Diversidade funcional do fitoplâncton no oceano Atlântico Tropical Oeste

Abstract

As the composition of the phytoplankton community affects the functioning of aquat- ic ecosystems, it is important to know which factors govern its dynamics. Because they are easily dispersed, given their small size and high abundance, it is reasonable to assume that local conditions can act as environmental filters, favoring groups of species that share similar adaptive characteristics. There are a few data on the spatial distribution of the phytoplankton community in the Western Tropical Atlantic and there are no studies on the functional diver- sity of phytoplankton relating the physical and chemical characteristics of the photic zone in this latitudinal gradient, under the influence of the ITCZ (5° to 10° N). Therefore, the present study evaluated the spatial distribution of phytoplanktonic functional groups in the Tropical Atlantic during two campaigns, in 2017 under La Niña influence and in 2018 under El Niño influence, through qualitative and quantitative analysis of nanoplanktonic and microplankton- ic organisms, biovolume and functional traits. Functional groups were formed and statistical- ly tested, and despite the similarity between the values of biomass and phytoplankton density in the 2017 and 2018 observations, the phytoplankton community showed different composition, with greater abundance of diazotrophic cyanobacteria in 2017 and an increase in functional richness (FRic), under the La Niña event, when comparing to 2018 campaign, under a El Niño event. The increase in surface water temperature and weakening of divergence and consequently of nutrient input to the euphotic zone, translated as variations in the N/P ratio, during the El Niño event affected the abundance of functional traits and species richness at ITCZ. In a biogeographic context, the environmental gradients observed varied little when compared to temporal variations (years under the influence of La Niña or El Niño), however, the divergence observed under the influence of the ITCZ in 2017, created a discrete gradient in surface water temperature and nitrogen availability. Temperature acted as an environmental filter, affecting the traits distribution of diazotrophic cyanobacteria that gives these organisms an adaptive advantage and affects nitrogen concentrations in the mixed layer.