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sediment and so might be expected to respond
more positively. These environments, however,
are highly variable in form and are characterized
by very different sediment dynamics and accre-
tion rates. River-dominated mangroves with high
sediment inputs may accrete at rates that are suf-
ficient to keep pace with sea-level rise (Woodroffe
1990). Those associated with smaller river catch-
ments (with lower sediment influx) or areas sub-
ject to seasonal sediment influx may undergo
landward migration. Tide-dominated mangrove
settings also vary significantly in terms of tidal
influence and sediment flux and, in such settings,
shoreline response will again be highly site specific
(Wolanski & Chappell 1996).
Predicting mangrove response to sea-level rise
is further complicated by the fact that future
sea-level rises will occur not only from condi-
tions of relative stability (Woodroffe 1990), but
also across relatively low-lying land (where the
spatial extent of sea-level increases is signific-
antly higher). Although this will increase the
rate at which tidal inundation and associated
changes in sediment characteristics and salinity
levels occur (Semeniuk 1994), from a positive
perspective it may also facilitate landward mig-
ration of mangroves. Indeed, in cases where
sediment accumulation rates are high and the
seaward margins of the swamps are maintained,
the spatial extent of mangrove swamps may
actually increase. This, however, will be inhib-
ited in areas where urbanization of the coastal
fringe has occurred and under these conditions,
regardless of sediment influx, progressive man-
grove degradation is likely.
(Wilkinson 1996). For example, major changes
in sea-level (of 100
m) occurred at various
times during the Pleistocene. In addition, there
is some evidence to suggest that Holocene sea-
levels underwent rapid 'jumps' at several points
over the past 16,000 years (Toscano & Macintyre
2003) and that rapid rates of sea-level rise (up to
20 mm yr
−1
) characterized the early Holocene
(Wilkinson 1996). Neither appear to have had
long-term detrimental effects on reef or man-
grove communities, although massive changes
in geomorphology and structure have obviously
occurred as the world's sedimentary systems mig-
rated across the continental shelves. Similarly,
atmospheric CO
2
concentrations and temper-
atures have fluctuated significantly through
the Pleistocene (Wilkinson 1996) without any
apparent long-term negative impact on reef and
mangrove communities.
Over the coming century, different sedimen-
tary responses may, however, result from the
combination of natural variation and anthro-
pogenic influence. Although coral and mangrove
communities and their associated sedimentary
environments are capable of responding to
natural changes in physical and environmental
factors, future responses may be influenced by
the additional stresses imposed by a range of
anthropogenic-related factors. In coral reefs,
modified rates and patterns of reef carbonate
production may, for example, occur as a result
of changes in reef community composition driven
by a range of factors including overfishing and
pollution. Similarly, in mangrove swamps,
mangrove mortality and substrate erosion may
occur due to sediment contamination and land-
use change. These additional pressures may
alter the ways in which reef and mangrove sys-
tems respond to climatic and sea-level change.
Our understanding of how both reef and man-
grove systems are influenced by specific anthro-
pogenic stressors, the mechanisms and rates
of recovery and adaptation potential, and the
longer term impacts on rates of carbonate
production and accretion (in coral reefs) and
sediment accumulation (in mangroves) remain
poorly understood.
+
9.6.3 Climatic and sea-level change in relation to
increased anthropogenic influence
Although global climate changes and related
increases in sea-level are often discussed from
the context of increasing ecosystem disturbance,
there is an important caveat to such speculation.
Predicted changes in temperature, CO
2
con-
centrations and sea-level through to 2100 are
unlikely to approach the scales of change that
have occurred in the recent geological past
