Phytoplankton assemblages across a NW Mediterranean front: Changes from winter mixing to spring stratification
Paraules clau:Phytoplankton, hydrographic front, NW Mediterranean
ResumA series of stations located along the transect between Barcelona and the Balearic Islands were sampled to study the composition of the phytoplankton community, during June 1984, March 1 985, June 1 985 and June 1 986. In March, the concentration of phytoplankton cells presented a maximum in the upper part of the water column. The Catalan front, near the continental slope of the Catalan coast side, separated a diatom-dominated assemblage in the coastal zone from a flagellate-dominated assemblage offshore. In the spring cruises, the most characteristic feature of the phytoplankton distribution was the presence of a deep chlorophyll maximum (DCM) which corresponded to increased phytoplankton cell numbers. The influence of the front on the phytoplankton distribution was well marked in early June ( 1 984), but in the cruises carried out later in the season, warm waters extended over the frontal zone. The distribution of the phytoplankton assemblages showed some regularities: during the spring cruises, diatom assemblages with Chaetoceros spp., Nitzschia spp. and Thalassiosira spp. formed patches in the DCM layer, while a flagellate and dinoflagellate assemblage occupied the upper part of the water column. In May 1 984 and May 1 986 a third assemblage with Coscinodiscus radiatus, Dictyochafibula and Oxytoxum margalefi formed a background population in the DCM level. Thus, in spite of high environmental variability, persistent groupings of species could be recognized.
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