SEASONAL-VARIATIONS OF NUTRIENTS, SESTON AND PHYTOPLANKTON, AND UPWELLING INTENSITY OFF LA CORUNA (NW SPAIN)

This study describes the main seasonal stages in oceanographic conditi ons and phytoplankton off La Coruna (Galicia, NW Spain), during 1991 a nd 1992, based mainly on monthly cruises near the coast. Upwelling con ditions were studied using an upwelling index calculated from local wi nds. The Galician coast is affected by a long upwelling season for mos t of the year. The upwelling pulses interact with the thermal stratifi cation-mixing cycle of surface waters, primarily affecting the dynamic s of phytoplankton. In addition, the presence of water masses of diffe rent salinity in the subsurface layers changes the stratification of t he water column. The less-saline North Atlantic Central Water (NACW) w as normally associated with upwelling events during summer. However, o n several occasions during the study, the presence of Eastern North At lantic Water (ENAW) of subtropical origin was observed with salinities up to 36.22 and temperatures between 13 and 14 degrees C. Observation s were grouped into five main stages related to the degree of surface stratification and characteristics of phytoplankton communities. These stages were recognized in both annual cycles, and were termed: winter mixing, spring and autumn blooms, summer upwelling, thermal stratific ation and special events (red tides and downwelling). A homogeneous wa ter column was the main characteristic of the winter stage, with high nutrient concentrations and low phytoplankton biomass. Eastern North A tlantic Water appeared at the end of this stage, which lasted from Nov ember to February. The spring and autumn blooms occurred along with we ak thermohaline gradients at the surface, producing high phytoplankton concentrations. Favourable upwelling conditions and the presence of E NAW in a subsurface layer were the factors that most likely induced ea rlier blooms, while thermal gradients developed at the surface could h ave been more important for later blooms. Upwelling events during summ er were related to a reduction in the depth of the surface mixed layer as the pycnocline moved upwards, and can produce significant phytopla nkton accumulations. These summer blooms interrupted the thermal strat ification stage, characterized by low nutrient and phytoplankton conce ntrations at the surface. The dominant phytoplankton in the study was composed mainly of diatoms, especially during blooms. However, a proli feration of red-tide dinoflagellates was observed along with weak upwe lling conditions in late summer. Also, in late summer, strong downwell ing conditions caused the accumulation of warmer shelf waters inshore, inducing the sinking of particulate matter produced at the surface. ( C) 1997 Academic Press Limited.