Aquatic eddy covariance pH and O2 fluxes, a technique for observing calcification in benthic ecosystems

Calcifying organisms, including scleractinian corals and coralline algae, play a pivotal role in supporting benthic habitats and their associated ecosystem functions. However, many of them are threatened by ocean warming and acidification caused by anthropogenic CO₂ emissions. Our understanding of their capacity to adapt to changes in their natural environment remains limited. To address this knowledge gap, we introduce a non-invasive method to quantify calcification from the simultaneous measurement of H⁺ ion and O₂ fluxes utilizing the aquatic eddy covariance technique. Because calcification is a net source of H⁺ ions, it can be quantified as a source of H⁺ ions in excess of those generated by organic carbon metabolism. To examine the effect of ocean acidification on calcification by coralline algae epiphytes, we measured H⁺ and O₂ fluxes at a seagrass meadow at a CO₂ vent and at a control meadow, 670 m away. At both meadows we found that opposing flows were enriched in vent CO₂. Additionally, vent CO₂ diffused upwards through sediments at both sites. Because of this, we were unable to completely separate the calcification signal (non-metabolic H⁺ ion production) from the persistent background signal of vent CO₂. However, we use these data as a demonstration of how H⁺ and O₂ eddy covariance can reveal a subtle time-varying signal consistent with calcification in a benthic ecosystem. Based on the results of this study, H⁺ and O₂ eddy covariance can quantify changes in benthic calcification over time, and therefore it can support better-informed management of scleratinian corals, coralline algae, and other benthic calcifiers.