Seagrass habitat metabolism increases short term extremes and long term offset of CO2 under future ocean acidification
Significance The impacts of ocean acidification in nearshore estuarine environments remain poorly characterized, despite these areas being some of the most ecologically important habitats in the global ocean. Here, we quantify how rising atmospheric CO2 from the years 1765 to 2100 alters high frequency carbonate chemistry dynamics in an estuarine seagrass habitat. We find that increasing anthropogenic carbon reduces the ability of the system to buffer natural extremes in CO2. This reduced buffering capacity leads to preferential amplification of naturally extreme low pH and high pCO2(s.w. events above changes in average conditions, which outpace rates published for atmospheric and open ocean CO2 change. Seagrass habitat metabolism drives these short term extreme events, yet ultimately reduces organismal exposure to harmful conditions in future high CO2 scenarios. The role of rising atmospheric CO2 in modulating estuarine carbonate system dynamics remains poorly characterized, likely due to myriad processes driving the complex chemistry in these habitats. We reconstructed the full carbonate system of an estuarine seagrass habitat for a summer period of 2.5 months utilizing a combination of time series observations and mechanistic modeling, and quantified the roles of aerobic metabolism, mixing, and gas exchange in the observed dynamics. The anthropogenic CO2 burden in the habitat was estimated for the years 1765 2100 to quantify changes in observed high frequency carbonate chemistry dynamics. The addition of anthropogenic CO2 alters the thermodynamic buffer factors (e.g. the Revelle factor) of the carbonate system, decreasing the seagrass habitat's ability to buffer natural carbonate system fluctuations. As a result, the most harmful carbonate system indices for many estuarine organisms [minimum pHT, minimum arag, and maximum pCO2(s.w. change up to 1.8x, 2.3x, and 1.5x more rapidly than the medians for each parameter, respectively. In this system, the relative benefits of the seagrass habitat in locally mitigating ocean acidification increase with the higher atmospheric CO2 levels predicted toward 2100. Presently, however, these mitigating effects are mixed due to intense diel cycling of CO2 driven by aerobic metabolism. This study provides estimates of how high frequency pHT, Oarag, and pCO2(s.w. dynamics are altered by rising atmospheric CO2 in an estuarine habitat, and highlights nonlinear responses of coastal carbonate parameters to ocean acidification relevant for water quality management.
Proceedings of the National Academy of Sciences