UPWELLING-DOWNWELLING SEQUENCES IN THE GENERATION OF RED TIDES IN A COASTAL UPWELLING SYSTEM

Differences in temporal and spatial hydrographic conditions, water cir culation patterns derived from temperature-salinity properties, phytop lankton community composition and distribution were studied in 4 Ria s ystems (flooded tectonic valleys) in Galicia, NW Spain, from 18 to 21 September 1986. The Rias are affected by upwelling cycles which introd uce nutrient-rich Eastern North Atlantic Water (ENAW). During upwellin g relaxation periods, the Rias are prone to red tide outbreaks, especi ally during autumn. In the northern most Ria (Muros), after an upwelli ng event on 18 September followed by a weak downwelling, a low chlorop hyll a (chl a) maximum occurred over the shelf which corresponded to t he distribution of a large dinoflagellate/red tide species community i dentified by principal component analysis (PCA) and cluster analysis o f species. This community was identified in all of the other Rias stud ied, but at different locations. With stronger downwelling on 21 Septe mber 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 dinof lagellate/red tide community. The chl a maximum in all Rias was predom inantly due to Heterosigma carterae. The increase in Gymnodinium caten atum cell numbers, from the northern to the southern Rias, corresponde d to stronger downwelling events. It is proposed that upwelling-downwe lling sequences, enhanced by the presence of inlets and embayments act ing as catchment concentration zones, are important mechanisms for gen erating red tide blooms in coastal upwelling systems.