Microplankton composition off NW Iberia at the end of the upwelling season: source areas of harmful dinoflagellate blooms

13 pages, 9 figures, 2 tables.The Rías Baixas of Galicia are 4 bays on the NW Iberia Peninsula that experience harmful dinoflagellate blooms toward the end of the upwelling season, in late summer to early autumn. In order to identify the source areas of initial bloom populations, we studied the hydrographic regime and the microplankton composition in the NW Iberian margin at the end of summer 1991. Three distinctive oceanographic features were recognised as possible sources of initial populations: (1) the onset of a northward stream, the Iberian Poleward Current (IPC), which can transport seed populations to the region; (2) a subsurface chlorophyll maximum (SCM); and (3) weak upwelling at the coast, which can promote the growth of dinoflagellates. While the SCM held populations of the small dinoflagellates Heterocapsa niei and Prorocentrum minimum, larger dinoflagellates Prorocentrum micans, Dinophysis acuminata, Gymnodinium catenatum and the raphidophycean Heterosigma akashiwo, which habitually form blooms in the Rías Baixas, were only found in upwelled coastal waters, mostly off the Rías. These species occurred with a microplankton assemblage composed of other large dinoflagellates and diatoms, which is characteristic of the final summer upwelling events in the Rías. It is hypothesised that the Rías Baixas can be both the source and the target of harmful dinoflagellate blooms. Summer upwelling, through cyst resuspension and subsequent germination in the nearby stratified waters, would provide the initial populations. Downwelling, which promotes the accumulation of dinoflagellates in the interior of the Rías, would ensure blooms and hence replenish the sediments with new cysts.Financial support for this work came from the EU projects MAST-CT90-0017 (The control of phytoplankton dominance) and HABILE (EVK3-CT- 2001-00063). B.G.C. was funded by a CSIC-ESF I3P fellowship and I.G.T by a FCT (Portuguese Foundation for Science and Technology) doctoral fellowship. This is a contribution to the GEOHAB Core Research Project – HABs in upwelling systems.Peer reviewe