Wetlands Australia 31: Are changes to coastal wetlands having a carbon benefit?
Authors: Jeffrey J Kelleway & Neil Saintilan (Department of Environmental Sciences, Macquarie University)
Although mangrove wetlands are well-known and highly visible ecosystems, few people would realise that these coastal wetlands are undergoing profound change as a result of climate change and sea-level rise, including the estuaries where many of our major cities are situated.
Coastal wetland ecosystems are sentinels of the impacts of climate change on the natural environment, with a growing body of work reporting profound change in the character of these wetlands globally.
Sea-level rise, together with changes in temperature, and potentially atmospheric CO2 concentration, are implicated in the global trend of mangrove proliferation at the poleward limits of their distribution, which we have documented on five continents.
Mangrove encroachment of saltmarsh in Towra Point Nature Reserve Botany. Photo:Jeffrey Kelleway
Along the sub-tropical and temperate coastlines of mainland Australia (and elsewhere), much of this mangrove expansion has been at the expense of another coastal wetland – coastal saltmarshes. This climate-driven shift in vegetation structure not only has implications for the flora and fauna which inhabit the wetlands, but may alter the supply of natural services that mangroves and saltmarshes provide for all of us. These include storm protection, carbon sequestration and provision of popular recreational resources such as coastal fisheries and birdwatching opportunities, among others.
Through our research we have determined that the encroachment of mangroves into saltmarsh has the benefit of enhancing a wetland’s carbon sequestration capacity. That is, mangroves remove additional carbon dioxide from the atmosphere, which may be stored in the woody biomass of mangroves, or their saline, oxygen depleted soils for decades through to several thousands of years, helping to mitigate greenhouse gas emissions. The accumulation of organic carbon as buried mangrove root material also helps to build the elevation of the wetland as sea-levels rise.
These findings have important implications for coastal zone management. While we have demonstrated the benefits of mangrove encroachment in sequestering additional carbon and enhancing wetland resilience to sea-level rise, the impacts of encroachment upon biodiversity values (including shorebird habitat use) and other ecosystem services remains largely unquantified.
Jeff Kelleway sampling mangrove sediments to determine their carbon sequestration capacity. Photo: Karen Lamont
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