Geoscience Reference
In-Depth Information
(and uncertainties) regarding sea-level are outlined
in Chapter 1, but mean sea-level rises of around
50 cm by 2100 are predicted (Wilkinson 1996)
and equate to annual increases of around 4 mm
yr
−1
. In the context of published coral growth
rate data, which can be as high as 10 -12 mm
yr
−1
, these predicted changes thus appear rela-
tively insignificant. Actual reef accretion rates
(the rate at which the reef grows vertically) are
determined not only by coral growth, however,
but also by factors such as the rate of bioerosion
and sediment production, and actual rates of
net reef accretion are far lower (Stoddart 1990).
In addition, reef accretion rates vary spatially
across individual reef systems and so different
reef subenvironments (reef flat, shallow reef-
front, deep reef-front) will have varying potential
to maintain their position relative to sea-level
(Spencer 1995).
Depending upon the time-scales over which
reef accretion trends are viewed, it is possible
to reach very different conclusions about the
response of reefs to sea-level change. Recent
accretion rates suggest that although those areas
of reef that are characterized by fast-growing
branched corals can accrete at rates that exceed
all but the highest sea-level rise estimates
(Fig. 9.23a), accretion rates for reefs as a whole
(i.e. encompassing lagoon through to deep reef-
front environments) fall below even the lower
sea-level rise scenarios (Spencer 1995). In con-
trast, longer term (geological time-scale) data
from coring studies indicate that many reef
systems have accreted at rates close to or above
those predicted over the next century (Fig. 9.23b).
Such data, however, include growth that occurred
during the early Holocene when sea-levels were
rising rapidly (around 5 mm yr
−1
between 10.5
and 7.7 kyr BP in the Caribbean; Toscano &
Macintyre 2003) and it is unclear how modern
reefs would respond to such changes. Based on
Holocene reef accretion records the threshold
rates beyond which reefs are unable to main-
tain pace with sea-level change lie at around
8 -10 mm yr
−1
(Spencer 1995). One benefit of
sea-level rise may be stimulation of reef growth
in areas that have reached sea-level and stabilized
over the past 5000 years (Wilkinson 1996).
Linked to the response of coral reefs to sea-
level rise is the question of how low-lying car-
bonate islands and rubble cays may respond.
These islands comprise semi-consolidated sand
and rubble sitting atop reef platforms, and com-
monly perceived threats include direct erosion
(or submergence) and increasing saltwater intru-
sion into island aquifers (Wilkinson 1996). Recent
modelling studies, however, suggest consider-
able complexity in island response to sea-level
rise, related in large part to changes in sediment
supply (Kench & Cowell 2002). Reduced sedi-
ment supply, combined with increased sea-levels,
may result in increased shoreline displacement
rates on atolls. Reef flats typically produce little
sediment, however, and the most significant dis-
placement rates are caused by changes in the
littoral sediment budget. These may occur as
the result of a range of additional anthropogenic
stressors (see section 9.6.3) and thus magnify
the impacts of sea-level change.
The ability of mangrove shorelines to maintain
their current positions and geometries in response
to sea-level rise appears equally dependent upon
the rate of sea-level change and local sediment
dynamics, and is likely to be highly site-specific.
Stratigraphical studies through Holocene man-
groves suggest that many of the large modern
mangrove swamps did not exist during the early
Holocene (Woodroffe 1990). This is attributed
to the rapid rates of sea-level rise (in the order
of 10 mm yr
−1
) and to differences in shoreline
geomorphology that prevented sediment accu-
mulation and restricted mangroves to isolated
fringing communities. Research indicates that
modern mangroves are likely to be eroded if sea-
level rises exceed 0.8 - 0.9 mm yr
−1
(Ellison &
Stoddart 1991), but this will depend upon local
sedimentation rates.
Mangroves that currently develop on low
lying carbonate islands, which are character-
ized by autochthonous sedimentation and typic-
ally exhibit low accretion rates (
0.8 mm yr
−1
),
may experience either expansion or contraction
depending on local rates of sediment production
and accumulation (Ellison & Stoddart 1991). In
contrast, both river- and tide-dominated man-
groves can potentially receive large amounts of
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