21 Jan 2022
21 Jan 2022
Status: this preprint is currently under review for the journal BG.

Assessing the influence of ocean alkalinity enhancement on a coastal phytoplankton community

Aaron Ferderer1, Zanna Chase1, Fraser Kennedy1, Kai G. Schulz2, and Lennart Thomas Bach1 Aaron Ferderer et al.
  • 1Institute for Marine and Antarctic Studies, Ecology & Biodiversity, University of Tasmania, Hobart, TAS, Australia
  • 2Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW, Australia

Abstract. Ocean alkalinity enhancement (OAE) is a proposed method to counteract climate change by increasing the alkalinity of the surface ocean and thus the chemical storage capacity of seawater for atmospheric CO2. The impact of OAE on marine ecosystems, including phytoplankton communities which make up the base of the marine food web, are largely unknown. To investigate the influence of OAE on phytoplankton communities we enclosed a natural plankton community from coastal Tasmania for 22 days in nine microcosms during a spring bloom. Microcosms were split into three groups, (1) the unperturbed control, (2) the unequilibrated treatment where alkalinity was increased (+495 ± 5.2 µmol/kg) but seawater CO2 was not in equilibrium with atmospheric CO2, and (3) the equilibrated treatment where alkalinity was increased (+500 ± 3.2 µmol/kg) and seawater CO2 was in equilibrium with atmospheric CO2. Both treatments have the capacity to increase the inorganic carbon sink of seawater by 21 %. We found that simulated OAE had significant but generally moderate effects on various groups in the phytoplankton community and on heterotrophic bacteria. More pronounced effects were observed for the diatom community where silicic acid draw-down and biogenic silica build-up were reduced at increased alkalinity. Observed changes in phytoplankton communities affected the temporal trends of key biogeochemical parameters such as the organic matter carbon-to-nitrogen ratio. Interestingly, the unequilibrated treatment did not have a noticeably larger impact on the phytoplankton (and heterotrophic bacteria) community than the equilibrated treatment, even though the changes in carbonate chemistry conditions were much more severe. This was particularly evident from the occurrence and peak of the phytoplankton spring bloom during the experiment, which was not noticeably different from the control. Altogether, the inadvertent effects of increased alkalinity on the coastal phytoplankton communities appear to be justifiable, relative to the enormous climatic benefit of increasing the inorganic carbon sink of seawater by 21 %.

Aaron Ferderer et al.

Status: open (extended)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-17', Anonymous Referee #1, 18 Apr 2022 reply

Aaron Ferderer et al.

Aaron Ferderer et al.


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Short summary
Ocean alkalinity enhancement has the capacity to remove vast quantities of carbon from the atmosphere but its effect on marine ecosystems is largely unknown. We assessed the effect of increased alkalinity on a coastal phytoplankton community when seawater was 1) equilibrated and not equilibrated with atmospheric CO2. We found that the phytoplankton community was moderately affected by increased alkalinity and equilibration with atmospheric CO2 had little influence on this effect.