Effect of Intensity and Mode of Artificial Upwelling on Particle Flux and Carbon Export

Abstract

Reduction of anthropogenic CO2 emissions alone will not sufficiently restrict global warming and enable the 1.5°C goal of the Paris agreement to be met. To effectively counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required. Artificial upwelling has been proposed as one such carbon dioxide removal technique. By fueling primary productivity in the surface ocean with nutrient-rich deep water, it could potentially enhance downward fluxes of particulate organic carbon (POC) and carbon sequestration. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions vs. one singular addition) on POC export, sinking matter stoichiometry and remineralization depth. We carried out a 39 day-long mesocosm experiment in the subtropical North Atlantic, where we fertilized oligotrophic surface waters with different amounts of deep water. The total nutrient inputs ranged from 1.6 to 11.0 μmol NO3– L–1. We found that on the one hand POC export under artificial upwelling more than doubled, and the molar C:N ratios of sinking organic matter increased from values around Redfield (6.6) to ∼8–13, which is beneficial for potential carbon dioxide removal. On the other hand, sinking matter was remineralized at faster rates and showed lower sinking velocities, which led to shallower remineralization depths. Particle properties were more favorable for deep carbon export in the recurring upwelling mode, while in the singular mode the C:N increase of sinking matter was more pronounced. In both upwelling modes roughly half of the produced organic carbon was retained in the water column until the end of the experiment. This suggests that the plankton communities were still in the process of adjustment, possibly due to the different response times of producers and consumers. There is thus a need for studies with longer experimental durations to quantify the responses of fully adjusted communities. Finally, our results revealed that artificial upwelling affects a variety of sinking particle properties, and that the intensity and mode with which it is applied control the strength of the effects.

Artificial upwelling leads to a large increase in surface dissolved organic matter concentrations

“…The fertilization with nutrient-rich deep water caused an accumulation of DOC with no visible decrease during the duration of the experiment. The magnitude of the excess DOC was comparable to the carbon that sunk out of the mesocosms as POC (Baumann et al, 2021b). Thus, DOC represents a pool of major importance for carbon sequestration and its fate will affect the efficiency of artificial upwelling in this regard.…”

“…No deep-water was added to M5 (Control) and ambient waters outside of the mesocosms were monitored during regular sampling (Atlantic). A detailed description of the experimental set up and sampling procedures can be found in Baumann et al (2021b).…”

“…Particulate organic carbon (POC) in the water column was measured using a CN analyzer (Euro EA-CN,HEKAtech). See Baumann et al (2021b) for details. Chl-a and POC data are available at the PANGAEA repository (Baumann et al, 2021a).…”

“…Given these values, most of the DOC increase must have originated from a source other than dissolved primary production. Considering the high accumulated particulate primary production, values (Figure S6) and that more than a half of it was retained in the water column during the experiment (Baumann et al, 2021b), the channeling of carbon from the particulate to the dissolved fraction likely accounted for the rest of the DOC increase.…”

“…Processes such as extracellular enzymatic activity on particles and gel structures (Arnosti, 2011), grazing and sloppy feeding (Steinberg and Landry, 2017) or viral lysis (Breitbart et al, 2018) result in release of DOM. The large amounts of particles (including transparent exopolymer particles) that were formed following the diatom bloom (Baumann et al, 2021b) provided the substrate for prokaryotes to consume POC, and this could have contributed to the production of DOC. Moreover, viral lysis can be key to phytoplankton bloom termination, resulting in major releases of DOC (Breitbart et al, 2018;Highfield et al, 2014).…”