Indian Ocean ecosystem

• Category: Marine and Oceanic Biomes.
• Geographic Location: East Africa, South Asia, and Australia.
• Summary: The Indian Ocean connects approximately one-third of the world’s human population and offers vast richness of habitat, yet it is under threat from unregulated runoff and pollutants as well as climate change.

The Indian Ocean is the third-largest of the world’s five main oceans, and comprises approximately one-fifth of the total ocean area of the world. The Indian Ocean rim region is home to around one-third of the world’s human population. It is bordered by the Indian subcontinent (from which it derives its name), and the southeast Asian Peninsula; Indonesia; Australia; East Africa; and the Arabian Peninsula of western Asia. The southern portion is bordered by the Southern Ocean.

Major marginal seas and lobes, or extensions, of the Indian Ocean include: the Arabian Sea, Red Sea, Persian Gulf, Bay of Bengal, Andaman Sea, and Great Australian Bight. The Indian Ocean has four access waterways of great significance: the Suez Canal (Egypt), Bab el Mandeb (Djibouti-Yemen), the Strait of Hormuz (Iran-Oman), and the Strait of Malacca (Indonesia-Malaysia).

Factors such as the area’s ecological and topographical complexity have led scientists to conclude that the Indian Ocean is generally characterized by unique species and high levels of biodiversity. This is supported by the high level of endemism—species found nowhere else on Earth—that has been recorded in adjacent, shallow habitats.

Climate and Resources

The majority of the continental shelves of the Indian Ocean are somewhat narrow, with an average width of only around 124 miles (200 kilometers), except off the western coast of Australia, where the continental shelf extends to more than 620 miles (1,000 kilometers). Given its vastness, the area’s climate is also diverse. The area north of the Equator has a monsoon climate with semiannual reversing winds, which can bring violent rains to India from June to September. Just south of the Equator is the trade-winds zone, where weather conditions are more consistent year round.

Climate zones in this region include tropical, temperate, sub-Antarctic, and Antarctic zones. A large part of the Indian Ocean lies within the tropical and temperate climatic zones, where numerous corals and other organisms that depend upon coral reefs are found. The tropical shore areas are home to ecologically important mangrove thickets that provide breeding and nursing grounds for many marine species. During the winter in the near-Antarctic areas farthest south, surface ice forms. However, the Indian Ocean is overall the warmest of the Earth’s oceans.

Another important feature of the Indian Ocean is the depth of many of its trenches, including the Diamantina Fracture Zone, also known as the Diamantina Trench or Diamantina Deep, and the Sunda Trench, formerly known as the Java Trench. These are deep areas of complex topography, featuring troughs to depths ranging from 24,245 to 26,401 feet (7,390 to 8,047 meters) for the Diamantina Trench and 23,813 to 25,345 feet (7,258 to 7,725 meters) for the Sunda Trench. Ridges that extend from the seafloor up to 13,123 feet (4,000 meters) in height also provide unique and varied deep-water habitats across much of the southern part of the region's ocean landscape.

The Indian Ocean's trench areas are of great ecological and economical importance, as their cold, nutrient-rich upwellings support a range of commercial fisheries focused on albacore and bluefin tuna, sharks, whiting, snapper, salmon, rock lobster, deep sea crab, abalone, and other marine species. Tuna coral reefs provide another rich area for large-scale fishing. Madagascar, for example, has extensive and diverse reefs off its coasts, as does India in the Gulf of Mannar reef constellation. Other areas with important reef zones include the Andaman Sea; the Arabian Sea and Gulf of Oman; and the Maldives, Mauritius, Reunion, Seychelles, and Comoros Islands with shallow fringing reefs.

Marine reptiles found in the Indian Ocean include the green, olive ridley, hawksbill, leatherback, loggerhead, and flatback turtles. All species of sea turtles found in this region were considered endangered or vulnerable. Marine mammals here feature the endangered dugong, spinner dolphin, humpback dolphin, killer whale, sperm whale, blue whale, and the southern elephant seal.

Large numbers of birds live in many island, coastal, and estuarine environments around the Indian Ocean. These range from resident species such as the Mauritius parakeet and kestrel; to shorebirds and waders like heron and ibis; to the far-traveling albatross, lesser frigate, and tern.

Threats

Coral reefs around the world are under the threat of depletion due to many factors, such as global climate change; direct human pressures, both commercial and recreational; and inadequate governance, awareness, and political will, something that is especially true in the Indian Ocean. Overexploitation of fishery resources has been reported off the shores of Tanzania and Mauritius, while the coral reefs near Kenya, Tanzania, and Mauritius also suffer from destructive fishing practices. Trampling of coral by fishermen has degraded many reefs in the region.

