Geoscience Reference
In-Depth Information
tidal inundation. Such processes may be enhanced
by the high levels of particulate organic matter
that are often associated with sewage or agri-
cultural discharges and may speed up anaerobic
reactions, increase rates of dentrification and
have an impact on mangrove growth. Although
mangroves may actually benefit from increased
nutrient inputs (and exhibit increased produc-
tivity), nutrient enrichment of soils may have a
more negative impact by altering soil chemistry
and increasing the anaerobic nature of the sedi-
ment (Field 1998).
The extent to which metal enrichment of sedi-
ments has a direct impact on the mangrove com-
munity is not clear. Some studies report uptake of
metals within mangrove roots, often to levels that
exceed surrounding sediments (e.g. MacFarlane
et al. 2003) but these do not appear to have an
impact on mangrove growth. In contrast, uptake
within leaves appears low (Machado et al. 2002),
but in the context of metal export from man-
groves this is significant given the high amounts
of leaf detritus that are exported from some
mangrove systems (see section 9.2.4.2).
Reef sediments also have potential to accumu-
late metal and chemical contaminants, and there
is evidence of marked spatial variations related
to sediment grain size. On the inshore Great
Barrier Reef higher metal concentrations occur,
for example, within fine-grained, clay-rich har-
bour sediments compared with coarse-grained
carbonate-quartz sands (Esslemont 2000). At
these sites, remobilization and transport of
fine-grained, metal-rich sediments onto adjacent
coral reef areas has occurred periodically due
to harbour dredging. Incorporation of metals
into coral skeletons may occur via (i) absorption
of dissolved metals by coral tissue or during
feeding, (ii) particulate trapping in cavities or
(iii) direct deposition on coral skeletons through
polyp damage (Fallon et al. 2002). Such uptake
has documented ecological impacts upon coral
fertilization and coral larvae success (Reichett-
Brushett & Harrison 1999), although long-term
effects on skeletal organisms remain poorly
documented.
In extreme cases, inputs of contaminants also
have potential to cause changes in carbonate
sediment assemblages via progressive sediment
dilution. In north Jamaica, bauxite dust from
a loading terminal built in the mid-1960s has
accumulated within sediments in Discovery Bay,
a semi-restricted coastal embayment fronted
by fringing reefs and previously dominated by
carbonate sediments. Recent studies (Perry &
Taylor 2004) have shown that bauxite now com-
prises upwards of 35% of the sediment in the
most heavily impacted areas and some 20 -25%
of the sediment across much of the southern/
central area of the bay (Fig. 9.20).
9.4.4 Impacts of contaminant input (oil,
chemicals, metals)
Common contaminant inputs into nearshore and
intertidal environments include a range of heavy
metals, as well as organochlorine pesticides and
oil residues. These may be derived from indus-
trial, urban and agricultural discharges, or from
contaminant dumping and refuse sites. Contam-
inant transfer may occur either in suspended or
dissolved form, with widespread distribution
in tidally influenced settings being enhanced by
diffusive flow mechanisms (Wolanski et al. 1992).
Transfer through sediments can occur due to
seepage and runoff (Clark 1998). Contaminant
accumulation is aided, particularly in mangroves,
by the fine-grained nature and high organic
content of the sediments (Harbison 1986; see
also Chapter 7). Mangrove sediments act both
as physical traps for fine particulate and trans-
ported contaminants, and as chemical traps where
metal sulphides precipitate from solution. Such
precipitation is driven by a range of bacterial
sulphate reduction reactions in the anaerobic
sediments. Although there is the potential for
precipitated metals to be remobilized into sur-
ficial sediments and overlying waters (Harbison
1986), precipitation of metal sulphide is environ-
mentally significant as a mechanism of immob-
ilizing metal contaminants.
Gradients of metal contamination within sedi-
ments are common through mangrove swamps
(Soto-Jiménez & Páez-Osuna 2001), with the
highest metal concentrations tending to occur in
areas rich in fine particulate and organic matter.
Search WWH ::




Custom Search