Geoscience Reference
In-Depth Information
changes in sediment flux, some significant changes
are also linked to anthropogenic disturbance.
Many such disturbances may have an immediate
impact on the reef or mangrove community, but
some may also have longer term sedimentological
impacts. In the context of coral reefs, this relates
to changes in rates and patterns of carbonate
production, in the case of mangroves to the sub-
strate destabilization that can follow community
degradation, and in both environments to pro-
gressive sediment contamination. A key issue in
terms of disturbance relates to the need to place
observed environmental or community changes
in the context of natural or background commun-
ity dynamics, i.e. change should not necessarily
be regarded as a consequence of anthropogenic
disturbance. This section is not intended as a
comprehensive review of anthropogenic influ-
ences but outlines, via selected examples, some
of the main impacts (and uncertainties) of human
disturbance in relation to reef and mangrove
sedimentology. Useful reviews of potential anthro-
pogenic disturbances to mangroves and coral
reefs are provided, respectively, by Ellison &
Farnsworth (1996) and Grigg & Dollar (1990).
In south west Sri Lanka a range of human-
related activities have promoted rapid coastal
retreat (average retreat 1.1 m yr
−1
; Preu 1989).
Activities include large-scale fluvial sand extrac-
tion, which prevents sediment (normally sup-
plied to the beaches) from reaching the coast.
In addition, reef degradation resulting from
channel blasting and coral mining has led to
modified nearshore current dynamics and locally
high (up to 4 m yr
−1
) rates of shoreline retreat.
Shoreline erosion rates of 0.5-1 m yr
−1
have
also been reported from the Marshall Islands,
where aggregate extraction has reduced sediment
supply to the lagoon rim, and causeway and
channel constructions have interrupted sediment
transport (Xue 2001).
Changes in land-use and the transition of
mangrove land for agricultural or development
purposes also have an impact on nearshore sedi-
ment dynamics. In particular, loss of mangrove
swamp has an impact on the spatial extent of tidal
influence. This may reduce the tidal prism leading
to modified current dynamics, reduced ebb-tide
flow and sediment export (see section 9.2.5.1),
and thus channel siltation (Wolanski et al. 1992).
Land modification for the purposes of shrimp or
rice farming also has longer term impacts beyond
the immediate loss of mangrove substrate. The
sediments in such areas typically become highly
saline and acidic, and this often inhibits natural
recolonization. This in turn leaves areas of
bare, degraded substrate which are more prone
to erosion from runoff and waves (Ellison &
Farnsworth 1996).
9.4.1 Impacts of coastline modification and
resource extraction
Modification of the coastal fringe as a result
of land-use change, construction or resource
extraction can exert a major influence on sedi-
ment dynamics and coastline stability (see also
Chapters 7 & 8). Change may occur either
directly due to sediment removal, leading to
changes in local sediment budgets, or indirectly
as a result of seawall or causeway construction,
which may modify local current dynamics and
restrict longshore sediment transport (Preu
1989). In reef environments, progressive habitat
degradation resulting from coral mining, sand
extraction or channel blasting may modify local
current pathways, and in mangrove settings,
land conversion for agriculture may remove
areas from tidal influence and thus alter the
area over which tidal influence is exerted. Such
changes can lead to modified patterns of sedi-
ment transport, erosion and deposition.
9.4.2 Impacts of increased sediment flux
Although high rates of sediment discharge can be
regarded as natural inhibitors or influences on reef
and mangrove development (see section 9.1.3),
increased or modified rates of sediment influx
are a widely cited example of anthropogenic dis-
turbance (Grigg & Dollar 1990). This has been
attributed to poor land management practices
(which may increase soil erosion and runoff ) and
to marine dredging and dumping. Two key factors
need, however, to be highlighted in relation to
sediment inputs into nearshore environments:
