Characterization of microphytoplankton associations on the Amazon continental shelf and in the adjacent oceanic region

Abstract

This study aimed to characterize microphytoplankton functional associations on the Amazon continental shelf and in the nearby ocean environment, taking into account the influence of environmental changes caused by the plume of the Amazon River. Two cruises were conducted during the dry period, and 34 locations were sampled at two depths (surface and depth chlorophyll maximum - DCM) to identify the microphytoplankton present as well as to determine salinity, temperature and inorganic nutrients. A total of 144 taxa were identified in the region and were grouped into the phyla Bacillariophyta (75.70%), Cyanobacteria (19.42%), Miozoa (4.68%), Ochrophyta (0.19%) and Haptophyta (0.01%). The cell biovolume ranged from 3.27 to 2.3 × 105 mm−3 L−1 at the surface and from 1.43 to 1.0 × 103 mm−3 L−1 at the DCM, with the difference between depths being significant (U = 1052; p = 0.04). The microphytoplankton associations grouped based mainly on the availability of nutrients at the surface of the continental shelf were mainly characterized by the presence of functional associations of diatoms, where salinity was lower and more nutrients were available, and the oceanic region was characterized by groups of dinoflagellates and Trichodesmium spp., which prevailed in more saline, oligotrophic oceanic waters. The microphytoplankton associations at the DCM were mostly composed of groups of dinoflagellates. The environmental descriptors used in the present research revealed the important influence of the Amazon River plume and local currents (retroflection of the north current of Brazil-NBC) on various attributes of microphytoplankton distribution.