| Summary: | peer reviewed Coastal ecosystems are under threat from ocean acidification. Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and biogeochemical processes altering carbonate chemistry. Mangroves and saltmarshes are global biogeochemical hotspots sequestering large amounts of carbon in sediments and in the ocean following lateral carbon export (outwelling). Here, we investigate whether mangroves and saltmarshes drive or buffer coastal waters against acidification and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of tidal wetlands export more DIC than alkalinity, enhancing pH declines of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol/m2/d in mangroves and 57 ± 104 mmol/m2/d in saltmarshes) was the major fate of plant production. However, substantial amounts of fixed carbon remain unaccounted for in budgets. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant carbon sequestration mechanism enhancing the overall value of tidal wetlands as a nature-based solution to climate change.
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