Diatom dynamics in a coastal ecosystem affected by upwelling: coupling between species succession, circulation and biogeochemical processes

The typical phytoplankton succession scenario in coastal upwelling zones is high diatom growth during upwelling and flagellate dominance during water column stratification. Within the diatom/flagellate succession there exist short-term changes in diatom communities that are caused by physical, chemi cal and biological processes. In this study, we used an improved 2-D kinema tic box model to assess the influence of these processes on diatom dynamics in an estuarine ecosystem affected by coastal upwelling. This model enable d us to separate hydrographic from biogeochemical processes occurring in th e estuary. Hydrographic variables, nutrient concentrations and phytoplankto n composition were determined over a 2 wk period in the Ria de Vigo, NW Spa in. Two major hydrographic phases were identified which coincided with a cl ear temporal and spatial, separation between 2 diatom assemblages: Thalassi osira spp./Skeletonema costatum and Chaetoceros spp./Cerataulina pelagica. During upwelling, horizontal (6.6 km d(-1)) and vertical (11.7 m d(-1)) con vective fluxes were high, causing a net input of NO3-, HPO42- and SiO4H4 Du ring this phase the Thalassiosira spp./S. costatum standing stock was high (> 20 mu mol C l(-1)). Hydrographic processes, however, affected the Thalas siosira spp./S. costatum assemblage more than biogeochemical processes and this resulted in the net loss of this assemblage from the Ria and its expor t towards the shelf. There was a significant correlation between the biogeo chemical Variations in this diatom assemblage and silicate, suggesting a st rong dependency of Thalassiosira spp./S. costatum on this nutrient. By comp arison, due to the higher carbon-specific net growth rate of the Chaetocero s spp./C, pelagica assemblage (0.35 d(-1)) during upwelling, this assemblag e maintained a high biomass in the Ria. Upwelling was followed by upwelling relaxation when horizontal (1.9 km d(-1)) and vertical fluxes (1.8 m d(-1) ) were reduced and nutrient levels diminished. During upwelling relaxation there was an accumulation of Chaetoceros spp./C. pelagica biomass (>18 mu m ol C l(-1)). Biogeochemical processes provoked a loss of Thalassiosira spp. /S. costatum due to rapid sedimentation and a net increase in Chaetoceros s pp./C. pelagica. It is suggested that the accumulation of Chaetoceros spp. is aided by a lower sinking rate whereas the selection of C. pelagica is mo re dependent on NO3- and HPO42- consumption. It is concluded that upwelling events in the Ria cause the exportation of Thalassiosira spp./S. costatum standing stock from the Ria towards the shelf, which will ultimately benefi t shelf pelagic and benthic fish communities. Upwelling relaxation events f avour the retention of a high standing stock of Chaetoceros spp./C. pelagic a, which is then directly available to the shellfish aquaculture of the Ria .