PHYTOPLANKTON BIOMASS AND PRODUCTION IN SHELF WATERS OFF NW SPAIN - SPATIAL AND SEASONAL VARIABILITY IN RELATION TO UPWELLING

Chlorophyll-a and primary production on the euphotic zone of the N-NW Spanish shelf were studied at 125 stations between 1984 and 1992. Thre e geographic areas (Cantabrian Sea, Rias Altas and Rias Baixas), three bathymetric ranges (20 to 60 m, 60 to 150 m and stations deeper than 200 m), and four oceanographic stages (spring and autumn blooms, summe r upwelling, summer stratification and winter mixing) were considered. One of the major sources of variability of chlorophyll and production data was season. Bloom and summer upwelling stages have equivalent me an and maximum values. Average chlorophyll-a concentrations approximat ely doubled in every step of the increasing productivity sequence: win ter mixing - summer stratification - high productivity (upwelling and bloom) stages. Average primary production rates increased only 60% in the described sequence. Mean (+/- sd) values of chlorophyll-a and prim ary production rates during the high productivity stages were 59.7 +/- 39.5 mg Chl-a m(-2) and 86.9 +/- 44.0 mg C m(-2) h(-1), respectively. Significant differences in both chlorophyll and primary production re sulted between geographic areas in most stages. Only 27 stations showe d the effects of the summer upwelling that affected coastal areas in t he Cantabrian Sea and Rias Baixas shelf, but also shelf-break stations in the Rias Altas area. The Rias Baixas area had lower chlorophyll th an both the Rias Altas and the Cantabrian Sea areas during spring and autumn blooms, but higher during summer upwelling events. On the contr ary, primary production rates were higher in the Rias Baixas area duri ng blooms in spring and autumn. Mid-shelf areas showed the highest chl orophyll concentrations during high productivity stages, probably due to the existence of frontal zones in all geographic areas considered. The estimated phytoplankton growth rates were comparable to those of o ther coastal upwelling systems, with average values lower than the max imum potential growth rates. Doubling rates for upwelling and stratifi cation stages in the northern and Rias Altas shelf areas were equivale nt, despite larger biomass accumulations during upwelling events. Low turnover rates of the existing biomass in the Rias Baixas shelf in upw elling stages suggests that the accumulation of phytoplankton was due mainly to the export from the highly productive rias, while the contri bution of in situ production to these accumulations was relatively low er.