Preprints
https://doi.org/10.5194/bg-2023-144
https://doi.org/10.5194/bg-2023-144
12 Sep 2023
 | 12 Sep 2023
Status: a revised version of this preprint is currently under review for the journal BG.

Investigating the effect of silicate and calcium based ocean alkalinity enhancement on diatom silicification

Aaron Ferderer, Kai G. Schulz, Ulf Riebesell, Kirralee G. Baker, Zanna Chase, and Lennart Thomas Bach

Abstract. Gigatonne-scale atmospheric carbon dioxide removal (CDR) will almost certainly be needed to supplement the emission reductions required to keep global warming between 1.5–2 °C. Ocean alkalinity enhancement (OAE) is an emerging marine CDR method with the addition of pulverized minerals to the surface ocean being one widely considered approach. A concern of this approach is the potential for dissolution products released from minerals to impact phytoplankton communities. We conducted an experiment with 10 pelagic mesocosms (M1–M10) in Raunefjorden, Bergen, Norway to assess the implications of simulated silicate- and calcium-based mineral OAE on a coastal plankton community. Five mesocosms (M1, M3, M5, M7 and M9) were enriched with silicate (~75 µmol L-1 Na2SiO3), alkalinity along a gradient from 0 to ~600 µmol kg-1, and magnesium in proportion to alkalinity additions. The other five mesocosms (M2, M4, M6, M8, M10) were enriched with alkalinity along the same gradient and calcium in proportion to alkalinity additions. The experiment explored many components of the plankton community, from microbes to fish larvae, and here we report on the influence of mineral based OAE on diatom silicification. Macronutrients (nitrate and phosphate) limited silicification at the onset of the experiment until nutrient additions on day 26. Silicification was significantly greater in the silicate-based mineral treatments, with silicate concentrations limiting silicification in the calcium-based treatment. The degree of silicification varied significantly between genera, and genera specific silicification also varied significantly between alkalinity mineral sources, with the exception of Cylindrotheca. Pseudo-nitzschia was the only genus affected by alkalinity, whereby silicification increased with increasing alkalinity during some periods of the experiment. No other genera displayed significant changes in silicification as a result of alkalinity increases between 0 and 600 µmol kg-1 above natural levels. Nor did we observe any indication of interactive effects between simulated mineral dissolution products and changes in carbonate chemistry. Previous experiments have provided evidence of alkalinity effects on diatoms underscoring the necessity for further studies under a range of boundary/environmental conditions to extract a more robust pattern of diatom responses to OAE. In summary, our findings suggest limited genus-specific impacts of alkalinity on diatoms, while also highlighting the importance of understanding the full breadth of different OAE approaches, their risks, co-benefits, and potential for interactive effects.

Aaron Ferderer, Kai G. Schulz, Ulf Riebesell, Kirralee G. Baker, Zanna Chase, and Lennart Thomas Bach

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-144', Anonymous Referee #1, 08 Nov 2023
    • AC1: 'Reply on RC1', Aaron Ferderer, 14 Dec 2023
  • RC2: 'Comment on bg-2023-144', Anonymous Referee #2, 24 Nov 2023
    • AC2: 'Reply on RC2', Aaron Ferderer, 14 Dec 2023
Aaron Ferderer, Kai G. Schulz, Ulf Riebesell, Kirralee G. Baker, Zanna Chase, and Lennart Thomas Bach
Aaron Ferderer, Kai G. Schulz, Ulf Riebesell, Kirralee G. Baker, Zanna Chase, and Lennart Thomas Bach

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Short summary
Ocean alkalinity enhancement (OAE) is a promising method of atmospheric carbon removal, however it's ecological impacts remain largely unknown. We assessed the effects of simulated silicate and calcium based mineral OAE on diatom silicification. We found that increased silicate concentrations from silicate based OAE increased diatom silicification. In contrast, the enhancement of alkalinity had no effect on community silicification and minimal effects on the silicification of different genera.
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