Propriedades Biogeoquímicas das Massas d'Água no Oceano Atlântico Tropical Oeste

Abstract

This study characterized the chemical properties of the water masses in the Western Tropical Atlantic Ocean according to their dissolved oxygen and inorganic nutrient concentration: nitrate, nitrite, phosphate and silicate. We collected full-depth water samples from 18 oceanographic stations along the 38°W transect, from 02°S to 15°N during the PIRATA-BR XVII and XVIII cruises, in November 2017 and 2018. In this region, the surface and subsurface circulation in the Atlantic Ocean displays complex seasonal patterns, under influence of the Intertropical Convergence Zone. The water samples were collected from Niskin bottles closed in ten different depths, stored frozen, and later analyzed through spectrophotometry. Besides that, the CTD-O2 data provided continuous salinity, temperature, and dissolved oxygen measurements, used to identify the water masses according to their thermohaline indexes. The same biogeochemical parameters from GLODAPv.2 database, sampled between 1990 and 2010 along the 35°W, from 03°S and 10°N, were also used to characterize the water masses. Six water masses were identified in the region based on their neutral density limits: Tropical Surface Water (TSW, γn < 24.448 kg m-3); South and North Atlantic Central Water (SACW and NACW, γn 24.448 – 26.815 kg m-3); Antarctic Intermediate Water (AAIW, γn 26.815 – 27.7153 kg m-3); North Atlantic Deep Water (NADW, γn 27.7153 – 28.135 kg m-3); and Antarctic Bottom Water (AABW, γn > 28.135 kg m-3). All water masses had very low nitrite concentration. TSW corresponds to an oligotrophic water that has low concentrations of other nutrients, and high oxygen concentration (~ 200 μmol/kg) due to gas exchange with the atmosphere; NACW and SACW have low nutrient concentration, acting as transition regions between TSW and AAIW; AAIW showed the highest concentration of phosphate (~ 1.35 μmol/kg) and nitrate (~ 30 μmol/kg) and low dissolved O2 concentration (~ 130 μmol/kg); NADW had the highest O2 concentration mean (~ 250 μmol/kg), and intermediate concentrations of other nutrients; AABW, on the other hand, was the water mass with the highest silicate concentration (~ 80 μmol/kg), as well as a high O2 (~ 240 μmol/kg), nitrate (~ 25 μmol/kg) and phosphate (~ 1.80 μmol/kg) concentrations. Finally, it was possible to verify that some water bodies have one or more nutrients and biogeochemical parameters that have specific concentrations, helping in their identification and characterization, such as the high nitrate and phosphate in AAIW, and high silicate in AABW, compared to central and surface water masses.