Atmospheric CO2 evasion, dissolved inorganic carbon production, and net heterotrophy in the York River estuary

Abstract

Direct measurements of the partial pressure of CO 2 (pCO 2) and dissolved inorganic carbon (DIC) were made over a 2‐yr period in surface waters of the York River estuary in Virginia. The pCO 2 in surface waters exceeded that in the overlying atmosphere, indicating that the estuary was a net source of CO 2 to the atmosphere at most times and locations. Salinity‐based DIC mixing curves indicate there was also an internal source of both DIC and alkalinity, implying net alkalinity generation within the estuary. The DIC and alkalinity source displayed seasonal patterns similar to that of pCO 2 and were reproducible over a 2‐yr study period. We propose that the source of inorganic carbon necessary for both the sustained CO 2 evasion to the atmosphere and the advective export of DIC is respiration in excess of primary production (e.g., net heterotrophy). The rates of CO 2 evasion and DIC export were estimated to provide an annual rate of net heterotrophy of ~100 g C m−2 yr−1. Approximately 40% of this excess inorganic carbon production was exported as DIC to the coastal ocean, whereas 60% was lost as CO 2 evasion to the atmosphere. The alkalinity generation needed to sustain the export of inorganic carbon, as HCO 3− , is most likely provided by net sulfate reduction in sediments. Accumulation of sulfide in the sediments of a representative site directly adjacent to the York River estuary is sufficient to account for the net export of alkalinity. The seasonality of net heterotrophy causes large variations in annual CO 2 and DIC concentrations, and it stresses the need for comprehensive temporal data sets when reporting annual rates of CO 2 evasion, DIC advection, and net heterotrophy.