Significance of nanophytoplankton photosynthesis and primary production in a coastal upwelling system (Ria de Vigo, NW Spain)

The Rias Baixas are reported to have the highest primary production along the NW coast of Spain due to the influence of upwelling, which occurs from April to October. Samples were taken over 2 wk periods at a single station in the Ria de Vigo during spring, summer and autumn 1993 to study the variation in the phytoplankton photosynthetic parameters (light limited slope, alpha(B) and light saturated chlorophyll specific rate, P-m(B)) and daily primary production in relation to phytoplankton size class, species and hydrographic parameters. The range in primary production for the Ria was 65 to 3689 mg C m(-2) d(-1) for total phytoplankton, 65 to 2533 mg C m(-2) d(-1) for net phytoplankton and 0 to 1650 mg C m(-2) d(-1) for nanophytoplankton. The data suggest that most of the variation in primary production in the Ria during the upwelling season was due to nanophytoplankton. Upwelling/downwelling cycles in the Ria therefore give rise to a larger variation in energy transfer through the microbial loop than through the classic, short, food chain. This is caused mainly by increases in nanophytoplankton alpha(B) coupled with high NO3 due to upwelling events. The data also suggest that there are 3 principal factors that give high primary production in the Ria. Periodic relaxation and stratification events linked to water column heating during upwelling/downwelling cycles coincided with periods of high productivity. Size class also influences the primary production; net phytoplankton had significantly higher primary production during the upwelling season than the nanophytoplankton and was responsible for 66% of the mean primary production. Species differences affected the primary production in the Ria; highest values coincided with blooms of Chaetoceros spp, and the lowest with dinoflagellate blooms of Gymnodinium catenatum.