Impacts of mesoscale eddies on the nitrate distribution in the deep-water region of the Gulf of Mexico

Mesoscale eddies play an important role in modulating the vertical distributions of nutrients and primary productivity in the ocean. In this study, we evaluated the role that eddies play in determining the vertical distributions of Nitrate + Nitrite (N + N), dissolved oxygen (DO), conservative temperature (CT), and absolute salinity (S_A) in the deep-water region of the Gulf of Mexico (GoM). We compared water-column profiles up to 1000 m depth and calculated the N + N stock in the euphotic layers within cyclonic eddies (CEs), anticyclonic Loop Current eddies (LCEs), and waters free of eddy influence (NOE). Stations were sampled based on a fixed grid during two oceanographic campaigns carried out under winter mixing and thermal stratification conditions and were classified as CE, LCE, or NOE stations based on a cluster analysis using dynamic height (DH) and the depth of the 25.5 kg m⁻³ isopycnal. Average profiles of the stations closest to the eddy cores (those with the highest or lowest dynamic heights and isopycnal depths of 25.5 kg m⁻³) were compared with those of NOE stations. Differences between structures were observed in multiple variables up to 1000 m, although the most notable differences occurred in the euphotic and upper mesopelagic zones (< 250 m). The average concentration of N + N between 0 and 50 m was ~0.03 μmol kg⁻¹, which was close to the detection limit, regardless of the presence or absence of eddies. The onset of the nitracline, defined as the depth at which the N + N concentration was 0.5 μmol kg⁻¹, was observed at ~56 m, ~ 82 m, and between ~100–120 m in CE, NOE, and LCE waters, respectively, which resulted in contrasting N + N stocks in the intermediate euphotic layer (50–100 m) among CE (158–208 mmol m⁻²), LCE (6–9 mmol m⁻²), and NOE (~ 28 mmol m⁻²) stations. As a result, fluorescence in the deep chlorophyll maximum was markedly higher in CE waters compared to that in NOE and LCE waters. In the summer campaign, a subsurface DO maximum between 30 and 50 m, which is characteristic of oligotrophic regions, was observed as a product of photosynthesis most likely associated with a positive anomaly of preformed N + N and positioned around the 24.3 kg m⁻³ isopycnal. In subsurface CE, LCE, and NOE waters, negative anomalies of preformed N + N were observed at different depths that remained between the same isopycnals (24.8–26.2 kg m⁻³), reaching up to −3.4 μmol kg⁻¹ in CEs. As has been suggested for other oligotrophic regions, this anomaly could be due to the uptake of N + N by migrating phytoplankton without DO production, the use of N + N by heterotrophic bacteria that are limited by dissolved organic nitrogen, the remineralization of transparent exopolymeric particles (TEPs) with high C:N ratios, or to a combination of these processes, which remain to be studied within the GoM.