the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Mar 2024
Technical note: Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP)
Abstract. Ocean Alkalinity Enhancement (OAE) aims to transfer carbon dioxide (CO2) from the atmosphere to the ocean by increasing the capacity of seawater to store CO2. The potential effects of OAE-induced changes in seawater chemistry on marine biology must be assessed to understand if OAE, operated at a climate relevant scale, would be environmentally sustainable. Here, we describe the design of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP) – a standardised OAE microcosm experiment with plankton communities to be conducted worldwide. OAEPIIP provides funding for participating laboratories to conduct OAE experiments in their local environments. This paper constitutes a detailed manual on the standardised methodology that shall be adopted by all OAEPIIP participants. The individual studies will provide new insights into how plankton communities respond to OAE. The synthesis of these standardized studies, without publication bias, will reveal common OAE-responses that occur across geographic and environmental gradients and are therefore particularly important to determine. The funding available to OAEPIIP and resulting data will be shared to maximise its value and the accessibility. The globally coordinated effort has potential to promote scientific consensus about the potential effects of OAE on diverse plankton communities. Such consensus, through inclusion of the global community, will provide a sounder base to facilitate political decision making whether OAE should be upscaled or not.
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RC1: 'Comment on egusphere-2024-692', Anonymous Referee #1, 16 Apr 2024
General comments:
This technical note describes a proposed intercomparison project in which standardized microcosm experiments will allow for study of natural plankton communities to two specific ocean alkalinity enhancement scenarios. Collection of the same response variables will allow for meta-analysis to improve understanding of potential environmental side effects of OAE on plankton communities. The methods described in this note, and the need for comparable OAE studies, are highly relevant to the scope of this journal and to advancing the field of marine carbon dioxide removal broadly. I recommend publication with minor revisions, as noted in specific comments below, with the caveat that this note could be significantly strengthened with an overview of recent studies on the environmental side effects of OAE, to place the suggested methods in context of similar work. Finally, these studies will all be completed in microcosms and there is limited discussion of extension to field experiments. This would be helpful context for those interested in participating and in using the eventual results for both field and modeling studies.
Specific comments:
- Paragraph 1 (Line 38-52) could use some additional references on OAE, particularly the NASEM 2021 report. A brief description of the different types of alkalinity sources would be useful as upfront context for interdisciplinary audiences.
- Line 54: Environmental drivers are referenced several times in this section but with limited explanation—drivers of CO2 exchange? Of environmental change, or climate change? Specificity with strengthen this section, especially as the examples range from ocean acidification research to climate models.
- Line 79: Two OAE scenarios leads me to believe that this considers 2 different alkalinity feedstocks, methods of alkalinity delivery, or significantly varying concentrations of alkalinity. Later sections highlight that this is in fact 1 control, 1 NaOH-based treatment (just after alkalinity dosing), and 1 equilibrated treatment (after air-sea exchange of CO2). This should be clarified upfront, that this is not two OAE scenarios, but essentially 2 timepoints in OAE—right after delivery and after equilibration.
- Line 88: This would be a good place for a comprehensive discussion of the potential environmental side effects under consideration, which aren’t touched on much throughout this manuscript.
- Line 89: capacity building of what? Of OAE? Of microcosm experiments? Of personnel and infrastructure?
- Figure 1: Some sense of scale here would be appreciated. Are there any sensors in these tanks? Are the microcosms sealed from the atmosphere? This comes across in the text but would be strengthened by inclusion in the figure.
- Line 110: Source of tanks? Part numbers? Are frames custom ordered? If yes, what are the details? I would expect a manual for microcosm setup to include specific details for replication without requiring the reader to dig through Ferderer 2022, in case the setup is not exactly the same. Later sections note that at least some of this equipment is available to participants—can non-funded labs wanting to replicate the methods also purchase this equipment from this group? If not, they need specific direction to replicate the experiment.
- Line 121: How are the microcosms cleaned? Soap and water? Acid? DI H2O? Useful details for a manual—this is in the supplementary but could be in the main text as well.
- Are microcosms open or closed to the atmosphere throughout the experiment? This should be made explicit, and a discussion of the amount of headspace would be helpful.
