Coral reefs are complex living structures, often referred to as the rainforests of the sea because of the immense biological diversity they support. Although they cover less than one percent of the ocean floor, these ecosystems provide habitat for an estimated 25% of all marine species, including thousands of fish varieties. Beyond their ecological significance, reefs deliver substantial economic benefits, generating billions of dollars annually through fisheries, tourism, and coastal protection. Human activities, both global and local, now impose severe pressures on these delicate systems, threatening their existence and the livelihoods that depend on them.
The Impact of Rising Ocean Temperatures and pH Changes
One of the most significant human impacts stems from the global rise in atmospheric carbon dioxide (CO2), which directly affects ocean chemistry and temperature. Elevated water temperatures are the primary cause of mass coral bleaching events, where coral polyps expel the symbiotic algae, known as zooxanthellae, living within their tissues. These algae provide the coral with up to 90% of its energy through photosynthesis, giving the coral its color and primary food source. When the coral expels the zooxanthellae under thermal stress, the tissue becomes transparent, revealing the white calcium carbonate skeleton beneath.
A bleached coral is not dead, but it is under immense stress and loses its main source of nutrition, making it highly vulnerable to starvation and disease. If high-temperature conditions persist, the coral will starve and die, leaving the reef structure to erode. A temperature increase of only about 1 degree Celsius above the typical summer maximum can trigger a bleaching event. This global thermal stress is compounded by the second major atmospheric impact: ocean acidification.
Ocean acidification begins when the ocean absorbs excess CO2 from the atmosphere, which then reacts with seawater to form carbonic acid. This acid releases hydrogen ions, which bond with carbonate ions naturally present in the water. Carbonate ions are essential building blocks for the calcium carbonate skeletons of reef-building corals. By binding up these carbonate ions, acidification reduces their availability, making it harder for corals to perform calcification.
This chemical change leads to slower growth rates and reduced skeletal density in corals. Acidification specifically impedes the process of thickening and reinforcing the skeleton, making the corals more brittle and susceptible to damage from storms or bioerosion. Severe acidification can even cause existing coral skeletons to dissolve, compromising the structural integrity of the reef over time. The combined effect of thermal stress and reduced skeletal building capacity makes reefs less resilient to local stressors.
Degradation Due to Localized Runoff and Nutrient Overload
In addition to global atmospheric changes, human activities on land create significant local threats to water quality. The clearing of coastal land for development, deforestation, and agriculture dramatically increases the volume of fine sediment washing into the ocean during rain events. This runoff increases water turbidity, reducing the amount of sunlight penetrating to the coral surface. Since coral relies on light for its zooxanthellae to photosynthesize, this reduced light availability starves the coral and is a direct precursor to localized bleaching.
When suspended sediment settles, it physically smothers the coral polyps, hindering their ability to feed and respire. Branching corals, with their complex structures, are particularly vulnerable to this physical accumulation. Coastal runoff also carries a heavy load of nutrients, primarily nitrogen and phosphorus, originating from sewage discharge, septic systems, and agricultural fertilizers. This influx of nutrients causes eutrophication, which disrupts the naturally nutrient-poor environment where coral reefs thrive.
The excess nutrients fuel the rapid growth of macroalgae, or seaweed, which can quickly outcompete and smother corals for space on the reef. This shift from a coral-dominant to an algae-dominant ecosystem fundamentally alters the entire reef food web. Nutrient pollution also makes corals more susceptible to disease outbreaks.
Direct Physical Destruction and Overexploitation
Some of the most immediate human impacts on reefs come from physical destruction and the removal of key organisms. Destructive fishing practices cause irreparable damage to the reef structure itself. Blast fishing, or “dynamite fishing,” uses explosives to kill fish by shockwave, which also shatters the calcium carbonate skeleton of the coral, reducing large sections of reef to rubble.
Another highly damaging method is cyanide fishing, where fishermen spray a sodium cyanide solution onto the reef to stun and capture live fish. When stunned fish retreat into crevices, fishermen often use hammers and crowbars to break the coral apart to retrieve them, causing extensive physical damage. Direct physical harm also occurs through coastal development, such as dredging and quarrying, and from the misuse of reef areas.
Recreational activities also contribute, with boat anchors dragging across and breaking coral colonies, and careless divers or tourists touching the fragile structures. The biological balance of the reef is further destroyed by overexploitation, which is catching marine animals faster than they can reproduce. The removal of herbivorous fish, such as parrotfish and surgeonfish, is particularly damaging because these species graze on algae. When these herbivores are removed, the algae they consume grow unchecked, accelerating the smothering process.