Spatio-temporal variability of the winter phytoplankton distribution across the Catalan and North Balearic fronts (NW Mediterranean)

The qualitative and quantitative composition of the phytoplankton assemblag es of the Catalano-Balearic Sea (NW Mediterranean) was examined in relation ship with frontal boundaries and short-term meteorological forcing originat ed by an intense wind storm. Data were obtained during February 1990, befor e and after strong NW winds, from several transects across the Catalan (con tinental side) and North Balearic (Balearic Islands side) shelf/slope front s. High chlorophyll concentrations and diatom dominance inshore of the Cata lan Front were typical of the winter-spring phytoplankton maximum. However, offshore of the Catalan Front, diatoms were scarce and the phytoplankton a ssemblage was dominated by coccolithophorids. In spite of weak density stra tification, a deep chlorophyll maximum (DCM) was well developed, before the storm, offshore of the Catalan Front. The wind event shifted the North Bal earic Front 15 km further offshore and appeared to cause a pulse of new pro duction, as suggested by increased oxygen concentrations in the vicinity of the DCM. The results presented here indicate the existence of a persistent association between the distribution of distinct phytoplankton assemblages and hydrographic characteristics of the studied area. Our results also sho w that, at least during an important part of the winter-spring bloom period of the Catalano-Balearic Sea, high phytoplankton biomass offshore of the C atalan Front is not associated with the diatom dominance typically found in coastal waters and other offshore areas of the NW Mediterranean. This find ing has biogeochemical implications in relationship with vertical export fl uxes of particulate matter.