Microplankton distribution in the Strait of Gibraltar: coupling between organisms and hydrodynamic structures

In this paper we present microplankton distribution patterns in the Strait of Gibraltar obtained during a cruise carried out in September 1997. Chloro phyll, microplankton abundance and biovolume increase from southwest to nor theast, a tendency that coincides with a gradual elevation of the interface depth in the same direction. The vertical distribution of biomass at the d ifferent stations shows deep maxima associated with the pycnoclines. The ge neration of internal waves and eventual intrusion of deep water on the sill can be decisive in the enrichment of surface water and plankton dynamics. The taxonomic structure agrees with the expected pattern, with a predominan ce of diatoms in the richer Mediterranean stations (especially in the north ) and a predominance of dinoflagellates and microzooplankton at the Atlanti c side. Phytoplankton distribution and salinity values found in the Strait of Gibraltar can be explained by a preliminary model that assumes a couplin g between diatom life cycles and hydrodynamic features. Thus, the cells in the surface waters of the eastern side of the Strait would aggregate and si nk below the interface, moving westwards and returning to the surface water s coupled with the rising and mixing of deep Mediterranean water at the sil l.