Structure of the microbial plankton community in the NW Iberian margin at the end of the upwelling season

At the end of summer, seasonal upwelling at the NW Iberian Peninsula relaxes and nutrient depleted oceanic waters conveyed by the Iberian Poleward Current (IPC) converge on the western shelf with outwelled waters from the four bays known as Rías Baixas. At that time, with virtual absence of diatoms and dominance of nanoflagellates in the autotrophic microbial plankton community, Synechococcus accounted for ~ 20% of the total autotrophic biomass (AB). This situation contrasts with that previously reported for spring upwelling (Crespo et al., 2011), when diatoms were present in upwelled waters on the shelf and Synechococcus accounted for a minor fraction (0.16 ± 0.13%) of AB. Primary production (624 ± 585 mg C m− 2 d− 1), 5-fold lower than in the spring upwelling, was particularly low (246 ± 104 mg C m− 2 d− 1) at the IPC, though higher values (1616 ± 389 mg C m− 2 d− 1) were measured on the western shelf in front of the Rías Baixas. Phytoplankton gross growth rates (μ) showed the same pattern of extremely low values in the IPC (μ = 0.06 d− 1) and the highest (μ = 0.27 d− 1) in the waters leaving the Rías Baixas. Whereas AB exceeded heterotrophic biomass (HB) in this upwelling system in spring, at the end of summer HB was higher than AB. However, both biomasses were linearly related with a HB:AB ratio of 1 and the excess of HB (intercept = 2.8 ± 0.5 g C m− 2) being similar to the rather constant bacterial biomass (2.7 ± 0.5 g C m− 2). This fact points to the key role played by bacteria during the oligotrophic conditions at the end of summer. Bacteria can maintain the functioning of the microbial community through processing dissolved organic matter and supplying food to heteretrophs and mixotrophs.

► The microbial plankton community in the NW Iberia was studied at the end of summer.
► This community structure is compared with that previously found in spring.
► Heterotrophic plankton dominated in this system at this time of the year.
► Coastal upwelling systems can behave as oligotrophic regions at certain times.
► The system may shift to be heterotrophic if upwelling intensity and length diminish.