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Unlock This CourseCoral reefs are among the most diverse and productive ecosystems on Earth, but they're also incredibly sensitive to changes in their environment. These underwater rainforests depend on very specific conditions to survive and thrive. Understanding these environmental factors is crucial for protecting coral reefs from threats like climate change and pollution.
Coral reefs are built by tiny animals called coral polyps that live in partnership with microscopic algae called zooxanthellae. This relationship is so delicate that even small changes in environmental conditions can disrupt it, leading to coral bleaching and death.
Key Definitions:
Coral reefs need warm water to survive, typically between 23-29°C. Even a 1-2°C increase above normal can cause mass coral bleaching events. This narrow temperature range explains why coral reefs are found mainly in tropical waters near the equator.
Several key environmental factors determine where coral reefs can grow and how healthy they remain. Each factor plays a vital role in maintaining the delicate balance that allows coral ecosystems to flourish.
Temperature is perhaps the most critical factor for coral reef survival. Corals have evolved to live within a very narrow temperature range and their symbiotic relationship with zooxanthellae is extremely temperature-sensitive.
23-29°C is the ideal temperature range. Within this range, corals can photosynthesize efficiently and build their calcium carbonate skeletons at maximum rates.
Temperatures above 30°C for extended periods cause coral bleaching. The zooxanthellae are expelled and corals lose their main food source and colour.
Temperatures below 20°C slow coral growth dramatically and can cause tissue damage. This limits coral reefs to tropical and subtropical regions.
Light is essential for coral reefs because the zooxanthellae inside coral polyps need sunlight for photosynthesis. This requirement limits coral growth to shallow, clear waters where sunlight can penetrate effectively.
Most reef-building corals are found in waters less than 50 metres deep, with the healthiest reefs typically in waters 10-25 metres deep. Factors that reduce light penetration, such as sediment runoff or algal blooms, can severely damage coral reefs.
The Great Barrier Reef experienced massive bleaching events in 2016, 2017 and 2020 due to rising sea temperatures. During the 2016 event, water temperatures were 1-2°C above average for several months, causing 29% of shallow-water corals to die. This demonstrates how sensitive corals are to temperature changes and highlights the threat of global warming to coral reef ecosystems worldwide.
The chemical composition of seawater directly affects coral health and growth. Two particularly important factors are salinity and pH levels.
Corals require stable salinity levels around 35 parts per thousand (ppt). Freshwater runoff from rivers can reduce salinity and stress corals, whilst areas with high evaporation can become too salty.
Ocean Acidification: As the ocean absorbs more carbon dioxide from the atmosphere, it becomes more acidic. This process, called ocean acidification, makes it harder for corals to build their calcium carbonate skeletons. The pH of seawater has already dropped from 8.2 to 8.1 since the Industrial Revolution and further decreases could severely impact coral reef growth.
Water movement is crucial for coral reef health as it brings nutrients, removes waste products and prevents sediment from settling on corals. However, too much wave action can physically damage coral structures.
Moderate water flow brings plankton and nutrients to corals whilst removing waste products and preventing sediment buildup on coral surfaces.
Severe storms and cyclones can physically break apart coral structures, though healthy reefs can recover from occasional storm damage over time.
Most reef-building corals grow in waters less than 50m deep due to light requirements, though some species can survive deeper in very clear water.
Human activities are increasingly affecting the environmental conditions that coral reefs depend on. Understanding these impacts is essential for developing effective conservation strategies.
Global warming is the greatest threat to coral reefs worldwide. Rising sea temperatures cause more frequent and severe bleaching events, whilst changing weather patterns affect rainfall and storm intensity.
Sea level rise, another consequence of climate change, can affect shallow coral reefs by changing light levels and water circulation patterns. However, healthy corals can often grow upwards to keep pace with slowly rising sea levels.
The Caribbean has lost over 80% of its coral cover since the 1970s due to multiple environmental stressors. Rising temperatures, hurricanes, disease outbreaks and pollution have combined to devastate once-thriving reef systems. Jamaica's reefs, for example, declined from 52% coral cover in the 1970s to less than 5% today, showing how quickly environmental changes can destroy coral ecosystems.
Land-based pollution significantly affects coral reef environmental conditions. Agricultural runoff containing fertilisers causes eutrophication, leading to algal blooms that block sunlight and reduce water quality.
Excess nutrients from fertilisers and sewage promote algae growth, which competes with corals for space and light. This can shift reef ecosystems from coral-dominated to algae-dominated states.
Sediment from coastal development and deforestation smothers corals and reduces light penetration. Corals cannot survive when buried under sediment and even small amounts can stress them by requiring energy to remove particles from their surfaces.
Protecting coral reefs requires managing both local and global environmental factors. Local management can address pollution, overfishing and coastal development, whilst global action is needed to tackle climate change.
Marine protected areas (MPAs) help maintain optimal environmental conditions by restricting harmful activities. Successful coral reef conservation also requires monitoring water quality, controlling coastal development and reducing greenhouse gas emissions to limit global warming.
Following severe bleaching in 1998 that killed 90% of corals in some areas, the Maldives has implemented successful restoration programmes. By maintaining good water quality, controlling tourism impacts and allowing natural recovery in protected areas, many reefs have shown remarkable resilience. Some areas now have coral cover exceeding pre-bleaching levels, demonstrating that coral reefs can recover when environmental conditions are properly managed.
Scientists use various methods to monitor coral reef environmental conditions, including satellite temperature monitoring, underwater sensors and regular diving surveys. This data helps predict bleaching events and assess reef health over time.
The future of coral reefs depends on our ability to maintain suitable environmental conditions. This requires both immediate local action to reduce pollution and long-term global efforts to limit climate change. Without urgent action, many coral reef ecosystems may not survive the environmental changes projected for this century.