The biogeochemical cycling of dissolved organic nitrogen in estuarine sediments

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Abstract
Benthic fluxes and pore‐water profiles of dissolved organic nitrogen and carbon (DON and DOC, respectively) were determined in seasonal studies at contrasting sites in Chesapeake Bay. Pore‐water dissolved organic matter (DOM) concentrations were elevated over bottom‐water values, generally increased with depth, and ranged from 15 to ~160 µM for DON and ~200–2000 µM for DOC. Pore‐water DOM concentrations and the C:N ratio of this material showed spatial (depth) and temporal changes that varied among the sites studied. These trends appeared to be related to differences in the types of sediment organic matter (SOM) undergoing remineralization, as well as differences in the biogeochemical processes occurring in the sediments (e.g., the presence or absence of bioturbation and bioirrigation). Measured DON fluxes ranged from essentially zero to ~0.4 mmol m−2 d−1, and together with benthic DOC fluxes were coupled to seasonal trends in temperature and SOM remineralization rates. On an annual basis, benthic DON fluxes were a small fraction (~3%) of benthic inorganic nitrogen fluxes. At an anoxic nonbioturbated site measured DON fluxes were essentially identical to calculated diffusive DON fluxes, whereas at a bioturbated and bioirrigated site, measured DON fluxes were much greater than calculated fluxes. The molar ratios of DOC to DON benthic fluxes ranged from ~2 to 6 and were lower than those of pore‐water DOM, which were >~10. This implies that DOM accumulating in these sediment pore waters was carbon‐rich compared with the DOM that was either remineralized or escaped the sediment as a benthic flux. These measured benthic DON fluxes and estimated DON fluxes from continental margin sediments combine to yield a lower limit for the integrated sediment DON flux to the oceans that is similar to a value estimated previously. These net DON inputs to the oceans are small compared with internal oceanic DON cycling rates, although sediment DON fluxes and riverine DON inputs are roughly of the same order. At the same time, our results also suggest that the DON escaping from these sediments may not be inherently refractory because of its observed low C:N ratio. This implies that estuarine sediments (and perhaps marine sediment in general) may not be a major source of “refractory” DON to the oceans.

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