Autotrophic and heterotrophic microbial plankton biomass in the NW Iberian upwelling: seasonal assessment of metabolic balance

Although it is assumed that small plankton cells prevail in the oligotrophic ocean and microplankton dominate in coastal upwelling zones, several signals point to a great importance of pico-and nanoplankton in upwelling systems. We studied the size distribution of autotrophic and heterotrophic microbial plankton biomass in shelf waters of the NW Iberian upwelling over an annual cycle. Both autotrophs and heterotrophs showed a seasonal evolution related to the hydrographic regime. The lowest total plankton biomass (3.8 +/- 0.9 g C m(-2)) was recorded in winter associated with the Iberian Poleward Current, while highest values occurred during the spring onset (10.5 +/- 3.4 g C m(-2)), summer upwelling (10.8 +/- 3.8 g C m(-2)) and summer stratification (9.3 +/- 2.0 g C m(-2)). Nano-and picoplankton dominated the microbial community, with the major variations in biomass occurring through the addition or disappearance of microplankton cells, mainly diatoms. Thus, the food web in this upwelling system should be considered multivorous, with the microbial loop (pico-and nanoplankton) as a background to which a diatom-based food web is added during upwelling. The estimated metabolic balance showed that the microbial community was autotrophic only during upwelling and spring onset, coinciding with the presence of diatoms. Heterotrophy was basically located in the picoplankton fraction. These results and the threshold of carbon fixation (2.5 to 3.5 g C m(-2) d(-1)) needed to maintain a balanced metabolism lead us to conclude that in a future scenario with low upwelling intensity and frequency, the microbial community in the NW Iberian upwelling would be heterotrophic.