Phytoplankton community structure in the “El Rincón” area, Argentine Sea (February 2011)

Abstract

The “El Rincón” area in the Argentine continental shelf (coastal sector between 39° S-41° S) is known as one of the main breeding areas of the Argentine anchovy (Engraulis anchoita) Northern stock. However, there is scarce information regarding the structure of the primary producers, base of the pelagic food webs. The aim of this study was to identify the different phytoplanktonic communities in a coastal section in the area (depth lower than 60 m) during February 2011. Mean surface chlorophyll a concentration was 0.72 mg m^-3, with the highest value in the innermost station (E34: 1.84 mg m^-3) and the lowest in the mid section (E31: 0.24 mg m^-3). In depth, similar concentrations were registered, except for the deepest station where the maximum value was reached on the thermocline (E29 32 m: 7.33 mg m^-3). In general, the chlorophyll fraction lower than 5 µm represented over 60% of the total concentration, with the exception of the thermocline of the deepest station (E29) where only 15% was registered. In coincidence with chlorophyll distribution, the ultraphytoplankton was the main fraction in surficial waters in the whole section. However, qualitative differences among the sampling stations were found. The major components were Synechococcus, picophytoeukaryotes, flagellates forms of Prymnesiophyceae, Chrysophyceae and Cryptophyceae (10⁵-10⁷ cells l^-1) and the cocolithophorid Gephyrocapsa oceanica (1.2 ´10⁶ cells l^-1) in the innermost station. As regards the largest fractions, on the surface of that last station the nanoplanktonic diatom Thalassionema nitzschoides (1.2 ´ 10⁵ cells l^-1) prevailed and on the thermocline of the deepest a bloom of microplanktonic dinoflagellate Neoceratium horridum (63 ´ 10³ cells l^-1) was verified. Results show that, in spite of the scarce variation in chlorophyll concentration, the size structure and composition of the phytoplankton community were notably different among stations. It is worth noting the impact N. horridum bloom may have on the pelagic food webs. The dinoflagellate, not very palatable for copepods, may cause the decrease of grazing pressure thus triggering massive population growth that could produce hypoxia events.