Gh. Tilstone et al., UPWELLING-DOWNWELLING SEQUENCES IN THE GENERATION OF RED TIDES IN A COASTAL UPWELLING SYSTEM, Marine ecology. Progress series, 112(3), 1994, pp. 241-253
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.