16.4.2 Coastal systems and resources
The coastlines of small islands are long relative to island area. They are also diverse and resource-rich, providing a range of goods and services, many of which are threatened by a combination of human pressures and climate change and variability arising especially from sea-level rise, increases in sea surface temperature, and possible increases in extreme weather events. Key impacts will almost certainly include accelerated coastal erosion, saline intrusion into freshwater lenses, and increased flooding from the sea. An extreme example of the ultimate impact of sea-level rise on small islands – island abandonment – has been documented by Gibbons and Nicholls (2006) in Chesapeake Bay.
It has long been recognised that islands on coral atolls are especially vulnerable to this combination of impacts, and the long-term viability of some atoll states has been questioned. Indeed, Barnett and Adger (2003) argue that the risk from climate-induced factors constitutes a dangerous level of climatic change to atoll countries by potentially undermining their sovereignty (see Section 16.5.4).
The future of atoll island geomorphology has been predicted using both geological analogues and simulation modelling approaches. Using a modified shoreline translation model, Kench and Cowell (2001) and Cowell and Kench (2001) found that, with sea-level rise, ocean shores will be eroded and sediment redeposited further lagoonward, assuming that the volume of island sediment remains constant. Simulations also show that changes in sediment supply can cause physical alteration of atoll islands by an equivalent or greater amount than by sea-level rise alone. Geological reconstructions of the relationship between sea level and island evolution in the mid- to late Holocene, however, do not provide consistent interpretations. For instance, chronic island erosion resulting from increased water depth across reefs with global warming and sea-level rise is envisaged for some islands in the Pacific (Dickinson, 1999), while Kench et al. (2005) present data and a model which suggest that uninhabited islands of the Maldives are morphologically resilient rather than fragile systems, and are expected to persist under current scenarios of future climate change and sea-level rise. The impact of the Sumatran tsunami on such islands appears to confirm this resilience (Kench et al., 2006) and implies that islands which have been subject to substantial human modification are inherently more vulnerable than those that have not been modified.
On topographically higher and geologically more complex islands, beach erosion presents a particular hazard to coastal tourism facilities, which provide the main economic thrust for many small island states. Ad hoc approaches to addressing this problem have recently given way to the integrated coastal zone management approach as summarised in the TAR (McLean et al., 2001), which involves data collection, analysis of coastal processes, and assessment of impacts. Daniel and Abkowitz (2003, 2005) present the results of such an approach in the Caribbean, which involves the development of tools for integrating spatial and non-spatial coastal data, estimating long-term beach erosion/accretion trends and storm-induced beach erosion at individual beaches, identifying erosion-sensitive beaches, and mapping beach-erosion hazards. Coastal erosion on arctic islands has additional climate sensitivity through the impact of warming on permafrost and extensive ground ice, which can lead to accelerated erosion and volume loss, and the potential for higher wave energy if the diminished sea ice results in longer over-water fetch (see Chapter 6, Section 6.2.5; Chapter 15, Section 15.4.6).
While erosion is intuitively the most common response of island shorelines to sea-level rise, it should be recognised that coasts are not passive systems. Instead, they will respond dynamically in different ways dependent on many factors including: the geological setting; coastal type, whether soft or hard shores; the rate of sediment supply relative to rate of submergence; sediment type, sand or gravel; presence or absence of natural shore protection structures such as beach rock or conglomerate outcrops; presence or absence of biotic protection such as mangroves and other strand vegetation; and the health of coral reefs. That several of these factors are interrelated can be illustrated by a model study by Sheppard et al. (2005), who suggest that mass coral mortality over the past decade at some sites in the Seychelles has resulted in a reduction in the level of the fringing reef surface, a consequent rise in wave energy over the reef, and increased coastal erosion. Further declines in reef health are expected to accelerate this trend.
