Phytoplankton size structure and water column metabolic balance in a coastal upwelling system:: the Ria de Vigo, NW Iberia

The metabolic balance between gross primary production (GPP) and community respiration and its relationship with the plankton community structure was investigated in the Ria de Vigo, in the coastal upwelling system of the NW Iberian Peninsula. Measurements of in situ GPP, dark community respiration (DCR), size-fractionated biomass, size-fractionated primary production (C-14 pp) and GPP from photosynthetic-irradiance incubations (C-14 PPP-1) were carried out in the Ria de Vigo between April 2004 and January 2005. Seasonal patterns of phytoplankton biomass and primary production were in phase with one another. The highest C-14 PP and biomass values, accounting mainly for the large-size phytoplankton (>20 mu m), were observed under upwelling-favourable conditions (spring and summer). Minimum values of C-14 PP and phytoplankton biomass were registered in winter with a shift to small phytoplankton dominance (>80%). Integrated water column DCR varied between a summer maximum of 132 mmol O-2 m(-2) d(-1) and a winter minimum of 31 mmol O-2 m(-2) d(-1). Community respiration was significantly related to primary production for the 2 upwelling cruises, pointing to a close coupling between these processes. However, no direct relationship was found for the other periods. Our results suggest that the water column metabolic balance inside the Ria de Vigo is dependent on phytoplankton size structure. The autotrophy degree of the system is high when the contribution of large-size phytoplankton is dominant, whereas it is almost in balance when the small phytoplankton fraction prevails.