Phytoplankton assemblages across a NW Mediterranean front: Changes from winter mixing to spring stratification

Abstract

A 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.