Influence of light and nutrients on the vertical distribution of marine phytoplankton groups in the deep chlorophyll maximum

Ecological traits of phytoplankton are being incorporated into models to better understand the dynamics of marine ecosystems and to predict their response to global change. We have compared the distribution of major phytoplankton groups in two different systems: in surface waters of the NW Mediterranean during key ecological periods, and in the DCM (deep chlorophyll maximum) formed in summer in the temperate NE Atlantic. This comparison disentangled the influence of light and nutrients on the relative position of diatoms, dinoflagellates, prymnesiophytes, pelagophytes, chlorophytes, Synechococcus and Prochlorococcus in these environments. Three clusters formed according to their affinity for nutrients: diatoms, chlorophytes and dinoflagellates as the most eutrophic groups; Synechococcus, pelagophytes and prymnesiophytes as mesotrophic groups; and Prochlorococcus as an oligotrophic group. In terms of irradiance, the phytoplankton groups did not cluster clearly. Comparing the nutrient and light preferences of the groups with their distribution in the DCM, dinoflagellates and chlorophytes appear as the most stressed, i.e. their position was most distant from their optimal light and nutrient conditions. Diatoms stayed in deeper than optimal irradiance layers, probably to meet their high nutrient requirements. On the opposite side, low nutrient requirements allowed Prochlorococcus to remain in the uppermost part of the DCM layer. The slight sub-optimal position of Synechococcus and prymnesiophytes with regard to their nutrient requirements suggests that their need for high irradiance plays a significant role in their location within the DCM. Finally, pelagophytes remained in deep layers without an apparent need for the high nutrient concentrations at those depths.