Biogeochemical Characteristics of Western Tropical Atlantic Ocean Water Masses

Water masses are commonly identified according to their conservative parameters. However, there are also studies that use non-conservative parameters, together with the conservative ones, to refine the water masses identification. The aim of this study was to analyze the chemical properties of the water masses in the western tropical Atlantic Ocean (WTAO) according to their inorganic nutrient concentration: nitrate-NO3–, phosphate-PO43–, and silicic acid-Si(OH)4, to set a regional descriptive framework of the water column in view of future comparative studies. We collected full-depth water column samples from 18 oceanographic stations from a latitudinal transect along 38°W, from 02°S to 15°N during the PIRATA-BR XVII and XVIII campaigns, in November 2017 and 2018. We have also used the regional data available from GLODAPv.2 data product to improve the water masses characterization. Six water masses were identified in the region based on their values of potential temperature, salinity, potential density, and neutral density observed in the study area according to the CTD-O2 data: Tropical Surface Water (TSW); South and North Atlantic Central Water (SACW and NACW, respectively); Antarctic Intermediate Water (AAIW); North Atlantic Deep Water (NADW); and Antarctic Bottom Water (AABW). Regarding the nutrient content within each water mass, our results showed that TSW corresponds to a surface oligotrophic water; NACW and SACW have intermediate nutrient concentration values between TSW and AAIW; AAIW showed the highest concentration of phosphate-PO43– (~ 1.35 µmol kg–1) and nitrate-NO3– (~30 µmol kg–1); AABW, on the other hand, was the water mass with the highest silicic acid-Si(OH)4 concentration (~ 80 µmol kg–1), as well as high nitrate-NO3– (~ 25 µmol kg–1) and phosphate-PO43– (~ 1.80 µmol kg–1) concentrations. Additionally, the water column between 300 and 650 m displays an increase in phosphate-PO43– concentrations north of 5oN, associated to a low dissolved oxygen area coupled to the North Equatorial Under Current (NEUC). Long-term, sustained hydrographic and ocean biogeochemistry observations are key to understand how climate change is affecting the ocean, and this study is a contribution to that.