Anthropogenic changes to seawater buffer capacity combined with natural reef metabolism induce extreme future coral reef CO₂ conditions.

摘要

Ocean acidification, via an anthropogenic increase in seawater carbon dioxide (CO₂), is potentially a major threat to coral reefs and other marine ecosystems. However, our understanding of how natural short-term diurnal CO₂ variability in coral reefs influences longer term anthropogenic ocean acidification remains unclear. Here, we combine observed natural carbonate chemistry variability with future carbonate chemistry predictions for a coral reef flat in the Great Barrier Reef based on the RCP8.5 CO₂ emissions scenario. Rather than observing a linear increase in reef flat partial pressure of CO₂ (pCO₂) in concert with rising atmospheric concentrations, the inclusion of in situ diurnal variability results in a highly nonlinear threefold amplification of the pCO₂ signal by the end of the century. This significant nonlinear amplification of diurnal pCO₂ variability occurs as a result of combining natural diurnal biological CO₂ metabolism with long-term decreases in seawater buffer capacity, which occurs via increasing anthropogenic CO₂ absorption by the ocean. Under the same benthic community composition, the amplification in the variability in pCO₂ is likely to lead to exposure to mean maximum daily pCO₂ levels of ca. 2100 micro atm, with corrosive conditions with respect to aragonite by end-century at our study site. Minimum pCO₂ levels will become lower relative to the mean offshore value (ca. threefold increase in the difference between offshore and minimum reef flat pCO₂) by end-century, leading to a further increase in the pCO₂ range that organisms are exposed to. The biological consequences of short-term exposure to these extreme CO₂ conditions, coupled with elevated long-term mean CO₂ conditions are currently unknown and future laboratory experiments will need to incorporate natural variability to test this. The amplification of pCO₂ that we describe here is not unique to our study location, but will occur in all shallow coastal environments where high biological productivity drives large natural variability in carbonate chemistry.