On the variability in the CO2 system and water productivity in the western tropical Atlantic off North and Northeast Brazil

Abstract A high-resolution nutrient biochemistry and carbonate system surface synoptic data set from the N-NE Brazilian continental shelf was reanalyzed to fill a gap in the time series of the carbonate system in the region and to allow us to perform a historical analysis of its evolution in recent years. We used data collected from 7 oceanographic cruises (n = 852) undertaken between March 1995 and September 2001 during the Brazilian Program “REVIZEE” in the North (N) and Northeast (NE) Economical Exclusive Zones of Brazil. Measured temperature and salinity data, which exhibited strong fluctuations (25.5 °C – 29.5 °C and 13.2–37.4 units, respectively), showed significant differences between the N and NE campaigns. The concentrations of dissolved inorganic nitrogen (DIN), PO 4 − and SiO 2 − were higher in the N region than in the NE region, mainly due to fluvial transport, and nitrogen: phosphorus (N:P) ratios of −1 ) values showed strong spatial variations that were mainly associated with the Amazon plume. The calculated p CO 2 values reached 423 μatm offshore in the NE region during boreal winter. The calculated sea-air CO 2 fluxes (average: +0.3 ± 1.7 mmol m −2  d −1 ; range: −1.2 to +2.0 mmol m −2  d −1 ) showed spatial and temporal variations, with negative values (sink) in the region of the Amazon River plume and positive values (source) offshore in the NE region (4°S to 12°S). The variability in the sea-air CO 2 fluxes in the N and NE regions was explained by variations in biological activity and the thermodynamic effect of temperature, respectively. The analysis of available data, complemented with those presented here, indicated that the surface water p CO 2 values showed a positive temporal trend (+1.10 ± 0.2 μatm yr −1 ) in the NE region during the period of 1987–2010. This rate of increase is lower than that verified to have occurred in the atmosphere (+1.72 ± 0.01 μatm yr −1 ) during the same period.