HUMAN DISTURBANCES TO CORAL REEFS

The current world population is estimated at 5.3 billion, of whom 3 billion lives in coastal areas. Much of this population is in tropical countries with fringing and offshore coral reefs, and by the year 2055 it is set to more than double. Such population pressure bring additional stresses to coral reefs.

EUTROPHICATION

Eutrophication is a process whereby water bodies, such as oceans receive excess nutrients that stimulate excessive plant growth and its effects upon coral reefs is perceived as an increasingly problem, with claims of damage to coral reef ecosystem by sewage and agricultural runoff. Effects of eutropihcation were described as causing substantial changes in community structure on the reefs. Such changes included coral species diversity and high abundances of filamentous algae at polluted sites, a reduction in skeletal growth and a reduction of reproductive effort in the corals species.

SEDIMENTATION

Sedimentation is one of the most ubiquitous human disturbances on coral reefs. It is also a disturbance that has regularly affected coral reefs in recent years, particularly in developing countries, as a result of dredging, land erosion and coastal engineering projects. Sediments may smother reef organisms and reduce light available for photosynthesis, but there appears to be no reliable way in which the effects of sedimentation can accurately be predicted for any reef site. The reef response to sedimentation will also very much depend on the reef setting, the hydrographic regime, the nature of the sediment, and the severity of its loading. For example, in the mid – 1980s another form of sediment stress on coral reefs caused concern. The potential problem was drilling fluids used to lubricate drill bits in the oil exploration industry. Experiment were carried out in the laboratory to evaluate effects of the drillings fluids on corals. In many of the experiments, a drilling fluid concentration of 100 ppm was required to produce a harmful effect. Such concentrations, however, would rarely be encountered in the field, a fact that highlights the difficulties of extrapolating the effects of such pollutants from laboratory studies to the real world.

OIL POLLUTION

Early observations on the effects of oil on coral reefs suggested that oil had little damaging effect on corals unless it comes into direct contact with coral surfaces. Often it is impossible to make comparisons between studies because of the number of variables involved – these would include the type and volume of oil spilled, the shelter or exposure of the site and water movement in the area or the nature of the reefs sites affected. Number of coral cover, and species diversity decreased substantially with increased amounts of oiling. Frequency and size of recent injuries on massive corals increased with level of oiling while growth of massive species was less at oiled reefs in the years of the oil spin than during the previous years. Estimated minimum times for recovery of the reef sites is 10-20 years on the assumption that no other events would further depress coral population.

CORAL MINING

Coral mining activities have caused extensive degradation of reefs in many countries; corals in fact have traditionally been used as building materials for example in some Indian Ocean’s islands such as the Maldives, where no rock or stone is available, and corals are removed by hand from the shallow reeftops (2-3 m depth) in lagoonal settings for use in the construction industry. Favored corals include also slow-growing species. A rapidly growing population together with the massive expansion of the tourism industry , resulted in great demand for construction material; the effects in the coral and fish communities have been profound, resulting in significant declines in coral cover, diversity and associated fish assemblage. Another cause of concern is the need to conserve reefs habitats on the outer edges of the atoll. Here the shallow reefs have an important protective role against the erosive forces of storm waves and potentially higher sea levels. Often as the supply of corals on shallow reeftops in the lagoon is exhausted, coral miners turn to shallow reef areas in the outer sides of the atoll to satisfy construction demands, a development that will only increase island vulnerability to storm influences.

HUMAN VERSUS NATURAL INFLUENCES ON CORAL REEFS

As the database of disturbances on coral reefs has built up over the last decade, scientists have tended to categorized disturbances as either human or natural. Although the causes of disturbances may fall into these two broad categories, often the reef responses are very similar. Some have described natural disturbances such as storms, mass bleachings as acute stresses, whereas human disturbances such as eutrophication and regular sedimentation have been considered as chronic influences.

PREDICTION OF RESPONSES OF REEFS TO CLIMATE CHANGE

The key factors that might be expected to affect coral reefs during a period of climate change are sea–level rise, increasing seawater temperature, increases in ultraviolet radiation, and possibly a strengthening of currents and storm activity.

SEA-LEVEL RISE

An average rate of global sea-level rise of about 6 cm per decade over the next century has been predicted. Sea-level rise will affect reef communities more profoundly than others. Reef flats constrained by present sea levels in protected waters might be expected to show increased diversity and productivity with progressively higher sea levels. However, in the short term (over the next decade), such changes will probably be swamped by natural variations in mean sea level.

EFFECTS OF INCREASED SEAWATER TEMPERATURE INCREASE

It has been argued that the potential effects of future temperature increases will depend on the scale of such changes. An increase in sea-surface temperature and its variability could produce an increasing frequency of bleaching, resulting in sublethal responses that might include reduced growth or reproductive potential of corals and increased partial mortality. Alternatively, more extreme temperature stress could lead changes in reef diversity and community structure, even if the extent of the ability of reefs corals and their zooxanthellae to genetically adapt to higher temperature are not known.

ULTRA VIOLET RADIATION

An increase in ultraviolet radiation would not be expected as a result of climate change, but the destruction of the stratospheric ozone layer by chlorofluorocarbons should be significant, then an elevation in ultraviolet (UVB) radiation levels could also act as an additional stress on coral reefs. In addition, negative influences that might accompany climate change, such as increased storms and changes in precipitations, all highlight the potential extra pressures on coral reefs in the next 40 years. Given the possible rapid decline in reef health that may result from synergistic interaction of such factors, it becomes apparent that much work needs to be done to minimize human influence to coral reefs worldwide.