Dynamics of dissolved organic and inorganic carbon in the Mid-Atlantic Bight

The Mid-Atlantic Bight (MAB) is a highly productive and dynamic continental shelf region on the eastern North American coast. The role of this ocean margin in sequestering atmospheric carbon dioxide and exporting it as dissolved organic carbon (DOC) is addressed. Results show a total of between 18.7 to 19.6 MTC of DOC are exported annually to the adjacent slope waters. Of this, about 30% is new production of DOC on the shelf (5.8 ± 1.7 MTC/year). This new DOC represents 8–15% of the primary production on the shelf. Mass balances show that biological production on the shelf far outweighs DOC inputs from adjacent estuaries (3% of DOC inputs). Total DOC inventories on the shelf increase from 5.7 to 6.1 MTC between March to August 1996 due to increasing productivity and stratification. On shelf residence times of DOC range between 21 to 103 which are well below estimated “semi-refractory” DOC half-lives, making DOC an effective vehicle for carbon transport.; Evaluation of total dissolved inorganic carbon (DIC) pools on the shelf yields a net consumption of DIC between 11 to 32 MTC/year (about 6% of DIC inputs). This DIC is sufficient to account for the carbon requirements for both dissolved and particulate carbon (DOC and POC) exports (11 ± 1 MTC per year). Air/sea flux estimates show that there is a shift from net inflow of carbon in March 1996 to net degassing in August 1996. This shift is driven by temperature changes in surface waters but is dampened by on-shelf productivity. The annual average air/sea influx is on the order of 7 ± 6 MTC/year. Bulk water flows deliver 97% of the DIC to the MAB, air/sea exchange delivers another 2% while the remaining 1% is from freshwater sources. Carbon sources for DOC and POC production are primarily derived from bulk waters while air/sea flux can supply only part of the carbon demand. The air/sea flux extrapolates to a global continental shelf CO₂ uptake of 1.3 GTC per year but is estimated to within a factor of 2. The export of DOC and POC from continental shelves to adjacent slope waters is estimated at 3 GTC per year, 70% of which is as DOC. The fate of this exported carbon is not known, however a portion is likely to be injected to deep waters where it enters a pool with a longer residence time as either respired CO₂ or as refractory DOC. Both DOC and DIC carbon assessments support the hypothesis that the MAB (and perhaps other continental shelf regions) is net autotrophic and indicate that continental shelves must be considered as important components in global carbon cycling models.