Phytoplankton size structure and primary production in a highly dynamic coastal ecosystem (Ría de Vigo, NW-Spain): Seasonal and short-time scale variability

Size-fractionated phytoplankton biomass and primary production, together with net community metabolism, were measured in a coastal ecosystem (Ría de Vigo, NW-Spain) during a full annual cycle (July 2001–July 2002). On a seasonal scale, this ecosystem was characterized by two distinct oceanographic conditions, namely upwelling and downwelling favourable seasons. During the upwelling season, total chlorophyll a (Chl a) and particulate organic carbon production rates (POC-pr) were in the range 36–129 mg Chl a m−2 and 89–834 mg C m−2 h−1, respectively, and were mainly accounted for (>80%) by the microphytoplankton size fraction (>20 μm). During the downwelling season, total Chl a and POC-pr were much lower (<27 mg Chl a m−2 and <97 mg C m−2 h−1, respectively), and the pico- (<2 μm) and nano- (2–20 μm) phytoplankton size fractions significantly increased their contribution to total Chl a (46–87%) and POC-pr (30–86%). The seasonal and short-time scale variability in the hydrographic conditions, in particular upwelling intermittency, provides a feasible explanation for the continuous dominance of large-sized phytoplankton during the upwelling period. Shelf water intrusions, continuous vertical mixing and the size-dependent limitation in light acquisition (package effect), suffered in a higher degree by larger phytoplankton, were likely to account for the shift in phytoplankton size structure during the downwelling period. During the upwelling season, community respiration represented a minor fraction of gross primary production (15–30%), which highlights the large export potential of organic matter by this ecosystem. On the contrary, community respiration accounted for a major fraction of primary production (85%) during the downwelling period, which suggests that most of the photosynthesised organic matter was remineralised within the ecosystem. Although the microbial plankton community of the Ría de Vigo exhibits a net autotrophic functioning throughout the year, the magnitude of the carbon flows and budgets seems to be dependent on phytoplankton size structure.