Wetlands Australia 31: Coastal wetlands among the most carbon rich of all habitats

Authors: Matthew Hayes (School of Biological Sciences, Villanova University) and Catherine Lovelock (School of Biological Sciences, The University of Queensland)

Globally, mangroves have some of the highest blue carbon stores per hectare of any ecosystem because of their high productivity, sediment accretion and low rates of decomposition of soil organic matter. Their capacity to sequester organic carbon makes them an important component of combatting climate change, but the fine-scale distribution of soil carbon, within regions, is not well characterised.

We assessed the distribution of soil carbon in Moreton Bay in southeast Queensland to find out how soil carbon varies among different habitats and across the intertidal zone. Moreton Bay is a large subtropical semi-open bay protected by the sand islands of Moreton, North Stradbroke and South Stradbroke Islands to the east, while the western side is comprised of a deltaic coastline from which five river systems flow into the bay. There are approximately 14,000 hectares of mangroves and 2,500 hectares of saltmarsh/saltpan habitat making this an ideal setting to investigate how carbon storage varies between river influenced and non-riverine influenced areas, over different habitats (salt marsh and mangroves) and also between sites dominated by different mangrove species.

Over a period of two years, more than 350 sediment cores were collected from a range of sites across Moreton bay, and over 2500 samples were analysed for their soil carbon content.

We found that sites influenced by rivers had much higher carbon stocks than non-riverine areas, such as those on the sand islands of Moreton and North Stradbroke Islands. Although salt marsh and mangroves were found to contain similar carbon stocks, there was a large difference in carbon stocks between species, with Red Mangroves (Rhizophora stylosa) and Grey Mangroves (Avicennia marina) having higher carbon stocks than other species. Based on our extensive survey we estimated that the coastal wetlands of the South East Queensland catchments (17, 792 ha of wetlands) house approximately 4,100,000–5,200,000 tonnes of sediment carbon, which is about the same amount of carbon storage estimated for the entire New South Wales coastline (3,900,000–5,600,000 tonnes).

Although coastal wetlands only make up about one percent of national vegetation cover across Australia, they account for up to five percent of all carbon storage across all of Australia’s ecosystems, underscoring the need to conserve and manage coastal wetland assets in order to enhance carbon sequestration in Australia’s landscape.

The impacts of global change, such as sea-level rise and fluctuating sea levels, increased drought conditions and extreme weather events are likely to have a substantial impact on ecosystem health along Australia’s coastlines and therefore are likely to lead to change in carbon sequestration in coastal wetlands. For example, rising sea-levels will lead to increased area of coastal wetlands if space is left for their expansion, but extreme drought is likely to reduce wetland productivity and carbon sequestration potential. Additionally, restoration of degraded wetlands provides opportunities to enhance carbon sequestration. The coastal wetland research community is focussed on providing science to support management of coastal wetlands that maximize their carbon sequestration services.

Mangroves of Eprapah Creek, Moreton Bay, Qld. Photo: Catherine Lovelock

Further information

Hayes, M. A., Jesse, A., Hawke, B., Baldock, J., Tabet, B., Lockington, D., & Lovelock, C. E. (2017). Dynamics of sediment carbon stocks across intertidal wetland habitats of Moreton Bay, Australia. Global change biology, 23(10), 4222-4234.

Sippo, J. Z., Lovelock, C. E., Santos, I. R., Sanders, C. J., & Maher, D. T. (2018). Mangrove mortality in a changing climate: An overview. Estuarine, Coastal and Shelf Science.

Runting, R. K., Lovelock, C. E., Beyer, H. L., & Rhodes, J. R. (2017). Costs and opportunities for preserving coastal wetlands under sea level rise. Conservation Letters, 10(1), 49-57.