AME 50:169-179 (2008)  -  DOI: https://doi.org/10.3354/ame01160

ABSTRACT: The metabolic balance between gross primary production (GPP) and community respiration and its relationship with the plankton community structure was investigated in the Ría 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 (¹⁴C PP) and GPP from photosynthetic-irradiance incubations (¹⁴C PP_P-I) were carried out in the Ría de Vigo between April 2004 and January 2005. Seasonal patterns of phytoplankton biomass and primary production were in phase with one another. The highest ¹⁴C PP and biomass values, accounting mainly for the large-size phytoplankton (>20 µm), were observed under upwelling-favourable conditions (spring and summer). Minimum values of ¹⁴C 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 O₂ m^–2 d^–1 and a winter minimum of 31 mmol O₂ 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 Ría 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.

KEY WORDS: Primary production · P/R balance · Microplankton size structure · Upwelling ·Ría de Vigo