Algal pigment patterns and phytoplankton assemblages in different water masses of the Río de la Plata maritime front

Abstract

The composition of phytoplankton assemblages were studied in three sections across the continental shelf between the Río de la Plata and the oceanic waters of the Subtropical Convergence, during late spring. Algal communities were examined using microscopy and HPLC-derived pigment concentrations. The CHEMTAX program was used to estimate the chlorophyll a (chl a) biomass of different algal classes. Trends in pigment ratios due to phytoplankton photo-adaptation and photo-acclimation were also examined. In order to accommodate the natural diversity of phytoplankton assemblages the original data have been split to represent five ecosystems. In addition, the pigment data for the Brazil Current ecosystem has been split by sample depth. High chl a concentrations were recorded in the outer estuary region (up to 15.5 mg m −3) and in the shelf-break front associated with Subantarctic waters (2–4 μg l −1). In contrast, chl a concentrations were relatively low over the continental shelf and in the oceanic region dominated by the Brazil Current, where the lowest values (0.1–0.2 μg l −1) were found. Both pigment patterns and microscopy-derived information showed five different phytoplankton assemblages spatially segregated by the prevailing environmental conditions. In the inner estuary assemblage green algae (54–56% of total chl a) were always the dominant group and most of the chl a, arises from chlorophyceans (40–49%). In a decreasing order, diatoms cyanobacteria and cryptophytes were also relevant. In the outer estuary assemblage diatoms and dinoflagellates were the dominant groups but cryptophytes and euglenophytes were present as sub-dominant groups. In the coastal and shelf region, the algal assemblage showed an almost total dominance (59.3–87.6%) of diatoms. The usual diatom-pigment pattern (chl c 1, chl c 2) group (diatom I), was the more abundant and widely distributed, but in some stations, diatoms containing chl c 2 and chl c 3 (diatom II) were present as dominant group. A more complex phytoplankton community dominated by coccoid and small flagellates (2–5 μm) predominantly comprised by chlorophyceans (up to 50%) and haptophytes (up to 62%) was found near the shelf-break front. This is the first time that high chl b concentrations associated to a bloom of a picoplanktonic (<3 μm) coccal chlorophycean was reported for this area. The Brazil Current assemblage showed the dominance (55.4–71.9%) of the picoplanktonic cyanobacteria Synechococcus spp. (32.3–45%) and Prochlorococcus spp. (41.4–10.4%). Haptophytes were also present as sub-dominant group being particular abundant at the deep chl a maxima. A sharp transition in photo-collectors/(chl a+Dv chl a) and photo-protectors/(chl a+Dv chl a) ratios at depth near the base of the euphotic zone was observed in the water column of this ecosystem. These results are discussed in relation to the complex environmental features of the region.