| Summary: | This research was carried out to assess phytoplankton diversity, distribution and ecology on the Pernambuco Continental Shelf and Oceanic region (lat. 7º 33' 00" S to 8º 41' 50" S and long. 34º 04' 47" W to 35º 01' 20" W). Samples were collected during the Joint Oceanographics Projects (JOPS II-Leg 5) by double oblique hauls with a baby bongo net 64 µm mesh size at depth between 14 and 150 m in inshore and offshore waters respectively, in seven transects, totaling 34 stations. The temperature and salinity characterised the tropical water masses. The dissolved oxygen was oversaturated in all stations. The nutrient-low concentrations showed an oligothrophic pattern in the whole area. 173 specific and infraspecific taxa were identified: Dinophyta (103 taxa), Bacillariophyta (61 taxa), Cyanobacteria (7 taxa), Chlorophyta and Chrysophyta (1 taxon). The family Ceratiaceae presented the highest species number (47 taxa). The cyanobacteria Trichodesmium erythraeum Ehrenberg, Oscillatoria spp. and the diatom Leptocylindrus danicus Cleve had higher frequence of occurrence and abundance. Species diversity varied from 0.71 to 3.46 bits.cell-1 and this low index was due to Trichodesmium erythraeum bloom and evenness from 0.14 to 0.65, showing an unstable pattern. The oceanic planktonic species were responsible for the higher richness with 78 taxa, corresponding to 58.39% of the total microphytoplankton. The species association presented two groups. The first one divided in two subgroups: one associated to the neritic/oceanic and oceanic planktonic species and the other to the oligotrophic indicator species. The second group was composed of some key species which were the most abundant and frequent in the area. The first three Principal Component Analyses (PCAs) explained 50.91% of the data variance showing that the area is structured by two groups: one offshore composed by oligotrophic indicators, and another nearshore influenced by continental fluxes.
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