Contrasting sediment and aquatic CO2 and CH4 effluxes across Australian tropical salt flat, saltmarsh, mangrove and tidally restricted wetlands

Coastal wetlands are globally important natural carbon sinks but can also act as sources of greenhouse gas emissions. Despite recent progress in quantifying carbon sources and sinks in these ecosystems, uncertainties remain over the climate benefits of restoration, particularly where greenhouse gas emissions may offset carbon gains. Here, we present paired measurements of sediment and aquatic CO2 and CH4 effluxes comparing vegetated (mangrove and saltmarsh) and unvegetated (salt flat) wetlands, as well as tidally connected and tidally restricted wetlands, in tropical monsoonal northern Australia during the cool dry season of 2025. Sediment CO2 and CH4 effluxes were significantly higher in saltmarshes and mangroves than in salt flats, which likely reflects greater organic matter inputs, root-associated processes, and higher sediment moisture in vegetated habitats. In addition, tidally restricted wetlands exhibited substantially higher aquatic CH4 effluxes than tidally connected systems, consistent with the observed salinity gradient. Sediment CH4 efflux data from salt flats in this study (median: 0.013, range: 0.007–0.015 nmol CH4 m−2 s−1) provide one of the first measurements of salt flat methane rates in Australia. These results fill emission factor gaps for tropical monsoonal coastal wetlands and reinforce the need to quantify both sediment- and water-air CO2 and CH4 fluxes in blue carbon assessments. More broadly, accounting for aquatic fluxes alongside sediment fluxes is essential for quantifying mitigation potential from restoring coastal wetlands and informing blue carbon management relevant to Australia's Nationally Determined Contributions implementation.

Estuarine, Coastal and Shelf Science

Estuarine, Coastal and Shelf Science

330

1096-0015

N/A

N/A

8