Artificial Upwelling Intensity and Mode Have a Major Imprint in Dissolved Organic Matter Dynamics

VIII International Symposium on Marine Sciences, 6-8 July 2022, Las Palmas de Gran Canaria, EspañaIn the face of climate change there is a need to reduce atmospheric CO2 concentrations. Artificial upwelling of nutrient-rich deep waters has been proposed as a method to enhance the biological carbon pump in oligotrophic oceanic regions in order to fuel carbon sequestration. However, the fate of the newly produced organic matter, and specifically of its resulting dissolved fraction, is not clearly understood. In the present work, nutrient-rich deep water was introduced to large scale (~44 m3) mesocosms in the oligotrophic subtropical North Atlantic with the aim of studying how the intensity and mode of artificial upwelling (large single pulses vs recurring smaller pulses) affects the dissolved organic matter (DOM) pool. Artificial upwelling yielded marked increases in the concentration and shifts in the characteristics of DOM. The magnitude of the observed changes was mostly related to the upwelling intensity: more intense treatments led to higher accumulation of dissolved organic carbon (>70 μM of excess DOC over ambient waters for extreme treatments), as well as increases in the concentration and average molecular weight of chromophoric DOM (CDOM) and the intensification of humic-like fluorescent DOM, suggesting transformation of the DOM pool. The artificial upwelling mode also affected DOM, with singular treatments overall resulting in higher CDOM quantities and molecular weight than recurring treatments. Our results indicate that under artificial upwelling, large DOM pools may accumulate in the surface ocean without being remineralised in the shortterm. This persistence could be associated with a combination of the molecular diversification of DOM due to microbial reworking, nutrient limitation and reduced metabolic capabilities of the prokaryotic communities inside the mesocosms. The present study highlights the importance of considering DOC when assessing the carbon sequestration potential of artificial upwellingPeer reviewe

Response of plankton community respiration under variable simulated upwelling events

Climate change is expected to alter the intensity and frequency of upwelling in high productive coastal regions, thus impacting nutrient fluxes, primary productivity and consequently carbon cycling. However, it is unknown how these changes will impact the planktonic (phytoplankton and bacteria) community structure, which affects community respiration (CR) and hence the carbon available for sequestration or transfer to upper trophic levels. Here we present results from a 37-day mesocosm experiment where we examined the response of CR to nutrient additions by simulating upwelling events at different intensities (low, medium, high and extreme) and modes (singular and recurring additions). We also analysed the potential contribution of different plankton size classes and functional groups to CR. The trend in accumulated CR with respect to nutrient fertilisation (total nitrogen added during the experiment) was linear in the two modes. Microplankton (mostly diatoms) and nanoplankton (small flagellates) dominated under extreme upwelling intensities and high CR in both singular and recurring upwelling modes, explaining >65% of the observed variability in CR. In contrast, prokaryotic picoplankton (heterotrophic bacteria and autotrophic cyanobacteria) explained <43% of the variance in CR under the rest of the upwelling intensities and modes tested. Changes in planktonic community structure, while modulating CR variability, would regulate the metabolic balance of the ecosystem, shifting it towards net-heterotrophy when the community is dominated by small heterotrophs and to net-autotrophy when large autotrophs prevail; although depending on the mode in which nutrients are supplied to the system. This shift in the dominance of planktonic organism will hence affect not only CR but also carbon sequestration in upwelling region