- Line 218: ‘Electrodialytical’ could be simplified to ‘electrochemical’, as methods producing NaOH-based OAE may be based on electrodialysis or electrolysis.
- Line 228: On magnesium and calcium oxide based alkalinity sources—comparatively clean may depend on sourcing (industrial processes vs mining). It would be helpful to expand this comment—‘comparatively clean to’ (mined minerals? Olivines? In reference to heavy metals?) and to add citations throughout.
- The intro (line 79) suggests 2 specific OAE scenarios, so I was expecting 2 alkalinity sources in section 2.2. Clarification in the intro and in this section would be helpful.
- Line 239—specify what longer is (decades vs centuries, for example). Other modeling studies could be cited here (e.g., Wang et al., 2023).
- Lines 247-259- discussion of the alkalinity addition chosen for this project is interesting. It would be very useful to place these estimates in context of the OAE studies that have been published and that are underway, which have ranged from 100-1000umol/kg.
- Line 263- are microcosms allowed to temperature-equilibrate before the alkalinity addition?
- Line 317- the section on seawater collection suggests gentle movements to avoid physical disturbance to plankton—how gentle is this manual stirring to break up Mg hydroxides?
- Table 1/ S2—appreciate the detail on measured parameters. It would be useful to list out required accuracy/ precision of measurements. Table S2 calls out Dickson standards for DIC/TA/pH, but some users may assume glass electrodes are good enough for pH. Should call out scale in which pH should be reported. Would also be useful to state desired T/S/ etc precision—i.e., is a handheld salinometer good enough?
- Line 368- provide an example of how short is too short and how long is too long.
- Line 416: Description of models, GAMs, GAMMs would benefit from citations (i.e., bump up line 424).
- Description of how specific parameters should be handled would be useful. Do you anticipate evaporation during these experiments, and if so, should carbonate parameters be normalized?
- Is there a reason dissolved oxygen is excluded from required parameters?
- Line 450: are just the microcosms supplied, or also the heat belts, frames, lights, etc?
- Does the $12000 include microcosms and publishing fees (line 450) or are those supplied additionally? If not, how much does that portion cost, i.e., how much does a participant have left over for analyses? Line 469 could be bumped up here to specify pub fees.
Technical corrections:
- Lines 42, 293, 302, 378: add equation numbers
- Line 95: Suggest rearranging to call out figure 1 in the text before presenting it.
Citation: https://doi.org/10.5194/egusphere-2024-692-RC1 -
RC2: 'Comment on egusphere-2024-692', Anonymous Referee #2, 17 Apr 2024
The OAEPIIP program proposed by Bach et al. would provide much-needed information about the potential environmental impacts of ocean alkalinity enhancement. The methodological approach they propose appears scientifically sound, but much of what they suggest in the way of experimental design has been published elsewhere (e.g., Ferderer et al., 2022; Iglesias-Rodríguez et al., 2023). Although Bach et al. do add valuable guidance for OAEPIIP participants – e.g., how to alkalinize seawater, recommendations for statistical analysis – I don’t find their overall methodological approach to be novel. The authors should also ensure that all relevant citations from Oschlies et al. (2023) are included in their manuscript. For example, Iglesias-Rodríguez et al. (2023) provide a list of recommended variables to be measured during OAE manipulations – including protocol references – that closely matches Table 1 of this manuscript; however, it was not cited.
References:
Ferderer, A., Chase, Z., Kennedy, F., Schulz, K. G., and Bach, L. T.: Assessing the influence of ocean alkalinity enhancement on a coastal phytoplankton community, Biogeosciences Discussions, 2022, 1–36, https://doi.org/10.5194/bg-2022-17, 2022.
Iglesias-Rodríguez, M. D., Rickaby, R. E. M., Singh, A., and Gately, J. A.: Laboratory experiments in ocean alkalinity enhancement research, https://doi.org/10.5194/sp-2023-7, 2023.
Oschlies, A., Stevenson, A., Bach, L. T., Fennel, K., Rickaby, R. E., Satterfield, T., Webb, R., and Gattuso, J.-P.: Guide to best practices in ocean alkalinity enhancement research, 2023.
Citation: https://doi.org/10.5194/egusphere-2024-692-RC2
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