Phytoplankton size structure and primary production in a highly dynamic. coastal ecosystem (Ria 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 (Ria 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 pm). 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 mu m) and nano-(2-20 mu m) 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 Rfa 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. (c) 2005 Elsevier Ltd. All rights reserved.