Geoscience Reference
In-Depth Information
have been used to try and mitigate the effects
of oiling. Evidence suggests that although dis-
persants may be useful in limiting the amount
of oil that reaches mangroves, and may limit
mangrove tree mortality, they are less effective
at preventing oil absorption into mangrove sedi-
ments, or subsequent patterns of oil weathering
and degradation (Duke et al. 2000). Some suc-
cess in remediation of oiled sediments has been
achieved through the use of bioremediation
techniques (Ke et al. 2003). These exploit micro-
organisms that occur naturally within mangrove
sediments, some of which are effective degraders
of hydrocarbons. Although rates of bacterial
oil degradation may be inhibited under normal
anaerobic sediment conditions, rapid degrada-
tion rates have been achieved by manipulating
bacterial population densities via active sediment
aeration and the application of slow-release
fertilizers (Ramsay et al. 2000).
levels are likely to enhance rates of photo-
synthesis leading to increased leaf production
and mangrove growth (Ellison & Farnsworth
1996), although the positive effects of increased
productivity may be restricted locally by sea-
level rise (see section 9.6.2). Elevated CO
2
levels
may have an impact on coral communities by
modifying marine aragonite saturation states
and thus reducing rates of coral calcification
(Kleypas & Langdon 2002). Recent modelling
studies through to 2069 predict that much of the
Pacific basin may become marginal (in terms
of calcification potential) for corals with respect
to aragonite saturation (Guinotte et al. 2003)
and would have major implications for reef
carbonate production.
Linked to increasing CO
2
levels are probable
increases in atmospheric and sea-surface tem-
peratures. In most mangrove settings predicted
temperature increases are not likely to be high
enough to cause direct mangrove mortality
(Ellison & Farnsworth 1996) and, in higher lati-
tude regions, may actually promote expansion
of mangrove swamps. Such expansions would
be dependent upon local hydrological constraints
and, in arid settings, conditions may actually
become increasingly marginal for mangrove
survival. Higher atmospheric temperatures and
increased solar radiation may also cause sediment
warming, leading to increased soil respiration,
organic matter decomposition, methane (CH
4
)
and hydrogen sulphide (H
2
S) release, and root
turnover. This would, in turn, modify the com-
position and chemistry of mangrove sediments,
although the precise impact on mangrove com-
munities is not clear. In contrast, coral reef
communities are likely to be influenced signi-
ficantly by increasing sea-surface temperatures
(Chadwick-Furman 1996). From a positive per-
spective such warming may permit an increase
in the latitudinal range of hermatypic corals,
and phases of reef 'switch-on' in higher latitude
carbonate shelf settings, whereas a negative con-
sequence may be increased coral bleaching. This
occurs as a physiological response to stress, and
involves coral shedding of the symbiotic algae,
leading to reduced coral growth and, if conditions
persist, mortality (Glynn 1996). The effects of
9.6
EFFECTS OF GLOBAL CLIMATIC AND ENVIRONMENT
CHANGE ON TROPICAL COASTAL SYSTEMS
Many of the predicted changes in global climatic
conditions (see Chapter 1) have the potential to
influence both coral reef and mangrove environ-
ments. Those with direct implications include
changes in atmospheric CO
2
concentrations,
increased atmospheric and sea-surface tempera-
tures, increased UV radiation, changes to patterns
of storm frequency and intensity, and increased
sea-level (Ellison & Farnsworth 1996; Wilkinson
1996). Depending upon the magnitude of these
changes and the local sedimentary regime, coral
reefs and mangroves may exhibit highly vari-
able, and often site-specific, responses. A crucial
point to emphasize is that environmental change
may have both positive and negative effects upon
coral reefs and mangroves.
9.6.1 Impacts of climate change on coral reef and
mangrove sedimentary systems
Changes in atmospheric CO
2
levels may have
both direct and indirect effects on coral reefs
and mangroves. In mangroves, increased CO
2
