Projected increase in carbon dioxide drawdown and acidification in large estuaries under climate change

Most estuaries are substantial sources of carbon dioxide (CO 2 ) to the atmosphere. The estimated estuarine CO 2 degassing is about 17% of the total oceanic uptake, but the effect of rising atmospheric CO 2 on estuarine carbon balance remains unclear. Here we use 3D hydrodynamic-biogeochemical models of a large eutrophic estuary and a box model of two generic, but contrasting estuaries to generalize how climate change affects estuarine carbonate chemistry and CO 2 fluxes. We found that small estuaries with short flushing times remain a CO 2 source to the atmosphere, but large estuaries with long flushing times may become a greater carbon sink and acidify. In particular, climate downscaling projections for Chesapeake Bay in the mid-21st century showed a near-doubling of CO 2 uptake, a pH decline of 0.1–0.3, and >90% expansion of the acidic volume. Our findings suggest that large eutrophic estuaries will become carbon sinks and suffer from accelerated acidification in a changing climate.