Fig. 1: Location of mangroves and saltmarshes with alkalinity (TA) and dissolved inorganic carbon (DIC) observations.

TA and DIC were measured in porewater and surface water at 38 mangrove- and 8 saltmarsh-dominated creeks and estuaries. The location of some sites was slightly adjusted to allow visualization (precise coordinates are available on Supplementary Table 1). The underlying global mangrove (blue areas)20 and saltmarsh (orange areas)21 distributions were retrieved from existing databases. The ecosystems overlap in many coastlines, such as Australia, China, and the Gulf of Mexico. The sample size scale refers to the number of measurements at each site.