Origins and temporal scales of hypoxia on the Louisiana shelf: Importance of benthic and sub-pycnocline water metabolism

Hypoxic-to-anoxic conditions (2–0 mg O 2 l − 1 ) occur in the bottom waters of the northern Gulf of Mexico on the Louisiana shelf west of the Mississippi river delta during late spring and summer where the rate of oxygen consumption exceeds its rate of input from physical transport plus photosynthetic generation. Although consumption of oxygen in the water column primarily via oxic respiration is an important process, the loss of oxygen at and near the seafloor may also be an important sink contributing to seasonal low oxygen conditions in the relatively shallow overlying waters in this region. Associated with the flux of oxygen into the sediments is the flux of nutrients out of the sediments from the remineralization of sedimentary organic matter via a number of possible electron acceptors. The nutrients that are released from the sediment can potentially stimulate further primary production. This can lead to generation of oxygen in the water column and production of organic matter, much of which can be transported to the seafloor where it again becomes a sink for oxygen. A non-steady-state data driven numeric benthic–pelagic model was developed to investigate the role of sediment and water-column metabolism in the development of hypoxia on the Louisiana shelf. The model simulations bare out the importance of sediment oxygen demand as the primary sink for oxygen at the beginning and end of a hypoxic event on the shelf, but once hypoxia has developed, the sediments, now isolated from the oxygen-rich surface waters, are driven into a more anoxic mode, becoming more dependent on sulfate and metal reduction. As a result, the bottom water near the pycnocline becomes the major sink for oxygen. Model simulations also suggest that there is a delay of several weeks between metabolite production (especially ammonium) and its efflux from the sediments. Thus the maximum sediment ammonium export occurs in September and October in time to fuel autumnal phytoplankton production, thereby continuing a biogeochemical cycle that expands the temporal and spatial scales of hypoxia on the Louisiana shelf.