Geoscience Reference
In-Depth Information
sedimentation and turbidity. The former is a
measure of downward sediment flux, the latter a
measure of suspended sediment load (Te 1997).
The two are not, however, directly correlated
because high turbidity conditions can occur in
areas with relatively low sedimentation rates
where wave-driven resuspension rates are high
(Thomas et al. 2003). Both factors, however,
have a potential impact on coral communities;
increased sedimentation leads to smothering and
burial, and to an increase in polyp stress, whereas
turbidity acts to inhibit light penetration and
thus restrict photosynthesis. As a result, sedi-
ment influx has been widely perceived as a key
threat to coral communities (see Rogers (1990)
and references therein).
Recent research, however, has highlighted
numerous sites where coral communities (and
reefs) not only develop but also persist, under
conditions of high turbidity and periodically
high sediment flux (Woolfe & Larcombe 1998;
Perry 2003; Smithers & Larcombe 2003). These
'reefs' are typically characterized by reduced
framework development (although not neces-
sarily reduced coral species diversity) and, as a
result, are somewhat distinct from the coral reefs
that can develop in clear-water settings. They
represent, however, locally important sites of
coral community development and bring into
question widespread assumptions about the nega-
tive effects of sedimentation. Coral reefs occur,
for example, at a number of sites along the
inshore regions of the Great Barrier Reef, despite
high rates of terrigenous sediment influx (Woolfe
& Larcombe 1998; Smithers & Larcombe 2003).
Furthermore, several studies present evidence
suggesting that coral communities have con-
tinued to develop under conditions of at least
periodically high turbidity and terrigenous sedi-
ment input for as much as the past 5000 years
(Johnson & Risk 1987; Perry 2005).
Although local increases in the amount of
sediment reaching coral reefs (especially in
areas where past inputs have been limited) are
undoubtedly likely to have a negative impact
on corals and to modify reef community struc-
ture, too little is known about the occurrence
of coral communities under natural conditions
of high turbidity and elevated sedimentation
rates to generalize about sediment input, coral
response and reef occurrence. Such generaliza-
tions are further complicated by the vastly dif-
fering regimes of sediment input and sediment
composition that characterize different coastal
sites. Sediment inputs vary between sites depend-
ing upon the frequency (seasonal, episodic),
longevity and volumes of terrigenous sediment
input, and sedimentation rates and turbidity
regimes vary depending upon grain size, degree
of sediment resuspension and tidal range. These
local variations will influence coral species
response to sedimentation in terms of sediment
rejection mechanisms and growth strategies.
Where sediments do influence coral growth, they
are likely to be significant from a sedimento-
logical perspective because rates and patterns of
reef carbonate production are directly influenced
by the composition of the reef community.
Studies in Indonesia have, for example, docu-
mented net erosion on a range of reefs subject to
high terrigenous sediment and nutrient inputs
(see also section 9.4.2) and this has been attri-
buted both to reduced coral cover and increased
rates of bioerosion (Edinger et al. 2000).
Despite their association with fine, organic-
rich sediments, mangroves are also potentially
susceptible to increased sedimentation. Ellison
(1998) reviews numerous examples of increased
sediment influx relating to dredge spoil dump-
ing, construction-related sedimentation, mining
and catchment deforestation. The primary im-
pacts on mangrove communities relate to burial
of aerial roots at rates sufficient to restrict soil-
gas exchange. This can cause mangrove mortal-
ity and thus loss of mangrove cover, and erosion
in areas subject to fluvial or marine reworking.
Sedimentation rates in excess of 1 cm yr
−1
are
likely to be detrimental, although different species
of mangroves exhibit different tolerances to
burial (Thampanya et al. 2002).
9.4.3 Impacts of increased nutrient input
The primary source of nutrients into shallow
marine and intertidal environments is from
anthropogenic sources (e.g. sewage effluent,
