Using CHEMTAX to evaluate seasonal and interannual dynamics of the phytoplankton community off the South-west coast of Portugal

Abstract

CHEMTAX was used to assess the relative contribution of the main phytoplankton classes to the total concentration of Chlorophyll a (Chl a) from the waters off SW coast of Portugal. Sampling campaigns were carried out during all seasons from 2008 to 2012, at three stations located 2, 10 and 18 km from the coast. Samples were taken from the surface, mid-Secchi and Secchi depth, for the determination of Chl a and other phytoplanktonic pigments by HPLC. Supporting data were also obtained including dissolved inorganic nutrients, salinity, transparency, temperature and upwelling indices. The CHEMTAX results were also related to microscopy counts and also spectral analysis of absorption of other samples from the same sampling campaigns. The pigment results showed that diatoms dominated from early spring to summer, coinciding with upwelling conditions, while cryptophytes, prymnesiophytes and prasinophytes dominated in autumn and winter, coinciding with seasonal stratification. Although the contribution of cyanobacteria to total Chl a was generally low, there were occasional sampling campaigns where it was exceptionally high, but these appeared not to be related to upwelling. Dinoflagellates and chrysophytes were minority groups although the pigment marker peridinin that was used to distinguish dinoflagellates was not adequate for distinguishing all the members of this group. CHEMTAX was particularly useful for discriminating between the smaller (0–20 μm) classes of the microplankton that could not be easily identified by microscopy.