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.