Coral reefs represent complex and vibrant underwater ecosystems, providing habitat for a vast array of marine species. These intricate structures also offer significant benefits to human populations, including coastal protection from storms and supporting fisheries that feed millions. Despite their immense value, these fragile environments face mounting pressures, particularly from various forms of overexploitation that threaten their very existence.
Direct Human Activities
Many direct human activities significantly contribute to the degradation and overexploitation of coral reefs. Overfishing, driven by both subsistence needs and the global seafood and aquarium trades, depletes fish populations that maintain the delicate balance of reef ecosystems. Removing herbivores, for example, can lead to unchecked algal growth, smothering corals. Destructive fishing methods further exacerbate this problem, causing widespread physical damage to the reef structure itself.
Practices like dynamite fishing involve detonating explosives underwater, which kills or stuns fish and shatters coral formations. Cyanide fishing, used for the live fish trade, involves spraying sodium cyanide solution onto corals to stun fish; this chemical also poisons corals and other reef organisms. Bottom trawling, though less common directly on coral reefs, can cause immense damage to deeper reef systems and surrounding seafloor habitats, dragging heavy nets that scrape away entire communities.
Beyond fishing, unsustainable coral harvesting poses another direct threat. Corals are collected for use in jewelry, souvenirs, and building materials, removing the slow-growing organisms that form the reef’s foundation. The collection of other reef organisms, such as shells and sea cucumbers, further stresses these ecosystems by removing species that play specific roles in the reef’s health and resilience.
Coastal Development and Tourism
Rapid coastal development also inflicts direct and often irreversible damage on coral reefs. Activities such as dredging, excavating seafloor sediments for channels or land expansion, can directly destroy reef structures and release vast plumes of sediment. Suspended sediment can smother nearby corals, blocking sunlight and reducing photosynthesis. Land reclamation, the process of creating new land from ocean areas, similarly leads to the direct loss of reef habitat and creates ongoing issues with sedimentation.
Unregulated or irresponsible tourism activities contribute to reef degradation. Boat anchors, dropped onto coral formations, can break off large sections, causing immediate damage. Direct contact by divers and snorkelers, even minor touches, can damage delicate polyps, leading to stress, disease, or death. The collection of coral fragments or shells by tourists as souvenirs also contributes to the depletion of reef resources and disrupts the natural ecosystem.
Pollution and Runoff
Various forms of pollution from land-based activities degrade water quality and harm coral reefs. Sediment runoff, often stemming from deforestation, agriculture, and construction, washes fine particles into coastal waters. Increased turbidity reduces light, hindering coral photosynthesis and growth, and can smother them.
Nutrient pollution, from agricultural runoff and untreated sewage, introduces excessive nitrogen and phosphorus into reef environments. These nutrients fuel the rapid growth of algae, which outcompete and overgrow corals, blocking light and space. Chemical pollutants, including pesticides, herbicides, and industrial discharges, can also directly poison coral polyps and other reef organisms, disrupting their physiological processes and overall health.
Plastic pollution is another threat, with debris accumulating on reefs. These plastics can abrade and damage coral tissues, creating entry points for pathogens and disease. Microplastics, smaller fragments resulting from the breakdown of larger plastics, can be ingested by corals and other filter feeders, potentially causing internal damage or reducing feeding efficiency.
Global Climate Change
Global climate change threatens coral reefs through two mechanisms: ocean warming and ocean acidification. Rising sea temperatures, a consequence of increased greenhouse gas concentrations, are the leading cause of coral bleaching events. During bleaching, corals expel the symbiotic algae (zooxanthellae) living in their tissues, which provide most of their energy and color. While corals can sometimes recover from mild bleaching, prolonged or severe events often lead to coral mortality, weakening the entire reef structure.
Ocean acidification occurs as the oceans absorb more atmospheric carbon dioxide, leading to a decrease in seawater pH. This change in ocean chemistry reduces the availability of carbonate ions, which corals need to construct their calcium carbonate skeletons. As the ocean becomes more acidic, it becomes harder for corals to grow and maintain their skeletons, making them more vulnerable to erosion and physical damage. This process also affects other calcifying organisms, such as mollusks and crustaceans, which are integral to the reef ecosystem.