Discarded fishing gear
Discarded fishing gear such as monofilament fishing lines, sinker and hooks can entangle corals and abrade polyp tissue leading to coral lesions and mortality. In popular cast fishing spots in Oahu, Hawaii, scientists recorded fishing lines on 65% of 129 cauliflower coral (Pocillopora meandrina) colonies surveyed with increasing percentages of entirely or partially dead colonies being found in areas of with high percentages of colonies with fishing lines. A strong relationship was also found between the percentages of surface area with fishing lines and the percentage of dead surface area.
The health and diversity of coral reefs are threatened by excess nutrients carried into the ocean from the terrestrial and coastal zone. Discharged sewage (treated and untreated) and industrial pollutants as well as surface run off containing traces of agricultural fertilisers, animal waste and sediments from deforested areas alter the natural balance of nutrients in the ocean. This influences the survival, growth, reproduction and recruitment of corals as well interactions of corals with other marine organisms.
Scientific studies into effects of nutrients on coral reefs have shown that high levels of dissolved inorganic nutrients, such as nitrogen and phosphorus, can reduce coral growth and calcification rates in some coral species by up to 50%. In other species, coral growth in high nutrient environments has been reported to increase although areas of new growth exhibit reduced skeletal density. The change in calcification rates of corals in high nutrient environments is believed to be caused by an increase in the number of zooxanthelle found in coral hosts which is triggered by the high concentrations of dissolve inorganic nitrogen. As the zooxanthelle densities rise, photosynthetic activity (which uses CO2) also increases limiting the availability of CO2 for calcification.
Increased levels of dissolved inorganic nitrogen (DIN as nitrate or ammonium) promote the rapid growth of species of algae such as crustose coralline algae and the macrophyta Lobophora variegate. Once dominant, these algae threaten reef health as they out compete corals for space for new growth and overgrow the existing reef framework. As the algae dies it sinks to the bottom of the ocean where it is decomposed by bacteria using dissolved oxygen. High levels of decomposition create further problems as the stratification (layering of waters of differing salinity and temperature) of ocean water limits oxygen replenishment at the seabed causing the formation a hypoxic (oxygen starved) zone. These hypoxic conditions affect the metabolism, respiration and disease occurrence in corals as well as other marine organisms that interact with the coral reef. To date, over 415 areas of the world have experienced symptoms of eutrophication and hypoxia.
Coastal erosion, development, deforestation and other terrestrial activities have led to increasing sediment loads being transported on to coral reefs via surface run off and river discharge. While large particles may deposited near the river mouth, fine grains can be transported on to coral reef systems as they can travel great distances in ocean currents. Within the reef framework, these sediment particles affect coral health as a result of mechanical abrasion, smothering and light attenuation (reduced light). The impact of this stress will depend on the coral species as well as the amount, type, grain size and duration of sediment cover. For example, scientific studies have shown that small grains of sediments with high organic and bacterial content cause greater stress to corals than particles with a large grain size.
Bioerosion: predation by coral eating species or ‘corallivores’
A total of 114 reef fish species including the butterflyfish, parrotfish, puffers, triggerfish, filefish, wrasses, and damselfish are known to eat live coral. The extent of damage to coral reefs caused by these corallivores varies as their feeding behaviours differ. For example, some corallivores species consume only the mucus of coral while others remove live tissue and portions of the underlying calcium carbonate skeleton. Almost half (53) of the 114 corallivore fish species are types of butterflyfish (Chaetodontidae). These fish are often referred to as reef ‘browsers’ as they remove coral tissue without damaging the underlying skeleton of the reefs. Butterflyfish consume a range of coral species including Porites, Acropora, Agaricites, Pocillopora, and Montipora and take an average of 7 bites per minute. Unlike butterflyfish, the Indo-Pacific giant humphead parrotfish removes live coral tissue and major portions of the underlying skeleton. Although these parrotfish forage on other prey, they breakdown more than 5 tonnes of reef carbonates per year. As well as the 114 marine vertebrate corallivores, there are also 51 invertebrate corallivores. These include species of annelids, arthropods, echinoderms and molluscs.
Bioerosion: Mechanical and chemical boring
Marine sponges play an important role in coral reef ecosystems as they bind and protect live corals on reef frame, recycle nutrients and provide a source of nutrition for other marine organisms. Although these activities contribute to a healthy reef system, sponges are also bioeroders and can account for more than 90% of boring activity on live and dead coral heads. As sponges burrow into the reef framework, possibly as a means for protection from predators, they release chemicals to dissolve grooves of calcium carbonate (CaCO3) to form small chips that they can mechanically remove from the substrate. This boring behaviour is a natural process and plays a key role in the carbonate budget as these chips create fine reef sediments (carbonate detritus). When bioerosion and accretion rates balance, boring by marine sponges is of little threat to the reef. However, if their boring activity exceeds the rate of accretion, the coral reef structure can become unstable. The most damaging effects of boring activity on corals reefs have been recorded by scientists in areas of high nutrients where marine sponge densities appear at their greatest. In general, seaweed-coral interactions cause few problems for coral reef systems when regulated by herbivorous feeders. However, when over fishing reduces grazing activity on seaweed beds, seaweed-coral interactions can have damaging effects on coral health.
