Phytoplankton pigments and community structure in the northeastern tropical pacific using HPLC-CHEMTAX analysis

This study investigated the relationship between phytoplankton biomass and taxonomic composition and hydrographic variables in the Northeastern Tropical Pacific for June 2015, March 2016, and September 2016. Hydrographic data were measured between surface and 100 m; samples were collected at the surface and the deep chlorophyll maximum (DCM) along one or two transects. Pigments were quantified by high-performance liquid chromatography; the CHEMTAX software was used to determine the relative contribution to chlorophyll a of the main taxonomic groups. Our results show that the studied region is characterized by a stable vertical distribution of phytoplankton biomass regardless of the season (winter, spring or summer). A subsurface maximum (DCM) is always observed close to the bottom of the mixed layer where there is a higher abundance of larger groups (diatoms and Prymnesiophytes) while at the surface community, composition was generally dominated by picoplankton (Cyanobacteria and Prochlorococcus), and occasionally Prymnesiophytes. However, during the spring cruise (June 2015), affected by the 2015–2016 El Niño, phytoplankton biomass at the DCM markedly decreased along with an increase in the abundance of Chlorophytes. On the other hand, in September 2016, there was an unexpected increase in phytoplankton biomass at the DCM, although stratification was strong, and Prymnesiophytes comprised 60% of the community at the surface. The evaluation of nutrient and light will be necessary in future studies to determine their role in this temporal variability. Finally, chemotaxonomy (HPLC/CHEMTAX) proved to be a valuable tool for describing phytoplankton distribution at group level in this region and its relationship with physical processes.