Role of across-shelf currents in the dynamics of harmful dinoflagellate blooms in the northwestern Iberian upwelling

11 pages, 9 figures, 1 tableHarmful dinoflagellate blooms are frequent in the Rías Baixas, which are made up of four open bays on the northwest part of the Iberian Peninsula. The relationship between surface currents resulting from wind forcing and the microplankton composition on the shelf offshore of the Rías Baixas was studied from May 2001 to April 2002. The aim was to assess the role of shelf currents in the dynamics of harmful dinoflagellate blooms in the Rías Baixas shelf system. In summer, upwelling-favorable northerly winds forced a variable but persistent southwestward flow on the shelf, which reverted to an onshore flow following the change to downwellingfavorable southwesterly winds in autumn. During winter and spring, there was an intense alongshore flow to the north. Diatoms and dinoflagellates were common components of the microplankton community in summer. Although diatoms increased their abundance during summer upwelling events, a dinoflagellate assemblage was also present on the shelf. This dinoflagellate assemblage acquired higher importance during the intermediate periods of upwelling relaxation and during the autumn downwelling in particular, when satellite images revealed the accumulation of phytoplankton in a narrow band close to the coast. A dinoflagellate bloom of Prorocentrum minimum (10,564 cells mL−1) and Karenia cf. mikimotoi (3,022 cells mL−1) developed in the interior section of the Ría de Vigo coincident with the onshore flow imposed by the autumn downwelling. As these two species were components of the dinoflagellate assemblage present on the shelf in summer, it is inferred that downwelling caused their accumulation in the interior section of the Ría de VigoThis work was funded by the Spanish DYBAGA project (MAR99-1039-C02-01) and the EU HABILE project (EVK3-CT-2001-00063). B.G.C. was supported by a CSIC-ESF I3P fellowshipPeer reviewe