Hurricanes/ typhoons/tropical cyclones/tsunamis
Hurricanes can threaten the structure of coral reef systems as a result of their powerful wave energy. The intensity of hurricane will also influence the amount of damage as it will determine the force of the waves. In general, studies of hurricane damage have shown that delicate branching corals species such as Acropora are more to prone to wave damage than species that have boulder and massive morphologies such as Porites.
Coral diseases damage the tissue and vital functions of polyps resulting in either partial or complete mortality. Commonly occurring diseases include: Black-band disease; Red-band disease; Brown-band disease; Yellow-band disease; White syndrome, Ulcerative white spot disease; Growth anomalies; Skeletal eroding band disease and; Pink-line disease.
The cause of many of these diseases remains unknown although scientists believe that coral disease is exacerbated by poor water quality resulting from human activity and increasing water temperatures. In light of this, some scientists believe that the expected rise in sea surface temperature associated with global climate change will increase host susceptibility, host range, pathogen survival and disease transmission.
The term ‘coral bleaching’ is used to refer to the loss of coral colour. This loss of colour is due to the expulsion of zooxanthelle symbionts from the corals host tissue in response to environmental perturbation. Environmental stresses linked to coral bleaching include long exposure to strong sunlight and high sea surface temperatures during the summer months.
In strong sunlight, the photosynthetic activity of zooxanthelle hosted within the coral tissue can increase, producing large concentrations of oxygen. If oxygen levels become too high, reactive oxygen species (ROS) can form and attack coral tissue. Whilst the zooxanthelle has a natural defence system against this, strong sunlight can damage other parts of their systems which prevent them from defending against attack from ROS.
As the ROS damage coral tissue and energy producing parts cells (mitochondrial membranes), a chain of chemical reactions occur which produces nitric oxide in zooxanthelle and coral host. Although the processes that lead to these chemical reactions remain a subject for debate among scientist, this nitric oxide appears to act as a trigger for zooxanthelle expulsion from the coral host.
Sea surface temperatures
High ocean temperatures have been implicated as an underlying cause of coral bleaching. Coral reefs and zooxanthelle live close to their upper thermal tolerance limits so even a small increase in
ocean temperatures could have a devastating effect. Past bleaching events have been associated with ocean temperature increases from as little as 1.0°C - 1.5°C above the normal seasonal maximum temperature in the region. Climate change models indicate that the average ocean temperatures could rapidly increase by 1.8°C to 4°C with a predicted maximum of change of 6.4°C under high emission scenarios.
As carbon emissions from human activities cause atmospheric levels of CO2 to rise, the amount of CO2 absorbed and released by the ocean can become imbalanced. Atmospheric levels of CO2 have risen from pre-industrial levels of ~280 parts per million (ppm) to today’s level ~387 ppm with the ocean absorbing approximately 30% of CO2 from the atmosphere.
As CO2 is absorbed into the ocean from the atmosphere, it naturally dissolves in the seawater to form carbonic acid (H2CO3). As further hydrogen ions are loss, the dissolved CO2 can form bicarbonate (HCO3–) and carbonate (CO32–). As the ocean absorbs greater amounts of CO2, changes in the proportions of these three carbon forms which can increase ocean acidity and low concentrations of carbonate ions.
Corals and calcareous algae use carbonate and calcium ions to build their calcium carbonate (CaCO3) skeletons which form and strengthen the underlying structure of coral reef systems. As the availability of carbonate ions decreases, the production of coral skeletons will slow or form at a lower density than usual causing a weakness in the reef framework. If calcification rates in corals and coralline algae slow, erosion levels may exceed reef growth destabilising coral reef structures.
Sea level rise
Historically, the growth rate of corals has matched or exceeded the rate of rising sea surface levels. However, scientists fear that the predicted rates of sea level rise for the turn of the next century (?10 mm/yr) are greater than coral reef growth rates (~6 mm/yr). If the rate of sea level rise exceeds coral growth, coral reefs living at their lower depth limits could drown. The impact of sea level rise could become more serious if the calcification rates of coral slow as a result of ocean acidification. As well as the potential for drowning coral reefs, the rise in sea levels is expected to increase rates of coastal erosion and sedimentation. This in turn could pose a further threat to coral reefs.
Discarded fishing gear damaging corals © Endangered Species International
The health of coral reefs is threatened by excess nutrients carried into the ocean from the terrestrial and coastal zone. © Endangered Species International
A healthy coral in a low level of dissolved inorganic nutrients. © Pierre Fidenci
Spearfishing can be detrimental to already damaged corals. © Pierre Fidenci
Shrimps living in coral reef ecosystem can be affected by human-induced sedimentation. © Paddy Ryan
Sedimentation from deforestation and pollution killing coral reefs. © Endangered Species International
Yellow mask angelfish eating coral. © Paddy Ryan
The cause of many coral diseases remains unknown. © Pierre Fidenci
Sea urchins found in dead coral subsrate. © Endangered Species International
Coral losses its colour during coral bleaching. © Pierre Fidenci
A healthy coral reef supporting a high density of fish larvae. © Pierre Fidenci
Close up view of coral in the Coral Triangle. © Pierre Fidenci
Porites species with sea weed encrusted in its center. © Endangered Species International
Porites affected by a disease. © Endangered Species International
A beautiful Xestospongia coral under threat from ocean acidification. © Endangered Species International
A coral affected by overfishing and growth of species of algae. © Pierre Fidenci