Assessing impact – establishing historic and contemporary sediment accretion and carbon burial rates in Darwin Harbour mangrove forests.
Mangroves are amongst the most productive ecosystems on the planet and are a large store of carbon. Almost 20 000 ha of highly diverse mangrove forest occurs throughout the Darwin Harbour, a forest estate that currently stores 5.48 Tg (5.48 million tons) of carbon, 80% of which is stored in the mangrove sediment (Bai et al., 2012). Mangroves have an ability to actively and passively capture (accrete) oceanic and riverine sediment and suspended materials, carbon rich organic matter, which is buried and stored within the forest. Rates of accretion and surface elevation vary over long timescales as environmental processes such as natural subsidence, variability of climate (wet season rainfall and riverine discharge), tidal regime and disturbance processes (severe storms, cyclonic damage). Anthropogenic modification to flow dynamics and turbidity and sea level rise also influence accumulation or loss of sediment. Accelerated sedimentation rates can result in mangrove decline although mangrove sedimentation is thought to be keeping pace with local rises in sea level throughout most of the tropics (Alongi 2008). It would be interesting to see if this is the case in sea level rise ‘hotspots’ such as north Australia.
Understanding the spatial and temporal patterns of sedimentation rates is critical information for planning development and assessing risk to mangrove ecosystems given the development of near-shore infrastructure.
To effectively assess any impacts of future climate, sea-level or anthropogenic change, the long-term, natural variability of mangrove sedimentation rates needs to be established. This proposal aims to quantify the spatial variability of both contemporary and historic sedimentation rates (last 200 years) in Darwin Harbour mangrove communities to provide a robust baseline against which future rates of sedimentation can be compared. Accurate methods for monitoring surface elevation change will be coupled with known sediment carbon concentration and isotopic analysis of sediment cores to provide best-practice estimates of contemporary and historic sedimentation and carbon-burial rates.
Rod Surface Elevation Table Marker Horizon (RSET) monitoring stations provide an accurate, long-term methodology for the measurement of sediment accretion and carbon burial throughout the soil profile (Webb et al. 2013). Sedimentation and carbon burial rates specific to the dominant mangrove vegetative community types will be established across the harbour. At these sites, 2-3 m sediment cores will be taken for isotopic analysis (210Pb, 37Ce) to determine average annual rates of accretion and carbon burial. The method provides ‘state of the art’ estimation of sediment age and this analysis will be undertaken in collaboration with the Environmental Research Institute of the Supervising Scientist (eriss) and Australian Nuclear Science and Technology Organisation (ANSTO). RSET measurement networks have been established around the world (Webb et al. 2013) and data from a Darwin Harbour, a near-pristine mangrove tropical estate would provide invaluable data for global monitoring of mangrove health, sedimentation rates and carbon burial.
Alongi DM. (2008) Mangrove forests: resilience, protection from tsunamis, and responses to global climate change. Estuar. Coast. Shelf Sci. 76:1–13.
Bai L., Hutley L.B., McGuinness K.A., Livesley S.J. (2012) The colour of mud: spatial patterns of blue carbon storage in Darwin Harbour mangrove communities. Ecological Society of Australia Annual Conference, Melbourne, 3-7 December 2012.
Webb, E. L., et al. (2013) A global standard for monitoring coastal wetland vulnerability to accelerated sea-level rise. Nature