AP Syllabus focus:
‘Coral reefs are damaged by multiple factors, including increasing ocean temperature, sediment runoff, and destructive fishing practices.’
Coral reefs are highly diverse coastal ecosystems built by tiny animals called corals. Because reef organisms have narrow tolerance ranges, multiple human-driven stressors can quickly reduce reef growth, biodiversity, and ecosystem services.
Coral reefs and why they are vulnerable
Reef-building corals are colonial animals that deposit calcium carbonate skeletons and host symbiotic algae (zooxanthellae) that provide much of the coral’s energy via photosynthesis. Reefs thrive in clear, warm (but stable), shallow, sunlit waters, so changes to temperature, water clarity, and physical structure are especially damaging.
Key idea: multiple stressors interact
Reef damage often reflects combined impacts rather than one single cause. For example, heat stress can weaken corals, making them less able to recover from sediment burial or physical breakage.
Driver 1: Increasing ocean temperature
Rising sea surface temperatures increase the frequency and severity of heat stress events. Even short periods above a coral’s optimum range can disrupt the coral–algae partnership.
Coral bleaching: loss or expulsion of symbiotic algae (and/or their pigments) from coral tissue, causing the coral to turn pale and reducing its energy supply.
Bleaching does not always kill coral immediately, but it reduces growth and reproduction and increases susceptibility to disease and mortality if stressful temperatures persist.
How heat damages reefs (mechanism)
Thermal stress impairs algal photosynthesis and increases harmful byproducts (oxidative stress).
Corals expel or lose algae, leading to energy shortages.
Reduced energy means less calcification, slower reef building, and lower resilience.
Repeated bleaching events can shift reefs toward algae-dominated states as corals fail to recover.
Driver 2: Sediment runoff
Sediment runoff from land (often intensified by deforestation, agriculture, construction, and poor erosion control) increases turbidity and deposits particles on reefs.
This NASA Earth Observatory image shows sediment-rich river plumes spreading from land into coastal waters and across reef areas. The discoloration highlights how runoff can increase turbidity, reducing light penetration and stressing corals by limiting photosynthesis and increasing particle deposition. Source
Corals are adapted to clear water; sediment disrupts both light availability and the physical condition of coral surfaces.
Major impacts of sediment on corals
Reduced light penetration lowers photosynthesis by symbiotic algae, decreasing energy available for growth.
Smothering/burial of coral polyps can cause tissue death if sediments are not removed.
Corals expend energy producing mucus and actively clearing sediments, leaving less energy for growth and reproduction.
Sediments can carry attached pollutants and nutrients, compounding stress even when the sediment itself is not toxic.
Why sediment problems can persist
Fine sediments can remain suspended, keeping water cloudy long after storms or runoff events.
Deposited sediments can be resuspended by waves, creating chronic turbidity and repeated exposure.
Driver 3: Destructive fishing practices
Certain fishing methods directly break reef structures or alter reef food webs. Damage can be immediate (physical destruction) and long-term (loss of key species that maintain reef balance).
Examples of destructive practices and effects
Blast fishing (explosives):
Shatters coral skeletons and flattens complex reef habitat.
Kills non-target species and reduces shelter for juvenile fish.
Bottom-contact gear (e.g., trawls in reef areas, heavy nets/anchors):
Breaks corals, abrades surfaces, and reduces three-dimensional structure.
Cyanide fishing (to capture live fish):
Stuns fish but can poison corals and other reef organisms, increasing mortality.
Food-web disruption (even without direct breakage)
Removing herbivorous fish can allow fast-growing algae to overgrow corals.
Loss of reef structure reduces habitat, lowering biodiversity and fishery productivity.
Recognising reef damage in the environment (what to look for)
Increased frequency of bleached (pale/white) coral colonies after warm-water events
Cloudy water and visible sediment plumes near river mouths or construction zones
Large areas of rubble or broken coral framework in regions using destructive gear
Shifts from coral cover to macroalgae dominance after repeated disturbances
FAQ
Some recover in weeks to months if temperatures return to normal quickly.
Repeated or prolonged heat stress can cause starvation, disease, and mortality over months.
Hillslope agriculture with poor soil cover
Road building and coastal construction
Logging/land clearing that removes stabilising vegetation
Complex reefs create shelter and feeding niches.
When reefs flatten into rubble, juvenile survival drops and species richness often declines.
Maintain vegetated buffer strips and wetlands
Use silt fences and retention basins at construction sites
Time earthworks outside heavy-rain seasons
Limited enforcement capacity and economic pressure can sustain harmful practices.
Community-based monitoring and alternative livelihoods can improve compliance.
Practice Questions
State two drivers of coral reef damage. (2 marks)
Increasing ocean temperature (1)
Sediment runoff (1)
Destructive fishing practices (1) (credit any two)
Explain how increasing ocean temperature and sediment runoff can each damage coral reefs. (6 marks)
Higher temperatures cause heat stress to corals (1)
Heat stress leads to coral bleaching (loss/expulsion of symbiotic algae) (1)
Bleaching reduces energy supply/calcification, lowering growth or survival (1)
Sediment runoff increases turbidity, reducing light for photosynthesis (1)
Sediment can smother/bury corals or force energy use for sediment clearing (1)
Reduced growth/reproduction/resilience increases likelihood of mortality or reef decline (1)
