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 were similarly low. In comparison, measured export rates of sinking particulate nitrogen from the euphotic zone were 2 – 3 orders of magnitude higher.
We reconciled these disparate findings using regional scale dynamics inferred independently from (A) remote-sensing products and (B) a regional biogeochemical model and found 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.
On average, elemental fluxes into and out of a system need to be in balance if the system is at (or close to) steady state. If the input and outputs are not in balance, then the system must be either gaining or losing material–processes that can only continue for finite periods of time. Since the open-ocean Gulf of Mexico, and the ecosystem there, is extremely nitrogen limited, it is interesting how the sources and sinks of nitrogen compare.
Summary of the environment we were studying.
Here I wanted to show both the study region as well as the field data we collected (colors match between the left and right panels). Not surprisingly, nitrate (the major form of nitrogen as a nutrient) becomes nearly undetectable as we approach the surface. This is because phytoplankton will utilize it and light to form organic molecules thereby reducing the nutrient concentrations.
Of particular interest to this study is the final column, which shows the export flux. Here I am showing rates of carbon removal from the system (as sinking particles), but we also measured removal rates of nitrogen. Since most of the biological processes in the ocean take place at or near the surface, this vertical transport is extremely important because once a particle sinks out, it is unlikely to be seen again for a very, very long time.
An unbalanced budget?
From our field data we are able to constrain many of the sources and sinks of nitrogen in these waters.
So looking vertically at the water column there’s a serious mismatch between sources (~10 umol N m-2 d-1) and sinks (>> 100 umol N m-2 d-1). Where is the nitrogen coming from?