Resilience and recovery of Coral Reefs and Coastal Habitats

 

By far the most coral reefs are located between latitudes 30º N and S, and of these nearly 92% are located in the Indo-Pacific (Spalding et al., 2001). This region was also severely hit by the El Niño event of 1998. The Seychelles and Comoros were hit hard (Wilkinson, 2002). Another more localized bleaching event occurred in 2002 in those areas. There is extensive documentation that  changes in the salinity of water, and in  particular run-off of silt, nutrients, sewage and other forms of coastal pollution associated with agricultural production, logging, land reclamation, clearing of vegetation for industrial and coastal development may isolate, kill, or deplete coral reefs (McCook 1999, Nyström et al. 2000, Bellwood et al 2004). However, these factors also serve as an essential role in hindering recovery of coral reefs following storms or severe temperature events resulting in bleaching of coral reefs.

Coastal development in terms of settlements, resort or industrial development reduces the diversity of the coastal vegetation and destroys significant areas, such as mangroves. These ecosystems play an essential role in limiting silt and nutrient outflows to the nearshore marine environment, including run-off of sewage animal waste and top soil during the heavy tropical rains or from rivers.

 
 
Figure 11. Recolonization of corals following bleaching along the coasts of Mahé island, Seychelles. With increasing development, silt, land reclamation and sewage cover corals with sand and algae, slowing down recolonization substantially. The Baie Ternay and Port Launay sites were hit badly by the bleaching events, but have been rapidly recolonized by soft and leather corals, as well as Porites sp. Correspondingly, the sites near heavy development have shown little or no recovery in the same period, suggesting that their ability to recover is much less (Source: Wilkinson, 2002, Global unit SO survey data with Nature Seychelles).   Figure 12. The impacts of coastal development and clearing of coastal vegetation in the beach crest on costal vegetation diversity within 5-25 m of the beach. Development often results in the clearing of coastal vegetation, which then again may increase run-off into rivers, creeks and into the ocean directly during heavy rains or as a direct result of coastal erosion. Most important, however, is likely marine pollution originating from land based sources. (Data from Mahé, Praslin and La Digue, Seychelles 2002-2006).
A survey by a UNEP team in collaboration with Nature Seychelles monitored the re-colonization of coral reefs following the bleaching event in 1998, and the later smaller ones in 2001-2003. The results revealed a remarkable relationship between coastal infrastructure development (roads, settlements and buildings) and loss of coastal vegetation diversity, and also great differences in the recovery rate of bleached corals (see box).

 
Figure 13. The Huang He (Yellow River) is the muddiest river on Earth and is China’s second longest river, running 5 475 km (3 395 miles) from eastern Tibet to the Bohai Sea. The Huang He’s yellow color is caused by its tremendous load of sediment, composed primarily of mica, quartz, and feldspar particles. The sediment enters the water as the river carves its way through the highly erodable loess plateau in north-central China (Loessial soil is called huang tu, or “yellow earth,” in Chinese). Centuries of sediment deposition and dike building along the river’s course has caused it to flow above the surrounding farmland in some places, making flooding a critically dangerous problem. Where the Huang He flows into the ocean, sediments are continuously deposited in the river delta, where they gradually build up over time. Between 1979 and 2000 - as these satellite images show - the delta of the Huang He river expanded dramatically. Several hundred square kilometres of newly formed land were added to China’s coast during this period.   Figure 14.Sundarban, the largest mangrove forest of the world, is situated in the southwestern part of Bangladesh and in the West Bengal state of India. Guarded by the Bay of Bengal, Sunderban is an excellent example of the coexistence of human and terrestrial plant and animal life. Despite high population pressure and environmental hazards such as siltation, cyclone flooding and sea level rise, the areal extent of the mangrove forest has not changed significantly in the last 25 years. In fact, with improved management, the tiger population has increased from a mere 350 in 1993 to 500-700 in 2000. Ecotourism is increasing as well. However, while sufficient data is not available, several reports suggest that forest degradation has been occurring in many parts of Sundarban. The Sundarban mangrove forests are also becoming more vulnerable due to the significant rise of shrimp farming in the region. The increase of shrimp farming has negatively affected agriculture and also contributed to the loss of mangrove forests during the past two decades.

The Status of the coral reefs in the Indian Ocean and in particular the Seychelles prior to and following the major bleaching event of 1998, where sea surface temperature in some areas exceeded 34º C, has been published in several reports. The Seychelles have had a high level of environmental awareness with many protected areas and good policies and legislation, although on the east side of the main island, Mahé, a substantial share of the coastal and marine ecosystems (seagrass and marine algal beds, reefs, etc.) have been covered by land reclamation and damaged by coastal development and pollution. Coral bleaching and mortality in 1998 was more severe in the Seychelles and Comoros than in the south (Madagascar, Mauritius and Reunion, where there was much more rapid recovery and much smaller extent of the bleaching. Since 1998 there has been several localized bleaching events in Reunion (2001), and on Rodrigues and the Seychelles (March 2002). Large proportions of the hard corals, such as Acropora species, died. For example in the St. Anne Marine National Park and Bay Ternay more than 95% of the corals were dead by 1999. However, in surveys in the following years, cover by soft corals and stony coral species like Porites sp. increased rapidly in Bay Ternay, the MPA the least affected by coastal development on the main island of Mahé. In the following years coral cover nearly doubled annually, reaching 20% in 2002, though not of the slow-growing branchy corals like Acropora and Pocillopora sp. 

A task force team with the assistance of  Nature Seychelles monitored coastal vegetation in 2004 and coral coverage from 120 dives on 22 sites in 2006. Dives were confined to coral reefs with visible structure remaining, minimum five plots of 3x3 meter per site at 2-6 m depths. Coral cover, live coral cover, no. of day-active fish species, rubble cover, bottom topographic ruggedness, coastal distance and various measures of coastal development incl. settlements, infrastructure and vegetation diversity were recorded. The monitoring revealed that around the entire island, recovery varied substantially (from 5-70% recovery of the fastest growing corals) not just with impacts of wave erosion, but particularly with coastal development and pollution. Many dead reefs were overgrown with algae caused by higher nutrient contents near developed areas, and surface run-off of silt, which apparently reduced re-colonization rate substantially. Acropora corals seemed to have survived mainly in sites with either cooler water, more current and in sites less exposed to development and pollution on the East coast. The results confirm findings and claims worldwide that land-based pollution, reclamation, clearing of coastal vegetation and poor sewage control can damage reefs (Burke et al., 2002). More importantly, they demonstrate that protection of coastal land areas around marine protected areas is essential for reducing local pollution and facilitating recolonization of corals. On Mahé island, and other populated granitic islands, curbing coastal development and capping land reclamation may become essential for improving resilience of marine ecosystems by, for example, securing the recolonization and healthy growth of damaged reefs.

< Previous  |   Next >