Biological thresholds for marine carbon dioxide removal (mCDR): the effect of changes in carbonate chemistry

Abstract. Marine carbon dioxide removal (mCDR) encompasses a variety of approaches to actively remove CO₂ from the atmosphere, which, given current and projected emissions, is necessary to keep global average temperature increases to less than 2 °C. In addition to the removal of CO₂ from the atmosphere, mCDR projects would have effects on the marine environment, including changes in ocean carbonate chemistry. These changes in carbonate chemistry can affect marine organisms through multiple physiological pathways. Although research on the effects of ocean acidification over the last fifteen years has advanced understanding of the effects of low pH/high CO₂ conditions on marine organisms, much less is known about organismal response to the high pH/low CO₂ or high alkalinity conditions that can be generated by some mCDR methods. We created a database of available information on marine species response to the carbonate chemistry conditions that can be generated by mCDR with a focus on identifying carbonate chemistry thresholds at which biological responses occur. The database contains 310 studies, from which we estimated 276 thresholds. In addition to all mCDR studies available to date, we reviewed studies not explicitly designed to address mCDR. These studies used natural and artificially manipulated variations in carbonate chemistry, including increasing pH and alkalinity, to explore basic physiological and biological responses as well as approaches to mitigate ocean acidification. In analyzing the database, we considered a variety of biological responses, both positive and negative, and considered sensitivity by taxa, exposure duration, treatment method and other factors. Using practical definitions of a threshold, including one based on the treatment level leading to the first statistically significant biological response, we describe the distribution of pH and alkalinity threshold values. The data reveal a high tolerance for high pH (>12) and high alkalinity for some species responses under some experimental conditions, but negative response to experimental conditions only slightly above ambient (pH 8.3) for other species responses. A positive effect of pH above ambient levels was observed in six cases, just 3 % of the total. We summarize results by presenting the distributions of threshold estimates collected from individual studies. The uncertainty and variation in organismal response represents a challenge for the development of management guidelines for the developing mCDR industry. Building on our review of existing studies, we suggest several paths forward for improved biological threshold estimates for mCDR-relevant carbonate chemistry conditions.