Size matters – small estuaries fringed by blue carbon ecosystems deliver disproportionately high dissolved carbon to the ocean

Blue carbon ecosystems, e.g., mangroves and saltmarshes, may help to mitigate climate change by storing atmospheric carbon. Most blue carbon studies focus on carbon burial in soils. However, lateral exports from blue carbon systems (termed outwelling) represents an additional carbon sink. We investigated dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and alkalinity (TA) outwelling rates as well as CO₂ emissions from 18 estuaries fringed by mangroves and saltmarshes. On average, estuaries exported 344 ± 150 mmol m^-2 y^-1 DIC and 120 ± 55 mmol m^-2 y^-1 DOC. Carbon outwelling rates were ~20-times higher than estuarine CO₂ emissions (33 ± 20 mmol m^-2 d^-1). Blue carbon ecosystems contributed 19 – 41 % to the overall carbon export to the ocean. In addition to wetland cover, estuary size and runoff were key drivers impacting carbon outwelling rates. Overall, small estuaries exported disproportionally high carbon loads, contributing up to 80% of the carbon outwelling from Australian estuaries. To improve global carbon budgets, it is crucial to consider carbon outwelling from small estuaries fringed by blue carbon ecosystems.