Are mangroves drivers or buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect

Mangrove forests are hot spots in the global carbon cycle, yet the fate for a majority of mangrove net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved CO2 [CO2*] within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore, the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26° latitudinal gradient in Australia and calculated the exchange of DIC, alkalinity, and [CO2*] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m−2 d−1 across the six systems, ranging from import of 97 mmol m−2 d−1 to an export of 85 mmol m−2 d−1. If the Australian transect is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C yr−1, which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m−2 d−1and an export of 117 mmol m−2 d−1 with an estimated global export of 4.2 ± 1.3 Tmol yr−1. A net import of free CO2 was estimated (−11.4 ± 14.8 mmol m−2 d−1) and was equivalent to approximately one third of the air-water CO2 flux (33.1 ± 6.3 mmol m−2 d−1). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.