A influência da deposição atmosférica da poeira mineral da Patagônia na biomassa fitoplanctônica do setor Atlântico do Oceano Austral

Abstract

The Southern Ocean is the larger ocean region where the macronutrients needed for primary production remain in high levels through the year. This condition is known as High Nutrient Low Chlorophyll (HNLC) and is conditioned, largely, by the relative shortage of micronutrients, particularly iron. Several experiments proved that the supply of iron to this system enhances biological production, increasing carbon fixation and, eventually, its exportation to deep waters. This phenomenon received much attention in the last 20 years due to its possible influence in the climate, through carbon cycle. The inverse relationship between the atmospheric CO2 concentration and the mineral dust flux observed on the Central Antarctic glacial records suggest that atmospheric deposition may be an important source for the supply of micronutrients. However, the contribution of mineral dust deposition for the primary production in this region remains to be demonstrated and its possible hole in the climate system it s not yet conclusive. In the case of Atlantic Southern Ocean, that s influenced by Patagonia, the low modern flows of mineral dust and the low iron solubility associated with aluminum-silicate structure led many authors to state that oceanic sources of micronutrients are more determinants. However, experimental evidence are lacking. In the present work, we approach the study of fertilization of Atlantic Southern Ocean by Patagonian dust employing two different tools: (1) orbital remote sensing of mineral aerosols and chlorophyll-a on inter-annual scale; and (2) a fertilization experiment with Patagonian dust, carried through in Drake Passage, considering estimated flux for the modern era and for the last glacial. After twelve days of bioassay, the dust addition treatments showed increase on chlorophyll-a and cell abundance beyond controls levels. Intermediary and higher levels of addition didn t differ between each other regarding the intensity of biological response, separating only of the lower addition treatment. These results indicate that even modern Patagonia dust flux is capable of providing micronutrients that are scarce in the water column, with potential to deflagrate a bloom. Through remote sensing analysis we have identified a region with high correlation between dust and chlorophyll-a, that s located between the Subtropical Front and the Polar Front, extending from Argentina to south of Africa. This region differs from waters south of the Polar Front by means of a deeper mixed layer, lower silicate concentrations, low diatom biomass and, is estimated, greater iron physiological stress and lower iron oceanic supply. Together, these properties seem to create conditions to which biological response would be more sensible to dust deposition. These results cast new light over controls on modern primary production in the region and over the phytoplankton climatic regulation in the Southern Ocean, mediated by Patagonian dust deposition.