Vórtices da Corrente das Agulhas e suas implicações na distribuição e ecologia do picoplâncton, nanoplâncton e microplâncton no Oceano Atlântico Sull

Abstract

From the southwestern termination of the Agulhas current, anticyclonic eddies are emitted and drift across the South Atlantic Ocean. The rings formation of the Agulhas Current retroflection plays an essential role in the thermohaline circulation, climate, and the plankton dispersion and ecology. This study is based on FORSA cruise, from Cape Town (South Africa), crossing the Atlantic to Arraial do Cabo (Brazil) in June 2015, where three eddies from Agulhas current of different ages (E1, seven months; E3, eleven months; E5, twenty-four months). In the samples destined to the evaluation of the planktonic community the following analyzes were carried out: i. picoplankton density and biomass (bacterioplankton and picoeukaryotes); ii. analysis of density, biomass and composition of nanoplankton protists and iii. density, biomass, composition and functional diversity of microplankton protists. The picoplankton was analyzed using flow cytometry, while the nano and microplanktonic communities were analyzed using bright-field and phase contrast microscopy. The stations were: one station control outside, and three stations inside the eddy (at the border, midway, and center). The youngest eddy, E1 showed the lowest temperatures and salinities and higher fluorescence, and the highest concentration of DIN, especially nitrite, and phosphate. The communities' picoplankton changes in the older eddies who lost vorticity and a mixture of nutrients that alter the picoplankton community. As for the autotrophic picoplankton, Prochlorococcus sp. and Synechococcus sp. were abundant in the observed eddies, the first presenting the highest densities and the second the highest biomass. These two genera demonstrated competition in the eddies environment, with a negative correlation observed between densities. Heterotrophic bacteria were the group with the highest density and biomass in almost all stations and depths, reinforcing that bacteria play a vital role in the organic matter remineralization, mainly in the eddies center.The nanophytoplankton groups' density demonstrated that phytoflagellates were dominant in E1, but cyanobacteria were dominant in all the other stations over phytoflagellates and dinoflagellates. The higher densities of cyanobacteria in eddies E5 (2 x 105 cells.L-1) reinforce the importance of these autotrophic organisms for the oligotrophic tropical ocean. Concerning the microplankton community, a increase in species richness was evident with a decrease in eddy intensity. The microplankton contained mainly mixotrophic dinoflagellates and cyanobacteria adapted to oligotrophic conditions. This fits the neutrality theory, once the functional strategy was the same, where resources are scarce, and the selection of species that best use the source of nutrients or use evolutionary advantages to live in an oligotrophic environment.