the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Responses of microbial metabolic rates to non-equilibrated silicate vs calcium-based ocean alkalinity enhancement
Abstract. This study contributes to the inaugural exploration of non-equilibrated Ocean Alkalinity Enhancement (OAE). The manipulation of Total Alkalinity (TA), involving silicate and calcium-based ∆TA gradients ranging from 0 to 600 µmol · L-1, was conducted without prior CO2 sequestration, under natural conditions and at a mesocosm scale (~60 m3). The resulting impact included an increase in pH and a decrease in pCO2, sustained across the experiment, as full natural equilibration via sea-gas exchange did not occur. Implemented in a neritic system under post-bloom conditions, a midway mixing event was simulated. Following an inorganic nutrient addition, discernible delays in bloom formation, as indicated by the Gross Production (GP) and Net Community Production (NCP) rates, as well as by the chlorophyll-a (Chla) concentrations, in relation to the ∆TA gradient, were observed. Notably, the delay was more pronounced for the calcium treatment set compared to the silicate one, where low TA treatments exhibited earlier responses than high TA ones. This delay is likely attributed to the previously documented, species-specific negative relationships between high pH/lowCO2 levels and phytoplankton growth rates. This study underscores the need for further investigation into the implications of this response pattern in terms of trophic transfer and seasonal suitability. Further, it is anticipated that a wider delay in bloom formation would be evident with a larger non equilibrated TA gradient. Thus, highlighting the importance of exploring variations in TA limits for a comprehensive understanding of the OAE’s impacts.
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Status: open (until 01 Aug 2024)
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CC1: 'Comment on egusphere-2024-1776', Steve Rackley, 22 Jun 2024
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It is questionable whether the TA perturbations studied here (138 to 550 umol/L) are relevant to non-equilibrated OAE deployments, except in a limited area around the addition site.
It seems a responsible "rule of thumb" that pCO2 should not be reduced below pre-industrial levels (except in that limited mixing zone) if ecosystem impacts are to be minimized or avoided. This typically occurs at a TA increase of around 80-90 umol/L, a little more than half the lowest level investigated here. Investigation of a TA perturbation of that order would have been helpful, as more relevant to anticipated OAE deployment conditions.
Citation: https://doi.org/10.5194/egusphere-2024-1776-CC1 -
AC1: 'Reply on CC1', Laura Marín-Samper, 28 Jun 2024
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Thank you very much for taking the time to provide feedback as a public commenter. We appreciate your insights and have carefully considered your concerns.
Our experimental setup was designed to simulate a real-world scenario where alkalinity is released at a point source, leading to progressively lower ∆TA levels as the source water dilutes. The ∆TA values in our study were selected to identify tipping points in the pelagic community response, which necessitated including higher ∆TA values.
We understand your concern regarding the relevance of the TA perturbations we studied, especially considering that such levels may only be pertinent to areas close to the addition site in non-equilibrated Ocean Alkalinity Enhancement (OAE) deployments. Your point about the importance of not reducing pCO2 below pre-industrial levels, except within a limited mixing zone, is well-taken. This is critical for minimizing ecosystem impacts, but it applies to the long-term, and large-scale. However, any permitting agency and the general public will want to know what environmental impacts are likely to occur from OAE applications, including close to the release site.
Although our study focused on higher ∆TA values to identify broader ecological tipping points, we agree that investigating a TA perturbation closer to 80-90 µmol/L would provide valuable insights for anticipated OAE deployment conditions. Future research should indeed consider such perturbations to ensure environmental safety in the long-term.
Thank you again for your valuable feedback.
Citation: https://doi.org/10.5194/egusphere-2024-1776-AC1
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AC1: 'Reply on CC1', Laura Marín-Samper, 28 Jun 2024
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