Dynamics of Primary Productivity and Oceanographic Parameters under Influence of the Amazon River Plume

Fluvial inflows carry significant amounts of nutrients, affecting especially continental platform regions, which act as a final recipient of these waters and have a major impact on the phytoplankton community structure. The dynamic structure resulting from this process is a mass of floating water with density lower than that of coastal waters, called a river plume.

Primary production and chlorophyll a concentrations are always dependent on synergistic effects among meteorological, physical, chemical and biological factors that limit or stimulate the activity of photosynthetic planktonic organisms. The aim of this work was to analyze the distribution of oceanographic parameters, chlorophyll a and primary productivity under the influence of the Amazon River plume, during the period of greatest extension of the Amazon plume.

Collections were carried out in the “Camadas Finas V” oceanographic campaign held in September/2014 at 16 stations along the continental shelf of the Amazon and adjacent oceanic region, aboard of the NHo 38 ship. At each station, surface vertical profiles (up to 400 m) of hydrodynamic, physical, chemical and biological parameters were measured. In addition, water samples were collected with Niskin bottles at the two depths selected for the determination of dissolved oxygen, nutrients, chlorophyll a and phytoplankton productivity.

The surface currents along the coast were of lower intensity in the northwest direction, while currents with higher intensities were observed in the east direction, mainly due to the retroflexion of the North Current of Brazil at this time of the year. The spatial influence of the plume was observed in the salinity, although the high precipitation in the region may also have contributed as a source of freshwater. However, the sampled region showed strong negative linear correlation of silicate, chlorophyll a and primary productivity with salinity. The primary productivity and chlorophyll a decreased their values as they moved away from the coast.

In conclusion, the Amazon River plume can reach and influence the oceanographic and biological parameters in a large area of this oligotrophic region. And the results also suggest that the export of material from the adjacent coastal region is a determinant factor of the productivity of the studied region, mainly due to the geographic extension of the plume of the Amazon River at this time of the year.

Article by Amanda Otsuka, et al, from Brazil and France.

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