Coastal Wetlands: Blue Carbon Sinks and Impacts of Land Use and Climate Change

Coastal wetlands such as mangroves, marshes, and seagrass meadows provide a number of ecosystem services including habitats for many species of fish and shellfish, storm protection, influences on water quality, aesthetics, and biodiversity.  Perhaps the least investigated, ecosystem service of coastal ecosystems is that of a significant global carbon sink.  Relative to many terrestrial and marine ecosystems, these contain among the greatest carbon densities on earth.  For example, the ecosystem carbon stocks in mangroves across the Asia Pacific have been found to range from 500 to 1200 Mg C/ha which is a CO₂ equivalence of 1603 to 8023 Mg/ha.  The large carbon stocks in mangroves  result from a combination of large-statured  forest (up to ~1m diameter trees) and organic soils up to 3 m deep or more. Because the numerous values of coastal wetlands are well known, it is ironic that rates of degradation largely relating to land use/land cover change are among the highest of any ecosystem on earth.  Dominant causes of degradation include conversion to aquaculture (shrimp), agricultural conversion, coastal development, and upstream activities that affect the timing and quality of freshwater flows.  Land conversion has resulted in the loss of over 35% of the World’s mangroves between 1980 and 2000. This loss translates to a very significant source of greenhouse gasses to the atmosphere.   The carbon emissions arising from conversion of tropical wetlands to other uses is exceptionally high and from stocks that have been accumulating in place for thousands of years.  Thus, the combination of very high C stocks, susceptibility to land cover change, and numerous ecosystem services makes tropical wetlands potentially ideal candidates for REDD+ or other mitigation strategies. Climate change impacts most strongly affecting coastal ecosystems and the people who depend upon them include sea-level rise, increasing soil salinity, changes in temperature, rainfall patterns, and increases in the number and severity of cyclones. Synergistic interactions of anthropogenic pressures and climate change have the potential to drastically affect the function of these ecosystems. Mitigation strategies that result in the protection/restoration of intact coastal ecosystems would have numerous co-benefits.  Intact ecosystems are most buffered and resilient to changes due to disturbances relating to climate change.  As such, mitigation strategies that protect coastal ecosystems are effective adaptation strategies to climate change.