Decadal variability in chlorophyll and primary ­production off NW Spain

Abstract

Long-term variability of phytoplankton biomass (chlorophyll a) and primary production (carbon fixation) in relation to changes in climatic and oceanographic factors was analysed for oceanic and shelf waters of the north and northwest Iberian Peninsula. Changes related to stratifica- tion in the water column and upwelling intensity were specifically addressed. The time series include measurements of primary production and biomass over 19 yr. Seasonally, the main mode of temporal variability of the series is the occurrence of phytoplankton blooms during the winter-spring and sum- mer-autumn transition periods. On the northwestern coast (Galicia), the blooms are more frequent and attain higher biomass and production values than on the northern coast (Mar Cantabrico) due to the longer duration and higher intensity of upwelling in the former region. Even for the longest series there is no significant long-term trend in biomass, but primary production increased in Galicia and decreased in the Mar Cantabrico at nearly equivalent rates when considering monthly series. The lagged relationships between external factors and phytoplankton biomass and production suggest non-linear interactions between phytoplankton and climate. Neither the increase in surface stratifica- tion nor the decrease in upwelling intensity has negatively affected primary production in this region following consistent and systematic patterns. Changes in the amount and proportions of nutrients during winter water-mass formation, and an increased use of nutrients remineralised in situ, are pro- posed as the main causes of decadal variability in primary production in this region.