Lateral Advection Supports Nitrogen Export in the Oligotrophic Open-Ocean Gulf of Mexico
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AbstractIn contrast to its productive coastal margins, the open-ocean Gulf of Mexico (GoM) is notable for highly stratified surface waters with extremely low nutrient and chlorophyll concentrations. Field campaigns in 2017 and 2018 indicate low rates of turbulent mixing, which combined with oligotrophic nutrient conditions, give very low estimates of diffusive flux of nitrate into the euphotic zone (< 1 mol N m-2 d-1). Estimates of local N2-fixation are similarly low. In comparison, measured export rates of sinking particulate organic nitrogen (PON) from the euphotic zone are 2-3 orders of magnitude higher (i.e. 462 1144 mol N m-2 d-1). We reconcile these disparate findings with regional scale dynamics inferred independently from remote-sensing products and a regional biogeochemical model and find that laterally-sourced organic matter is sufficient to support >90% of open-ocean nitrogen export in the GoM. Results show that lateral transport needs to be closely considered in studies of biogeochemical balances, particularly for basins enclosed by productive coasts.
