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

Most estuaries are substantial sources of carbon dioxide (CO2) to the atmosphere. The estimated estuarine CO2 degassing is about 17% of the total oceanic uptake, but the effect of rising atmospheric CO2 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 CO2 fluxes. We found that small estuaries with short flushing times remain a CO2 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 CO2 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. Large eutrophic estuaries with long flushing times may become a greater carbon sink and acidify, while small estuaries with short flushing times are likely to remain a source of carbon to the atmosphere, according to coupled hydrodynamic-biogeochemical model and generalized box model simulations.