Benthic–pelagic coupling in the Gulf of Mexico hypoxic area: Sedimentary enhancement of hypoxic conditions and near bottom primary production

• Benthic nutrient fluxes and water column ammonium reveal coupled biogeochemical cycles.
• The benthic pump prolongs persistent hypoxia in the N limited Gulf of Mexico hypoxic zone.
• A budget of measured nitrogen pools coupled with physical and chemical environmental parameters.
• Benthic–pelagic coupling sustains hypoxia, promoting OM production, and oxygen consumption.

The seasonal bottom water hypoxia that covers large portions of the Louisiana continental shelf (USA) can extend far beyond the nutrient-rich Mississippi River plume. The hypoxia usually persists under surface-water that is all but depleted of nitrogen and phosphorous. Steep near-bottom gradients of NH4+NH4+ during summer hypoxic conditions suggest that the sediments may become a net source of fixed nitrogen, potentially providing an important limiting nutrient to the system. Coupled measurements of benthic ammonium fluxes and water column NH4+NH4+ profiles are combined to estimate turnover time of ammonium below the pycnocline. A tight, statistical relationship between photosynthetically active radiation (PAR) and NH4+NH4+ reflects uptake by photosynthesis in the chlorophyll maximum, but utilization by nitrifiers and heterotrophic microbes could also be important at low levels of light, consuming oxygen (and thus contributing to hypoxia) and removing fixed nitrogen by denitrification.