Anthropogenic changes to seawater buffer capacity combined with natural reef metabolism induce extreme future coral reef CO 2 conditions

Abstract

Ocean acidification, via an anthropogenic increase in seawater carbon dioxide ( CO 2 ), is potentially a major threat to coral reefs and other marine ecosystems. However, our understanding of how natural short‐term diurnal CO 2 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 G reat B arrier R eef based on the RCP8.5 CO 2 emissions scenario. Rather than observing a linear increase in reef flat partial pressure of CO 2 ( p CO 2 ) in concert with rising atmospheric concentrations, the inclusion of in situ diurnal variability results in a highly nonlinear threefold amplification of the p CO 2 signal by the end of the century. This significant nonlinear amplification of diurnal p CO 2 variability occurs as a result of combining natural diurnal biological CO 2 metabolism with long‐term decreases in seawater buffer capacity, which occurs via increasing anthropogenic CO 2 absorption by the ocean. Under the same benthic community composition, the amplification in the variability in p CO 2 is likely to lead to exposure to mean maximum daily p CO 2 levels of ca. 2100 μatm, with corrosive conditions with respect to aragonite by end‐century at our study site. Minimum p CO 2 levels will become lower relative to the mean offshore value (ca. threefold increase in the difference between offshore and minimum reef flat p CO 2 ) by end‐century, leading to a further increase in the p CO 2 range that organisms are exposed to. The biological consequences of short‐term exposure to these extreme CO 2 conditions, coupled with elevated long‐term mean CO 2 conditions are currently unknown and future laboratory experiments will need to incorporate natural variability to test this. The amplification of p CO 2 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.

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