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

Abstract

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, chemical and biological processes. In this study, we used an improved 2-D kinematic box model to assess the influence of these processes on diatom dynamics in an estuarine ecosystem affected by coastal upwelling. This model enabled us to separate hydrographic from biogeochemical processes occurring in the estuary. Hydrographic variables, nutrient concentrations and phytoplankton composition were determined over a 2 wk period in the Ría de Vigo, NW Spain. Two major hydrographic phases were identified which coincided with a clear temporal and spatial separation between 2 diatom assemblages: Thalassiosira spp./Skeletonema costatum and Chaetoceros spp./Cerataulina pelagica. During upwelling, horizontal (6.6 km d-1) and vertical (11.7 m d-1) convective fluxes were high, causing a net input of NO3-, HPO42- and SiO4H4. During this phase the Thalassiosira spp./S. costatum standing stock was high (>20 µmol C l-1). Hydrographic processes, however, affected the Thalassiosira spp./S. costatum assemblage more than biogeochemical processes and this resulted in the net loss of this assemblage from the Ría and its export towards the shelf. There was a significant correlation between the biogeochemical variations in this diatom assemblage and silicate, suggesting a strong dependency of Thalassiosira spp./S. costatum on this nutrient. By comparison, due to the higher carbon-specific net growth rate of the Chaetoceros spp./C. pelagica assemblage (0.35 d-1) during upwelling, this assemblage maintained a high biomass in the Ría. 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 µmol C l-1). Biogeochemical processes provoked a loss of Thalassiosira spp./S. costatum due to rapid sedimentation and a net increase in Chaetoceros spp./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 more dependent on NO3- and HPO42- consumption. It is concluded that upwelling events in the Ría cause the exportation of Thalassiosira spp./S. costatum standing stock from the Ría towards the shelf, which will ultimately benefit shelf pelagic and benthic fish communities. Upwelling relaxation events favour the retention of a high standing stock of Chaetoceros spp./C. pelagica, which is then directly available to the shellfish aquaculture of the Ría.