Global change is also creating a number of other stress factors that are very likely to influence the health of coral reefs around islands, as a result of increasing sea surface temperature and sea level, damage from tropical cyclones, and possible decreases in growth rates due to the effects of higher CO2 concentrations on ocean chemistry. Impacts on coral reefs from those factors will not be uniform throughout the small-island realm. For instance, the geographical variability in the required thermal adaptation derived from models and emissions scenarios presented by Donner et al. (2005) suggest that coral reefs in some regions, such as Micronesia and western Polynesia, may be particularly vulnerable to climate change. In addition to these primarily climate-driven factors, the impacts of which are detailed in Chapter 6, Section 6.2.1, there are those associated mainly with other human activities, which combine to subject island coral reefs to multiple stresses, as illustrated in Box 16.2.
Box 16.2. Non-climate-change threats to coral reefs of small islands
A large number of non-climate-change stresses and disturbances, mainly driven by human activities, can impact coral reefs (Nyström et al., 2000; Hughes et al., 2003). It has been suggested that the ‘coral reef crisis’ is almost certainly the result of complex and synergistic interactions among global-scale climatic stresses and local-scale, human-imposed stresses (Buddemeier et al., 2004).
In a study by Bryant et al. (1998), four human-threat factors – coastal development, marine pollution, over-exploitation and destructive fishing, and sediment and nutrients from inland – provide a composite indicator of the potential risk to coral reefs associated with human activity for 800 reef sites. Their map (Figure 16.1) identifies low-risk (blue) medium-risk (yellow) and high-risk (red) sites, the first being common in the insular central Indian and Pacific Oceans, the last in maritime South-East Asia and the Caribbean archipelago. Details of reefs at risk in the two highest-risk areas have been documented by Burke et al. (2002) and Burke and Maidens (2004), who indicate that about 50% of the reefs in South-East Asia and 45% in the Caribbean are classed in the high- to very high-risk category. There are, however, significant local and regional differences in the scale and type of threats to coral reefs in both continental and small-island situations.
Figure 16.1. The potential risk to coral reefs from human-threat factors. Low risk (blue), medium risk (yellow) and high risk (red).
Source: Bryant et al. (1998).
Recognising that coral reefs are especially important for many small island states, Wilkinson (2004) notes that reefs on small islands are often subject to a range of non-climate impacts. Some common types of reef disturbance are listed below, with examples from several island regions and specific islands.
1. Impact of coastal developments and modification of shorelines:
- coastal development on fringing reefs, Langawi Island, Malaysia (Abdullah et al., 2002);
- coastal resort development and tourism impacts in Mauritius (Ramessur, 2002).
2. Mining and harvesting of corals and reef organisms:
- coral harvesting in Fiji for the aquarium trade (Vunisea, 2003).
3. Sedimentation and nutrient pollution from the land:
- sediment smothering reefs in Aria Bay, Palau (Golbuua et al., 2003) and southern islands of Singapore
- (Dikou and van Woesik, 2006);
- non-point source pollution, Tutuila Island, American Samoa (Houk et al., 2005);
- nutrient pollution and eutrophication, fringing reef, Réunion (Chazottes et al., 2002) and Cocos Lagoon,
- Guam (Kuffner and Paul, 2001).
4. Over-exploitation and damaging fishing practices:
- blast fishing in the islands of Indonesia (Fox and Caldwell, 2006);
- intensive fish-farming effluent in Philippines (Villanueva et al., 2006);
- subsistence exploitation of reef fish in Fiji (Dulvy et al., 2004);
- giant clam harvesting on reefs, Milne Bay, Papua New Guinea (Kinch, 2002).
5. Introduced and invasive species:
- Non-indigenous species invasion of coral habitats in Guam (Paulay et al., 2002).
There is another category of ‘stress’ that may inadvertently result in damage to coral reefs – the human component of poor governance (Goldberg and Wilkinson, 2004). This can accompany political instability, one example being problems with contemporary coastal management in the Solomon Islands (Lane, 2006).