Phytoplankton chemotaxonomy within contiguous optical layers across the western North Atlantic Ocean and its relationship to environmental parameters

A CHEMTAX analysis of the phytoplankton community composition is presented for the summertime northwest Atlantic Ocean from the Sargasso Sea to the New England continental shelf. To minimize the confounding influence of light intensity-related changes in internal pigment concentration (photo-acclimation), we compared CHEMTAX community composition within (but not between) five discrete optical depth layers (OL1-5) arranged vertically throughout and beneath the euphotic zone, and extending horizontally from the oligotrophic to mesotrophic regions. CHEMTAX model results revealed a gradient of increasing proportions of eukaryotes to prokaryotes in the landward direction. The oligotrophic oceanic waters were dominated about equally by Prochlorococcus and haptophytes with Synechococcus and prasinophytes being secondarily important, while the mesotrophic slope and shelf waters were dominated by haptophytes and prasinophytes, with Prochlorococcus undetectable in shelf waters. Redundancy analysis revealed strong and significant niche separations between eukaryotic and prokaryotic phytoplankton, with the cyanobacteria more common in warmer, saltier waters, and eukaryotic species more common in fresher, colder waters. Salinity was the strongest predictor of the phytoplankton community composition in OLs 2 through 5, but in the surface OL1 temperature was the strongest predictor. There were positive significant correlations between salinity and Prochlorococcus, and negative significant correlations between salinity and all eukaryote groups: pelagophytes, chlorophytes, haptophytes, diatoms, prasinophytes, cryptophytes, and dinoflagellates, in most cases the strength of the correlation increased with depth. Other environmental variables, PO43-, NO3- + NO2-, and vertical density stratification were all weak predictors of phytoplankton community composition.