Abstract
In 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 identified low rates of turbulent mixing, which combined with oligotrophic nutrient conditions, give very low estimates for diffusive flux of nitrate into the euphotic zone (< 1 µmol 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 µmol 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.
Highlights
In 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
Nutrients and organic matter can be carried into the offshore region, this flux is seldom included in new production (NP) calculations
Nitrification was not directly measured, ammonium-specific rates from other oligotrophic studies (0.02–0.5 d−1; ref. 3 and references within) suggest potential nitrate sources of 1 and 30 μmol N m−3 d−1, which could supply the majority of nitrate utilized by the phytoplankton community
Summary
In 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. We combine these in situ measurements with independent remote-sensing observations and a biogeochemical model to test alternate hypotheses that (1) locally upwelled nitrate, (2) N2-fixation, or (3) lateral transport support N export from oligotrophic oceanic waters of the GoM.
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