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SEDIMENTATION
Introduction
The earliest coral reef researchers recognized that coral reefs
were strongly inhibited wherever muddy freshwater enters the sea by
realizing that gaps in continuous fringing and offshore reefs faced
the river mouths. Negative impacts of rivers include effects of
freshwater and sediments. Freshwater can cause tissue bleaching, but
excessive sediment smothers and kills coral tissue and reduces light
levels and food supplied to the coral by symbiotic algae. Corals
differ greatly in their ability to resist sedimentation, with most
species being highly intolerant of even small amounts while a
minority are able to tolerate extremely muddy conditions, and a few
are even able to live directly in muddy bottoms. The sediment
tolerant corals are able to push sediment off their surface through a
variety of mechanisms, but these all require expenditure of metabolic
energy and when sedimentation is excessive they eventually reach the
point where they can no longer spare the energy to keep themselves
clean, and the affected tissue dies back.
Effects
The effects of sediment have greatly accelerated with regard to
freshwater for two reasons. First is expanded coastal dredging to
clear harbors and and provide beach sand and fill, which generates
huge muddy plumes in reefs which had formerly had clear water. Second
is increasing deforestation of coastal watersheds, causing
tremendously increased soil erosion from hillsides. Depending on
rainfall, topography, geology, soil types, and land management,
deforestation can result in up to thousand-fold increases in
sedimentation in near shore waters. As a result sedimentation is
taking a severe toll on almost all reefs near continental coastlines
and islands, unless they are exceptionally flat or extremely dry.
Sediment damage has a very long lasting impact because mud settles
out and is re-suspended many times before it leaves the reef. The
original plume will continue to act long after it seems to have
disappeared, because mud will be re-stirred by every storm, causing
clouds of sediment to slowly and episodically work their way down
coastlines, damaging reefs many times more before they are washed
away. The effects are severe around reefs of Central and South
America, South East Asia, East Africa, and high islands throughout
the Pacific, Indian Ocean, and Caribbean.
Classic studies of the impact of sedimentation on coral reefs
include the work of Jorge Cortes in Costa Rica, Gregor Hodgson at El
Nido, Palawan Island, the Philippines, and Tim McLanahan and David
Obura in East Africa. GCRA has not done studies specifically focused
on sedimentation, and urges those researching these effects in more
detail to check the papers mentioned above. GCRA has taken video of
sediment impacted reefs which will be added to this web site later
this year, to allow comparison of normal and sediment damaged reefs.
Assessment
Quantitative assessment of sedimentation is always difficult
because sediment concentrations and settling rates are extremely
variable, depending on the detailed history of rain, wind, and waves
at each site. One popular approach is to put out settling tubes,
vertically oriented cylinders from which trapped material is
periodically collected and weighed. The problem is that what is
caught depends on the water energy immediately before, since sediment
settles in and is repeatedly resuspended. Tremendous variability of
such measurements means vast amounts of data must be collected before
average values or trends in them can be meaningfully estimated. Most
studies often do not have time, resources, or people for the
measurements to cut through the inherent data scatter and noise, and
may often be collecting numbers from which little useful conclusions
about trends can be drawn.
We favor simpler approaches because such measurements, while
useful, can be tediously slow and prohibitively expensive to reach
meaningful conclusions. In our view the best proof that sediment is
affecting reef health is to look at the proportion of dead coral and
living coral tissue covered with fine sediment. Studies should be
carried out year round because sediment plumes are highly episodic,
linked to dredging, storm, or rain flood events and variations in
wave energy and direction. However it is very rare that the relative
importance of sediment covering live coral cannot be readily seen,
and so recording sediment cover of corals wherever it is significant
should be an important part of any reef assessment.
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