Microbial activities and organic matter degradation at three sites in the coastal North Atlantic: Variations in DOC turnover times and potential for export off the shelf

The Mid-Atlantic Bight is a highly productive region where considerable quantities of organic matter are processed, transformed, and respired. Much of this organic matter is in the form of dissolved organic carbon (DOC). We investigated the activities of heterotrophic microbial communities responsible for DOC processing at three sites in the coastal Mid-Atlantic Bight/northern end of the South Atlantic Bight: near the Chesapeake Bay mouth, off Cape Hatteras, and near Cape Lookout, NC. Activities of endo- and exo-acting extracellular enzymes (polysaccharide hydrolases, and glucosidase and peptidases, respectively), and bacterial protein production were measured in surface and subsurface waters at the three stations. Total suspended material, cell counts, and particle-associated enzyme activities were also quantified. Water masses at each station showed distinct physical characteristics (temperature, salinity, total suspended material), and differed notably in rates and spectrum of enzyme activities, as well as in the fraction of particle-associated activities. Despite similar cell counts, bacterial productivity differed by a factor of ca. 30 among depths and stations. Glucosidase and peptidase activities differed by a factor of ca. 40 among stations, but the differences in enzyme activities among stations did not scale with the differences in bacterial productivity. Theoretical turnover times of dissolved carbohydrates - calculated from total dissolved carbohydrate concentrations and polysaccharide hydrolase activities - range from weeks to months, suggesting that dissolved carbohydrates may be among the components of DOC that can be advected from the Mid-Atlantic Bight into the open ocean, connecting the carbon cycle in the ocean margin with the ocean's interior.