Search

Abstract: The aim of this study is to explore the contribution of living phytoplankton carbon to vertical fluxes in a coastal upwelling system as a key piece to understand the coupling between primary production in the photic layer and the transfer mechanisms of the organic material from the photic zone. Between April 2004 and January 2005, five campaigns were carried out in the Ría de Vigo (NW Iberian Peninsula) covering the most representative oceanographic conditions for this region. Measurements of particulate organic carbon (POC), chlorophyll-a (chl a), phaeopigments (phaeo), and identification of phytoplankton species were performed on the water column samples and on the organic material collected in sediment traps. The POC fluxes measured by the sediment traps presented no seasonal variation along the studied period ranging around a mean annual value of 1085±365mgm−2 d−1, in the upper range of the previously reported values for other coastal systems. The fact that higher POC fluxes were registered during autumn and winter, when primary production rates were at their minimum levels points to a dominant contribution of organic carbon from resuspended sediments on the trap collected material. On the contrary, fluxes of living phytoplankton carbon (Cphyto) and chl a clearly presented a seasonal trend with maximum values during summer upwelling (546mgm−2 d−1 and 22mgchl a m−2 d−1, respectively) and minimum values during winter (22mgm−2 d−1 and 0.1mgchl a m−2 d−1, respectively). The contribution of Cphyto to the vertical flux of POC ranged between 2% and 49% in response to the pelagic phytoplankton community structure. Higher values of Cphyto fluxes were registered under upwelling conditions which favour the dominance of large chain-forming diatoms (Asterionellopsis glacialis and Detonula pumila) that were rapidly transferred to the sediments. By contrast, Cphyto fluxes decreased during the summer stratification associated with a pelagic phytoplankton community dominated by single-cell diatoms and flagellates. Minimal Cphyto fluxes were observed during the winter mixing conditions, when the presence of the benthic specie Paralia sulcata in the water column also points toward strong sediment resuspension. [Copyright &y& Elsevier]

Abstract: The influence of suspended mussel culture on the benthic–pelagic coupling was evaluated in the Ría de Vigo, in the coastal upwelling system of the NW Iberian Peninsula, during the month of July 2004. Measurements of water column properties were carried out at three stations in the Ría de Vigo: under a mussel raft (1), and at two reference sites in the main channel (2) and in the inner part (3) of the Ría. Dissolved nutrients, dissolved oxygen and inorganic carbon benthic fluxes were measured by means of a benthic chamber at stations 1 and 3. Sediment traps were deployed at 6 locations to estimate vertical fluxes. The water column structure at the three study stations was very similar, characterized by upwelling conditions during the second week of July and a posterior stratification showing a strong thermocline, with a depth fluctuation mainly modulated by the shelf wind regime. Vertical POC fluxes underneath the mussel raft (3gCm− 2 d−1) were 3 times higher than those obtained between rafts and 10 times higher than in the main channel reference site. Dissolved oxygen, ammonium, silicate and phosphate benthic fluxes were significantly higher under the raft than at the inner Ría reference site. A 1D carbon budget showed the importance of benthic metabolism under the raft (2.3±0.8gCm−2 d−1), being higher than the organic carbon produced at the photic layer (0.7±0.3gCm−2 d−1), as a result of higher organic loading compared to the inner Ría reference site. The results show for the first time the important role that suspended mussel cultures play in the benthic–pelagic coupling in this coastal upwelling ecosystem. [Copyright &y& Elsevier]

Abstract: 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±585mgCm−2 d−1), 5-fold lower than in the spring upwelling, was particularly low (246±104mgCm−2 d−1) at the IPC, though higher values (1616±389mgCm−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=0.06d−1) and the highest (μ=0.27d−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±0.5gCm−2) being similar to the rather constant bacterial biomass (2.7±0.5gCm−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. [Copyright &y& Elsevier]