Upwelling-downwelling sequences in the generation of red tides m a coastal upwelling system

Differences in temporal and spatial hydrographic conditions, water circulation patterns derived from temperature-salinity properties, phytoplankton community composition and distribution were studied in 4 Ria systems (flooded tectonic valleys) in Galicia, NW Spain, from 18 to 21 September 1986. The Rias are affected by upwelling cycles which introduce nutrient-rich Eastern North Atlantic Water (ENAW). During upwelling relaxation periods, the Rias are prone to red tide outbreaks, especially during autumn. In the northern most Ria (Muros), after an upwelling event on 18 September followed by a weak downwelling, a low chlorophyll a (chl a) maximum occurred over the shelf which corresponded to the distribution of a large dinoflagellate/red tide species community identified by principal component analysis (PCA) and cluster analysis of species. This community was identified in all of the other Rias studied, but at different locations. With stronger downwelling on 21 September in the Ria de Vigo. Ria water and the chl a maximum were confined to the Ria interior, which corresponded to a shift in the large dinoflagellate / red tide community. The chl a maximum in all Rias was predominantly due to Heterosjqma carterae. The increase in Gymnodinium catenatum cell numbers, from the northern to the southern Rias, corresponded to stronger downwelling events. It is proposed that upwelling-downwelling sequences, enhanced by the presence of inlets and embayments acting as catchment concentration zones, are important mechanisms for generating red tide blooms in coastal upwelling systems.