Aspectos climáticos e regionais controladores da distribuição e fluxos da matéria orgânica dissolvida no oceano Austral

Abstract

Dissolved organic matter (DOM) is the largest organic stock in the oceans, linking the carbon and nutrient cycles. However, there are still significant gaps in understanding the role of DOM in polar environments, due to spatiotemporal sampling limitations. This study evaluated, over three chapters, the climate and regional aspects controlling the distribution and fluxes of DOC in the northern Antarctic Peninsula (NAP) and the Atlantic sector of the Southern Ocean between 1972 and 2022, considering a series of observational data on dissolved organic carbon and nitrogen (DOC and DON) and reconstructed DOC data based on the correlation between net DOC production and nitrate deficits due to planktonic production. Data collected by the Brazilian High Latitude Oceanography Group (GOAL), the Alfred Wegener Institute (AWI, Germany) and data from international repositories were used. DOC and DON concentrations ranged from 33.1-157.6 μmol kg–1 and 0.1-13.6 μmol kg–1, respectively. The wide sample coverage made it possible to identify significant regional differences in DOC distributions in the NAP during the austral summers of 1995 and between 2015 and 2019, due to the lability of DOC. It was also possible to observe that the advective fluxes of DOC in the NAP are 106 times greater than the diffusive fluxes. However, ocean fronts play an important role in increasing upward and downward diffusive fluxes of DOC. Along the Atlantic sector of the Southern Ocean, regional and seasonal differences in the distributions of DOC, DON and the DOC:DON ratio between 2004 and 2022 demonstrated the importance of South Georgia Island for marine biogeochemistry, during the austral spring, and the dynamics of the Antarctic Circumpolar Current (ACC) fronts for the production and export of DOM. The NAP and Atlantic sector of the Southern Ocean data were also combined with reconstructed data to assess the effects of the El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM) climate modes on DOC distribution between 1972 and 2022. SAM and ENSO events "in phase" affected the distributions of DOC, through the variability of the fluxes of the ACC and the Weddell Sea. However, the results for 1988 and 2017 also indicate the importance of assessing DOC contributions in "out-of-phase" ENSO and SAM events of record intensity. Therefore, this doctoral thesis contributes to understanding the role of DOM in the complex biogeochemical cycle of the Southern Ocean.