Overstated potential for seagrass meadows to mitigate coastal ocean acidification

Ricart et al. (2021, R21) explored the potential for seagrass meadows to act as a buffer of coastal ocean acidification (OA). This work relied largely on sensor-based pH measurements during 29 separate instrument deployments along the California coast, arguing that (1) seagrass aerobic metabolism consistently enhances local pH, and that (2) this OA-amelioration can persist for lengths of up to 21 days. While pH was often greater in seagrass meadows than in adjacent unvegetated regions, we are concerned with the extent to which this is attributed to seagrass ecosystem metabolism, and the interpretation that this OA-amelioration is sustained over daily and tidal timescales. By incompletely considering alternative explanations for elevated pH, irrespective of seagrass productivity, R21 offers insufficient support of OA-mitigation as a reliable ecosystem service of seagrasses. We therefore see the authors’ claim that seagrass ecosystems could be “leveraged as local management tools to mitigate the consequences of OA” as worthy of critical inspection.

First, there is a fundamental issue in R21 with the mathematical treatment of ΔpH (ΔpH = pHseagrass – pHnonvegetated), stemming from the fact that pH is on a logarithmic scale. To illustrate this issue, it is a simple fact that a ΔpH of 0.1 involves a change in [H+] that is 10 times larger when starting at pH = 7 than at pH=8. This effect is made abundantly clear by the linear relationship between pH and ΔpH (Figure 1A) present in the R21 dataset. As expected, pH also varies with salinity, with slopes that vary across sites (Figure 1B) and seasons (Figure 5 in R21). Such comparisons of ΔpH across sites, seasons, etc., are mathematically inappropriate, when made without also considering Δ[H+] (Fassbender et al., 2021). We therefore find the interpretation of ΔpH in R21 to be misleading because it does not consider the impact of initial pH on the size of ΔpH.

Van Dam B., Lopes C., Zeller M. A., Ribas-Ribas M., Wang H. & Thomas H., 2021. Overstated potential for seagrass meadows to mitigate coastal ocean acidification. Frontiers in Marine Science 8: 729992. doi: 10.3389/fmars.2021.729992. Article.

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