Mesoscale Drives Inorganic Nutrient Dynamics in Two Areas of the Mexican Off-Shore of the Gulf of Mexico

Cristian Hakspiel, Autonomous University of Baja California, Instituto de Investigaciones Oceanologicas, Enesenada, BJ, Mexico, Victor Froylan Camacho Ibar, Autonomous University of Baja California, Ensenada, BJ, Mexico and Augusto Valencia, Autonomous University of Baja California, Instituto de Investigaciones Oceanologicas, Ensenada, BJ, Mexico
The environmental variability and productivity in the upper and subsurface layers in deep waters of the Gulf of Mexico (GoM) are mainly controlled by the shedding of Loop Current eddies and other mesoscale and submesoscale local processes. However, in contrast to the northern GoM, nutrient dynamics in Mexican off-shore waters of the western and southern GoM has been poorly described. We report here the results of nutrient distributions (nitrate plus nitrite, phosphate and silicate) during three oceanographic cruises carried out in the off-shore areas of Perdido and Coatzacoalcos (areas concessioned for deepwater oil exploration and extraction) during March and September 2016 and June 2018. Our results indicate that mesoscale dynamics, more than seasonality, modulates nutrient distributions and their availability in the upper 150 m. We found a significant positive/negative correlation (P<0.05) between depth-integrated nutrient concentrations and indicators of cyclonic/anticyclonic circulation including sea level height anomalies, surface chlorophyll-a and the σθ = 25.5 isopycnal depth. The strength of this relationship decreased in March due to winter mixing, as well as for silicate in coastal stations where continental inputs may modify its spatial distribution. The slopes of the nitrate plus nitrite versus phosphate relationship do not provide enough information to conclude that there are obvious biogeochemical differences between samples influenced by cyclonic and anticyclonic eddies. However, these slopes showed large differences through depth, with samples around the nutricline (σθ 25.2 to 25.8) showing a mean value ~24.4 and deeper intermediate water masses (σθ 25.8 to 27.7) showing a mean value ~ 14.2, for all cruises and stations. The relatively high value at the nutricline suggests remineralization of settling organic matter with a high N/P ratio probably produced by diazotrophs.

Keywords: mesoscale eddies, macronutrients, Perdido, Coatzacoalcos, N:P ratio