Carbon outwelling across the shelf following a massive mangrove dieback in Australia: Insights from radium isotopes

Abstract

Mangrove soil carbon stocks are known to decrease following forest loss due to respiration and enhanced soil CO 2 emissions. However, changes in carbon outwelling to the coastal ocean due to mangrove forest disturbance have not been considered. In December 2015, an extremely large mangrove dieback event (∼7000 hectares, spanning 1000 km of coastline) occurred in the Gulf of Carpentaria, Australia. To assess the effect this dieback event had on carbon outwelling, we used radium isotopes and dissolved carbon measurements (dissolved organic carbon, DOC, dissolved inorganic carbon, DIC, and total alkalinity, TAlk) to estimate cross-shelf carbon transport from living and dead mangrove areas and to calculate the carbon losses from living and dead forest soils via SGD. Radium distributions imply cross shelf eddy diffusivity of 107.5 ± 26.9 and 104.6 ± 23.9 m −2 s −1 from dead and living areas and radium water ages reveal that mangrove carbon reaches 10 km offshore within 7 days. Outwelling rates from living and dead areas were explained by soil carbon losses via SGD. This study suggests a decrease in carbon outwelling to the ocean from dead forest areas compares to living areas by 0–12% for DOC, 50–52% for DIC and by 37–51% for TAlk ∼8 months after the dieback event occurred. Changes to oceanic carbon outwelling rates following mangrove loss are likely driven by a gradual depletion of carbon stocks from the sediment profile.