Seasonal variability of phytoplankton community structure in a coastal station of the Argentine continental shelf based on a chemotaxonomic approach

The composition, abundance and size structure of the phytoplankton community at a coastal time series station (38° 28' S-57° 41' W, EPEA, Argentina) was characterized by applying the chemotaxonomic approach. The seasonal variability of pigment diversity determined by high-performance liquid chromatography (HPLC, n = 171), nutrient concentration (n = 934), and temperature, salinity and degree of stratification of the water column was identified (n = 190 CTD profiles). The CHEMTAX program was used to estimate phytoplankton abundance in terms of contribution to chlorophyll a concentration of the different phytoplankton pigmentary types (PPTs). Two different pigment indices were compared to estimate phytoplankton community size fractions throughout the year, giving contrasting results. Water column was mostly mixed, with minimum temperatures between July and September and maximum between January-March (range: 8-23 °C). Nitrate was the limiting nutrient, with minimal concentrations at the end of summer. It’s range varied between 0.010-13.330 μM, while silicate ranged between 0.016-10.670 μM without major seasonal variations, and phosphate between 0.120-2.180 μM. Fucoxanthin, chlorophyll c2, 19’-hexanoyl-oxy-fucoxanthin, diadinoxanthin, chlorophyll b, chlorophyll c3, peridinin and, alloxanthin, were the most frequent phytoplankton pigments. The PPT DINO-1 (dinoflagellates with peridinin), haptophytes types: HAPTO-6, HAPTO-7 and HAPTO-P showed a seasonal cycle with peaks of abundance in autumn and spring, while the diatoms DIATO-1 was high during the whole year and DIATO-2 mainly during winter. A pigment profile of a group of prymnesiophytes possessing MVChl_c3 was described. It was evident that at this site PPTs having potentially toxic species bloom under different hydrological conditions. DINO-1 is likely to bloom in April (autumn), with temperatures close to 18 °C and weak stratification conditions, while October bloom (spring) occurs with lower temperatures of 10-12 °C. DINO-4 was noted during January and February (summer), when temperature was > 18 °C, salinity < 33.7, and the water column showed maximum stratification. In contrast, the maximum abundances of DIATO-2 occurred between August and September (winter), under completely mixed conditions, high nitrate concentration and low temperature of 10 °C. This work constitutes the first description of the variability of the abundance of the main PPTs in a coastal a time series station in the southwestern Atlantic Ocean shelf throughout the annual cycle, demonstrating the power of chemotaxonomy and CHEMTAX to perform descriptive analysis of a large number of samples.