The influence of mesoscale physical structures in the phytoplankton taxonomic composition of the subsurface chlorophyll maximum off western Baja California

The distribution of the subsurface chlorophyll maximum (SCM) layer, its taxonomic phytoplankton composition, and the maximum quantum efficiency of charge separation of PSII (Fv/Fm) was investigated in the west coast off Baja California during October 2003. SCM characteristics were described and related to the hydrographic regime and the mesoscale physical structures present during this period. Seven groups of phytoplankton were detected in the SCM based on chemotaxonomic analysis of pigment fingerprints: diatoms, haptophytes, pelagophytes, prasinophytes, cryptophytes, Prochlorococcus and cyanobacteria. The distribution of these groups was heterogeneous and closely related to the circulation patterns characterized by the interaction of subarctic and tropical water. Eddies and meanders were detected in the study area and these structures exerted a direct response in the depth, chlorophyll concentration, and photosynthetic competence of phytoplankton in the SCM. A cyclonic eddy characterized by a high chlorophyll concentration (1.6 mg m−3) and high values of Fv/Fm, (0.52) was detected in the northern zone of the study area. In the central zone, a cyclonic eddy (1.2 mg m−3); and other structure resembling a mode-water eddy was located northwest of Cedros Island. This structure presented the highest chlorophyll concentration (1.8 mg m−3) and high Fv/Fm (∼0.5). Chlorophyll concentration and the photosynthetic performance of the phytoplankton community was lower outside of these eddies. Cyanobacteria dominated the phytoplankton SCM community in these areas.