The response of phytoplankton production to periodic upwelling and relaxation events at the Iberian shelf break:: estimates by the 14C method and by satellite remote sensing

The primary productivity of the NW Iberian margin has been determined by a combination of C-14 incorporation experiments on natural phytoplankton assemblages and satellite remote sensing with the ocean colour sensor, SeaWiFS. The approach to the field determination of phytoplankton production was Eulerian, involving measurements on a fixed grid of stations covering the region from 41degrees30'N to 43degrees00'N and 09degrees00'W to 10degrees30'W. Three different laboratories made primary production measurements over a 3-year period. Each group used variations of the C-14 method-in situ incubations and two modelling approaches based on P-E parameter derivation. An intercalibration experiment showed acceptable agreement between the three estimates. Measurements by the three groups on eight cruises have been merged to provide estimates of regional productivity. However, the temporal coverage of the data was too limited to provide robust seasonal estimates since few experiments were done in the autumn and winter months. A second approach to the estimation of primary production utilised surface pigment concentrations derived from satellite remote sensing. Two models were used. Firstly, an empirical relationship was found between depth-integrated primary production and surface chlorophyll concentration, which explained 78% of the variance in the production estimates. The second approach used a semi-analytical model which incorporates irradiance and temperature as well as chlorophyll-derived parameters. Both models have been applied to SeaWiFS-derived, surface chlorophyll concentrations to estimate primary production during 1998, 1999 and 2000 for three regions-the shelf, the slope and the open ocean of the region of study. Using the semi-analytical model, the mean primary production for the 3 years is estimated to be 319 g C m(-2) year(-1) for the shelf, 280 g C m(-2) year(-1) for the slope and 217 g C m(-2) year(-1) for the adjacent open ocean. Interannual variation in production on the shelf ranged from 334 g C m(-2) year(-1) in 1998 to 301 g C m(-2) year(-1) in 2000. It is estimated that upwelling increased primary production, relative to that in the open ocean, by similar to50%. Satellite-derived estimates of the ratio of new production to primary production were 0.37, 0.28 and 0.40 for the shelf region during the upwelling seasons of 1998, 1999 and 2000. (C) 2002 Elsevier Science B.V. All rights reserved.