Control of Mesoscale Eddies on the Vertical Nitrate Distribution and the Nitracline Shape and Depth in Open Ocean Waters of the Gulf of Mexico

Depth and curvature of the nitracline are important features of the vertical nitrate distribution, and its implications on the supply of nitrate to the euphotic layer and its control on primary production. Results of a study of the control of mesoscale eddies on the vertical nitrate concentration ([NO₃]) in open waters of the Gulf of Mexico (GoM) are presented. We analyzed 35 vertical profiles obtained during the XIXIMI-5 oceanographic campaign carried out in Mexican waters in June 2016. Based on non-steric sea level anomalies and integrated [NO₃] from 0 to 150 m ([NO₃]_i), stations were classified as influenced by cyclonic (CEs, 15 stations) or anticyclonic (AEs, 12 stations) eddies or non-eddy influenced (NEs, 8 stations; used to determine the background profile). Nitracline depth and curvature were analyzed as a function of σ_t employing linear and second order polynomial fits. NO₃ concentration anomalies at 100m (above the euphotic layer depth, 120 < Zeu < 150 m) reveal four CEs: ⁓ 4.8, ⁓ 4.5, ⁓ 3.0 and ⁓ 2.0 μmol kg^-1, and anomalies at 150 m (below Zeu) reveal two LCEs: ⁓ -3.0 and ⁓ -4.0 μmol kg^-1. Mesoscale modifies the mean nitracline depth (CEs: 64 ± 8 m, NEs: 100 ± 3 m y AEs: 113 ± 11 m), and increases the number of profiles with non-linear nitracline curvature (NEs: 50 %, CEs: 64 % AEs: 80 %). Non-linearity is emphasized in AEs showing [NO₃]_i (115 ± 20 mmol m^-2) smaller than in CEs (621 ± 100 mmol m^-2) and in NEs (315 ± 50 mmol m^-2). A latitudinal regionalization also reveals a clear difference in [NO₃]_i and the percentage of non-linear profiles among northern (24 – 26 °N: 314 ± 180 mmol m^-2, 79 %), central (22 – 23 °N: 251 ± 100 mmol m^-2, 33 %) and southern (20 – 21 °N: 664 ± 176 mmol m^-2, 38 %) transects. Our results, as reflected by the precision of the confidence intervals (95 %), show that a classification of profiles based on mesoscale features rather than a latitudinal regionalization better describe the nitracline depth and curvature.