Description: This data set includes water column nitrate+nitrite and phosphate concentrations and ancillary CTD variables from the deep-water region in the southern Gulf of Mexico (GoM). These measurements were used to estimate water mass fractions using an Optimum Multi-parameter analysis that requires semiconservative parameters (NO and PO4* parameters) calculated with nitrate+nitrite, phosphate and dissolved oxygen concentrations. Water samples were collected with a rosette equipped with 12 20-L Niskin bottles during the XIXIMI-5 oceanographic cruise that took place during June 10–25, 2016. The southern region of the GoM (20–26º N, 86–97º W) comprises the deep-waters of the GoM, including the continental slope and abyssal plain. A total of 35 stations were sampled on board R/V Justo Sierra of the National Autonomous University of Mexico. Data columns include sampling Date/Time stamp in ISO format, Longitude, Latitude, Depth, Pressure, temperature and salinity from CTD. The CTD sensors were previously calibrated by the manufacturer, and the accuracy of the measurements was ± 0.001 ºC for temperature, ± 0.002 for salinity, and 2% for DO. Data from the DO sensor were calibrated with data from the Niskin bottle samples, analyzed with the microWinkler method. The accuracy and precision of the microWinkler method were 0.1% and ~ 1.3 µmol/kg, respectively. Nitrate+nitrite and phosphate analyses were performed with a SEAL-AA3-HR auto-analyzer (SEAL Analytical Ltd., Norderstedt, Germany), following the guidelines described in the GO-SHIP Repeat Hydrography Manual. Accuracy and precision were estimated with measurements of certified reference material (CRM) for nutrients (Lots CD and CC; Kanso Co. Ltd., Osaka, Japan). The Ocean Data Standards from the UNESCO were used to assign Quality Flags (QF) for nutrients. The limits of detection (3 SD, n = 11) for nitrate+nitrite and phosphate were 0.037 and 0.028 µmol/kg, respectively. The mean values obtained for nitrate+nitrite from the CD and CC lots were 5.514±0.012 µmol/kg and 30.958±0.047 µmol/kg, respectively. The sum of the certified values for nitrate and nitrate from the CD and CC lots is 5.516±0.050 µmol/kg and 30.996±0.240 µmol/kg, respectively. The mean values obtained for phosphate from the CD and CC lots were 0.447±0.010 µmol/kg and 2.092±0.012 µmol/kg, respectively. The certified values for phosphate from the CD and CC lots is 0.446±0.008 µmol/kg and 2.080±0.019 µmol/kg, respectively. This research has been funded by the Mexican National Council for Science and Technology - Mexican Ministry of Energy - Hydrocarbon Fund, project 201441. This is a contribution of the Gulf of Mexico Research Consortium (CIGoM, http://www.cigom.info). We acknowledge PEMEX's specific request to the Hydrocarbon Fund to address the environmental effects of oil spills in the Gulf of Mexico. Project: Implementación de redes de observación oceanográficas (físicas, geoquímicas, y ecológicas) para la generación de escenarios ante posibles contingencias relacionadas a la exploración y producción de hidrocarburos en aguas profundas del Golfo de México.

Description: The seasonal and interannual variability of chlorophyll in the Gulf of Mexico open waters is studied using a three‐dimensional coupled physical‐biogeochemical model. A 5 years hindcast driven by realistic open‐boundary conditions, atmospheric forcings, and freshwater discharges from rivers is performed. The use of recent in situ observations allowed an in‐depth evaluation of the model nutrient and chlorophyll seasonal distributions, including the chlorophyll vertical structure. We find that different chlorophyll patterns of temporal variability coexist in the deep basin which thereby cannot be considered as a homogeneous region with respect to chlorophyll dynamics. A partitioning of the Gulf of Mexico open waters based on the winter chlorophyll concentration increase is then proposed. This partition is basically explained by the amount of nutrients injected into the euphotic layer which is highly constrained by the dynamic of the winter mixed layer. The seasonal and interannual variability appears to be affected by the variability of atmospheric fluxes and mesoscale dynamics (Loop Current eddies in particular). Finally, estimates of primary production in the deep basin are provided.