Phytoplankton of the continental shelf on the equatorial southwestern atlantic - Chemotaxonomic approach

Abstract

The biomass and composition of the phytoplankton community were investigated in the continental shelf of the Equatorial Southwestern Atlantic (Ceará, NE Brazil), a region influenced by the semiarid climate and dominated by coral reefs. In addition, it is characterized by an intense hydrodynamic process, low nutrient input, constant high temperatures and no upwelling. Samples for marker pigments and dissolved inorganic nutrients (phosphate, silicate and dissolved inorganic nitrogen-NID) were collected in the dry period of 2019 at the subsurface (0.15 m) and at a second depth (8-14 m) equivalent to half of the total depth of each of the ten stations. Dissolved oxygen, water transparency, depth and vertical profiles of temperature and salinity were also obtained. The identification and quantification of pigments took place through High Performance Liquid Chromatography (HPLC) analysis and the data were processed by CHEMTAX, a chemical taxonomy program that aims to quantify the contribution of each group to total chlorophyll a concentrations. The assemblage was diverse and included the following groups: type-1 diatoms, type-2 diatoms, Synechococcus, Prochlorococcus, haptophytes, green flagellates, dinoflagellates and cryptophytes. The area had low biomass (mean 0.36 ± 0.15 mg.m-3), but higher values were observed close to some reefs. Diatoms (38.2%; 14.5 to 66.6%), picocyanobacteria (28.5%; 15 to 54.4%) and haptophytes (24.5%; 11.2 to 41.9%) were the main components of biomass and also stood out for the high variation in relative contribution values between samples. The picocyanobacteria and the haptophytes belonging to the pico and nano fractions, respectively, contributed with more than half of the total biomass (53%). The temperature (mean 27.8 ± 0.05°C) and salinity (mean 36.3 ± 0.14) presented small vertical and spatial variations, indicating homogeneity of the water column with regard to these parameters. Phosphate (0.32 ± 0.2 μM), NID (0.41 ± 0.3) and silicate (2.94 ± 5.6 μM) concentrations were low, but at stations E1 (closer to the coast) and E10 (most distant) relatively higher concentrations for the three nutrients were obtained. Both the average TRIX index value (2.5) and TRIX values for each station ensure that this is an oligotrophic environment. The type-2 diatom group had a negative correlation with the concentrations of NID and silicate, while the haptophytes were negatively correlated with phosphate, showing the consumption of these nutrients by these groups. In the group of type-2 diatoms, the genus Pseudo-nitzschia is inserted, whose presence in the region was confirmed by optical microscopy. This genus has ecological importance as it has species considered potentially toxic, which synthesize domoic acid, an amnestic toxin. In this context, the data in

this dissertation provide unprecedented information on the composition of the phytoplankton

community in an oligotrophic marine environment dominated by coral reefs located in a semi- arid and highly energetic region, in which high variations in the relative contributions of the

main groups were highlighted (diatoms, picocyanobacteria and haptophytes). Other important features are the conspicuous presence of the genus Pseudo-nitzschia and the large biomass contribution of smaller size class groups: pico (mainly Synechococcus) and nanoplankton (haptophytes).