Coral mining and coral sand mining in Mafia (Tanzania), Comoros, Mauritius, and Madagascar have caused damage to the reefs in the region. More than 551,156 tons (500,000 metric tons) of coral sand was excavated annually from Mauritius by the end of the twentieth century; most beaches in Comoros also have been scarred by this destructive activity. Exploitive collection and coral mining moved from the depleted areas off of East Africa to the Maldives Islands, where it was used for road construction, and to Sri Lanka and India, where tens of thousands of tons were removed annually for a variety of purposes. Though coral is no longer used for construction and Sri Lanka and the Maldives placed bans on the removal of coral, growing increasingly concerned about the environmental impact of coral mining into the twenty-first century, the practice continued to some extent illegally. Sri Lanka banned removing coral in 1983, and the Maldives introduced regulations to coal mining in 1992, and banned the practice of selling and removing coral from reefs in 1995.

Sedimentation from agricultural practices is yet another ecological concern throughout the region, one that affects the reefs off the coasts of the Seychelles, Dar es Salaam, and Zanzibar. Eutrophication is the overabundance of nutrients leading to unsupportable blooms of aquatic plant matter. Nutrient pollution occurs when excess nitrogen and phosphorus, essential for life, enter aquatic systems due to human activities, such as intensive agriculture and fertilizer use. This excess stimulates algal blooms, leading to reduced light penetration, loss of subaquatic vegetation, and shifts in aquatic species composition. Coral reefs are damaged as algae outcompete coral larvae, and harmful algal blooms become more frequent. Oxygen depletion creates hypoxic "dead zones," causing ecosystem collapse and mass fish kills. Nutrient pollution, linked to agricultural runoff, poses significant threats to coastal and marine ecosystems, drinking water quality, and biodiversity.

Seagrass beds that are found throughout the region—another vital habitat type—are also under pressure from intensive use of bottom traps and beach seine nets, fishing practices using explosives, sand mining, and dredging off coastlines and in shipping channels. The high degree of water turbidity has driven down the rate of growth of seagrasses off the western coast of India, while rapid fluctuations in salinity levels from river runoff have had deleterious effects on the beds near Bangladesh.

Other activities that exact a high environmental cost in the Indian Ocean include petroleum exploration from large reserves of hydrocarbons, especially in the Arabian Sea; deforestation, leading to higher runoff levels; coastal development, which undermines mangrove and seagrass bed habitat; and excessive levels of motorized tourism, which leads to persistently more pollution. Since 1963, approximately 50 percent of India’s mangroves have been destroyed. The Indian government established Sundarbans National Park in 1984 in order to protect one of the world’s biggest and richest mangrove areas.

The Indian Ocean provides major sea routes connecting the Middle East, Africa, and east Asia with Europe and the Americas. Given the significant amount of international trade that takes place throughout the region’s waters, the Indian Ocean is vulnerable to high levels of pollution caused by deliberate waste disposal at sea and by accidental oil spills. This poses a difficult-to-regulate threat to the direct survival of marine organisms and the ongoing buildup of toxins in the marine ecosystem.

Because the Indian Ocean is bordered by the fast-growing economies of nations such as India, Thailand, and Indonesia, there is a high risk that industrial, agricultural, and municipal waste and runoff will continue to be released into the Indian Ocean because of a lack of regionally coordinated enforcement of environmental protection standards. Each of these threats is likely to be multiplied or complicated by climate change. The direct impacts of global warming upon the Indian Ocean will likely include rising sea levels; rising air and water temperatures; more and heavier tropical cyclones; altered timing of monsoon season onset; and increasing acidity of seawater and the negative effects of this on coral, mollusk, and lower-food-web species reproduction. In 2016, the worst coral bleaching event on record, which began in 2014, spread to and drastically affected reefs in the Indian Ocean, particularly those off of Western Australia and the Maldives. According to some studies, as a result of the warmed ocean temperatures that were likely a result of climate change, between 60 and 90 percent of Maldive coral reefs were bleached following the event. Coral reefs need between ten and fifteen years to regain their coral cover. This time period for recovery is considering the reefs do not experience additional challenges and problems like pollution, human damange, or another bleaching event.

A study conducted in 2021 concluded that coral reefs in the Western Indian Ocean are at risk of being wiped out by the end of the twenty-first century. However, a 2024 study by the Wildlife Conservation Society revealed some promising results in coral resilience to rising temperatures as it relates to global warming. The study showed coral resilience to a 1.5 degree Celcius temperature increase, but did note that carbon mitigation was still something to be concerned about, and when temperatures increased beyond the 1.5 degree Celsius threshold, the coral did not respond favorably overall. The study highlighted the urgency of global greenhouse gas reductions to reduce the likelihood of extreme scenarios. The scientists hoped their study help other scientists map biodiversity with higher precision to improve efforts to protect ecosystems at risk from climate change.

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