The response of mangrove soil surface elevation to sea level rise

Coastal ecosystems such as mangroves can reduce risk to people and infrastructure from
wave damage and flooding. The continued provision of these coastal defence services by
mangroves is dependent on their capacity to adapt to projected rates of sea level rise. This
report explores the capacity of mangrove soil surfaces to increase in elevation in response to
local rises in sea level.
Historical evidence suggests that mangrove surface elevations have kept pace with sea level
rise over thousands of years in some places, such as Twin Cays, Belize. Rates of surface
elevation increase ranged between 1 mm/yr and 10 mm/yr in different locations and settings.
Key controls on this include external sediment inputs and the growth of subsurface roots.
Recent evidence based on measurements using the Surface-Elevation Table – Marker
Horizon methodology (from studies published between 2006 and 2011) suggest that
mangrove surfaces are rising at similar rates to sea level in a number of locations. However,
surface elevation change measurements are available for a relatively small number of sites,
and most records span short time periods. Longer term mangrove surface elevation datasets
are needed from more locations, and these need to be analysed relative to sea level changes
over the same periods of measurement.
Six sets of processes are known to influence surface elevation change in mangroves:
sedimentation/resuspension; accretion/erosion; faunal processes (e.g. burrowing of crabs);
growth/decomposition of roots; shrinkage/swelling of soils in the presence/absence of water;
and compaction/compression/rebound of soils over time and under the weight of soil/water
above. A variety of factors affect the rates of these processes, including the supply of external
sediment, the types of benthic mats that bind surface sediments together, vegetation
characteristics such as tree density and aerial root structure, nutrient availability to subsurface
roots, storm impacts, and several hydrological factors such as river levels, rainfall and
groundwater pressure. The sum of these processes results in surface elevation change.
The number and complexity of processes involved in surface elevation change create
significant challenges to the modelling and prediction of future elevation change in the face
of sea level rise. It is likely that negative feedbacks exist between sea level change and
surface elevation change, but evidence for these feedbacks is currently lacking. Such
feedbacks might enable mangrove soil surfaces to maintain their surface elevation with
respect to local sea level over the longer term. Threshold rates of sea level rise are also likely
to exist, beyond which mangrove surfaces are no longer able to keep up. An improved
understanding of the different processes and feedbacks involved in surface elevation change
will increase our ability to predict the response of surface elevation to sea level rise, and to
manage mangrove areas in ways that enhance their ability to keep pace with sea level rise.
Monitoring and management of mangrove areas is recommended to ensure continued
provision of coastal defence services into the future. In particular, sediment inputs need to be
maintained, mangroves should be protected from degradation, and space should be allowed
for mangroves to colonise landward areas. In many areas, short term anthropogenic losses of
mangroves represent a greater threat to the provision of coastal defence services by
mangroves than the longer term effects of sea level rise.

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