Coral reefs are diverse and productive ecosystems, providing habitat and sustenance for a quarter of all marine species. Since 1977, a global decline in coral health and coverage has been observed. This widespread degradation is a complex phenomenon driven by interconnected environmental changes and human pressures.
Understanding the Historical Context of Coral Health
The period around 1977 marks a turning point in the scientific community’s awareness of widespread coral reef degradation. While reefs faced localized pressures for centuries, the late 1970s and early 1980s saw broader declines and new threats emerge. Early reports of coral disease date back to 1965, with more diverse types appearing in Caribbean waters. By the early 1990s, research institutions recorded coral disease on major reef systems like the Great Barrier Reef.
This era coincided with a growing understanding that human activities were exerting global pressures on marine ecosystems. Scientists documented early signs of stress, such as mass bleaching events and changes in ocean chemistry. The increasing frequency and severity of these events showed an accelerating decline, resulting from compounding pressures that intensified over decades.
Overarching Global Environmental Changes
Global environmental changes, particularly increasing atmospheric carbon dioxide, are a main driver of coral decline. Rising sea temperatures, a direct consequence of global warming, cause coral bleaching. Corals host microscopic algae called zooxanthellae, which provide energy through photosynthesis and give them color. When water temperatures rise by one to two degrees Celsius above average for weeks, corals expel these symbiotic algae. This expulsion makes coral tissue transparent, revealing the white skeleton underneath, known as coral bleaching.
A bleached coral is not immediately dead, but it is weakened and more susceptible to starvation, disease, and mortality if stress persists. Mass coral bleaching events have become more frequent and intense globally, with notable events like the 2005 episode that caused half of the U.S. Caribbean reefs to be lost in a single year.
The ocean’s absorption of atmospheric carbon dioxide also leads to ocean acidification, a change in seawater chemistry that directly impacts corals. As the ocean absorbs more CO2, its pH decreases, making it more acidic. This shift reduces the availability of carbonate ions, essential for corals to build and maintain their calcium carbonate skeletons through calcification. Ocean acidification impedes the thickening of coral skeletons, resulting in less dense and more fragile structures. This makes corals more vulnerable to physical damage and erosion.
Studies show a measurable decrease in coral growth rates, with one report indicating a 14% reduction in calcification for the Great Barrier Reef since 1990. These two global stressors, warming and acidification, weaken corals physiologically.
Direct Human and Local Environmental Stresses
Localized human activities exert direct pressures on coral reefs. Land-based pollution, from coastal development, agriculture, and urban areas, carries sediment, excess nutrients, and chemical contaminants into reef environments. Sedimentation can smother corals, interfere with feeding and reproduction, and reduce light for symbiotic algae. Nutrient pollution from sewage and agricultural fertilizers fuels algal overgrowth, which outcompetes and suffocates corals.
Toxic substances, including metals, pesticides, and chemicals from industrial discharges and sunscreens, impair coral reproduction, growth, and physiological processes. Plastic debris also poses a physical threat, snagging on corals, blocking sunlight, or entangling reef organisms. These pollutants disrupt the balance required for coral health.
Overfishing and destructive fishing practices compromise reef ecosystems. Overfishing removes key species, particularly herbivorous fish like parrotfish and surgeonfish, which graze on algae and prevent overgrowth. Their absence can lead to unchecked algal proliferation, smothering the reef.
Destructive methods, such as dynamite and cyanide fishing, cause immediate and widespread damage. Dynamite fishing uses explosives that destroy large reef sections, creating rubble fields that take hundreds of years to recover. Cyanide fishing, used to stun fish for trade, directly poisons corals and other marine life. These practices decimate fish populations and physically shatter coral structures that provide habitat.
Physical damage from human activities, linked to coastal development and tourism, also contributes to coral decline. Boat anchors and chains can crush and dislodge corals, causing breakage and fragmentation. Boat groundings can also destroy large reef areas. Coastal development often leads to increased sedimentation and altered water flow, stressing nearby reefs.
Unsustainable tourism practices, such as direct contact by divers and snorkelers or non-reef-safe sunscreens, harm coral tissues and introduce chemicals. These physical impacts accumulate to weaken reef structure and health.
Coral diseases are also a concern, often exacerbated by other stressors. While some disease is natural, outbreaks are more frequent and severe when corals are stressed. Elevated water temperatures, nutrient pollution, and bleaching stress weaken corals’ immune responses, making them susceptible to infections. The prevalence of diseases, like white syndrome or black band disease, contributes to coral mortality and accelerates reef degradation.
The Interplay of Factors Driving Decline
The decline of coral reefs since 1977 is not due to a single cause but a complex web of interacting stressors, where the combined impact often exceeds individual pressures. Global changes, such as ocean warming and acidification, weaken corals, making them vulnerable to localized threats. For instance, heat stress from rising temperatures can induce coral bleaching, leaving corals compromised and susceptible to diseases.
Ocean acidification, by reducing skeletal density, makes corals more fragile and less able to withstand physical damage from storms or human activities like anchoring. Local stressors, in turn, can intensify global changes; poor water quality from pollution can worsen bleaching severity and reduce recovery. The removal of herbivorous fish through overfishing can lead to algal overgrowth, further stressing corals already struggling with warmer, more acidic waters.
This synergistic relationship means multiple stressors together create a greater negative impact than if each occurred in isolation. While some interactions may be additive or antagonistic, the overall trend shows a compounding effect that overwhelms corals’ natural resilience. Understanding this interplay is key to comprehending the decline of coral reefs globally.