide (pCO₂) and new dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) data, a model analysis is presented to identify and quantify the contributions of various terrestrial carbon inputs on SAB sea surface pCO₂. After removal of pCO₂ variations due to annual temperature variability and air–sea gas exchange from the in situ pCO₂, the temperature- and gas-exchange-corrected pCO₂ (TG-corrected pCO₂) is derived. Contributions from rivers, salt marshes, and the continental shelf to the TG-corrected pCO₂ are then calculated. Our findings demonstrate that although additions of CO₂ from within shelf waters (i.e., ΔpCO₂(shelf)) were the greatest of the three components and underwent the largest seasonal changes, ΔpCO₂(shelf) showed smaller onshore–offshore gradients than rivers and marshes. In contrast, CO₂ contributions from river (ΔpCO₂(river)) and salt marsh (ΔpCO₂(marsh)) components were greatest closest to the coast and decreased with distance offshore. In addition, the magnitude of ΔpCO₂(marsh) was about three-fold greater than ΔpCO₂(river). Our findings also revealed that decomposition of terrestrial organic carbon was an important factor regulating the seasonal pattern of pCO₂ on the inner shelf. Despite large uncertainties, this study demonstrates the importance of terrestrial inputs, in particular those from coastal wetlands, on coastal ocean CO₂ distributions.