Ocean acidification enhances carbon burial in seagrass meadows: new insights from CO2 vents

Seagrass meadows of Posidonia oceanica are well known natural carbon sinks, burying large amounts of organic carbon (C_org) in their underlying sediments. However, it is not well understood how C_org burial will be affected by ocean acidification (OA). While the primary production of P. oceanica is anticipated to benefit from a lower pH, it is unknown how C_org burial rates respond as these are influenced by additional abiotic and biotic factors that may interact with OA in various ways.

Here, we utilize natural CO₂ vents as OA analogs to evaluate C_org burial rates as a function of water column pH. We sampled 14 sediment cores across three different sites around the island of Ischia, Italy, comprising pH regimes from ambient (pH_T≈8.0) to low pH_T (7.5-7.8) and extreme low pH_T (6.6-7.2). We used concentration profiles of ²¹⁰Pb and C_org to estimate C_org burial rates of the past ~100 years. We concomitantly assessed signatures of stable isotopes 𝛿¹³C and 𝛿¹⁵N along the depth profiles to estimate the contribution from different primary producers to sediment C_org.

We found low C_org burial rates at ambient pH but rates became markedly elevated as a function of lowered pH, with extreme low pH sites displaying nearly an order of magnitude higher rates compared to ambient pH sites. Moreover, sediment cores collected within CO₂ vents revealed an increasing C_org burial rate over the last 30 years, whereas those outside of vent areas displayed a stable or decreasing pattern with time. We hypothesize that this reflects an increase in volcanic activity that has spurred elevated CO₂ release rates in recent decades, thus further decreasing seawater pH. Our findings suggest that seagrass meadows may represent a negative feedback between OA and C_org burial, with possible ramifications for their future climate change mitigation potential. We will discuss the potential mechanisms behind these findings.