Coral Reef Structure & Zonation
What is the shallow, protected area on a reef system that experiences low wave energy?
Lagoon
What is the name of the symbiotic algae that live inside coral tissue?
Zooxanthellae (genus Symbiodinium).
What is it called when corals turn white after losing their zooxanthellae?
Coral bleaching.
Which species of starfish is known for coral predation and causing major reef declines?
The Crown-of-Thorns Starfish (COTS).
What is the term for a shift from coral-dominated to algae-dominated reefs?
A phase shift.
Which reef zone absorbs the most wave energy and contains tough, robust corals?
The Crest
What do corals gain from their zooxanthellae?
Glucose, amino acids, and oxygen through photosynthesis.
What is the “bleaching threshold”?
The temperature at which corals begin to experience stress leading to bleaching.
How does nutrient runoff affect COTS outbreaks?
It increases phytoplankton, boosting food for COTS larvae, causing population booms.
Name one natural disturbance and one human-induced disturbance that reduce coral cover.
Natural: Cyclones
Human-induced: Nutrient runoff, overfishing, or climate change-driven heatwaves.
How does the reef slope differ from the reef crest in light and wave exposure?
The reef slope has more light variation and lower wave energy, while the crest gets intense wave action and full light.
Why are zooxanthellae essential for coral calcification?
They increase pH around the coral, helping coral deposit calcium carbonate (CaCO₃) more efficiently.
Why do extended periods of high SST reduce coral resilience?
They deplete coral energy reserves, slow growth, and reduce recovery ability.
How does reduced herbivory increase macroalgae and limit coral recovery?
Without grazers, macroalgae overgrow corals and prevent settling of coral larvae.
Why is coral recruitment essential for reef recovery after disturbances?
Recruitment brings new juvenile corals, allowing damaged areas to regrow.
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Why do coral growth forms change with depth?
Because light intensity, sedimentation, and water movement change with depth, selecting for different coral shapes.
What happens inside coral tissues during thermal stress that leads to bleaching?
Zooxanthellae become stressed, produce reactive oxygen, and are expelled, removing the coral’s colour and energy source.
How do sustained SST anomalies disrupt coral metabolic processes?
They damage coral enzymes, reduce photosynthesis, and impair cellular function.
How does rising global SST influence disturbance frequency on coral reefs?
It increases heatwaves, bleaching events, and strengthens cyclones, intensifying damage.
How does increasing SST shape long-term coral cover on the GBR?
More SST anomalies = more bleaching, slower growth, and long-term coral decline.
How does reef zonation reflect long-term equilibrium between abiotic factors and coral adaptations?
Each coral zone is shaped by stable combinations of light, wave energy, and sedimentation, with corals evolving growth forms and physiological tolerances suited for that zone.
Why is the coral–algal symbiosis an example of mutualism that becomes unstable under extreme conditions?
Both benefit normally, but heat stress causes the algae to harm the coral via toxic byproducts, turning the mutualism into a liability.
Why do repeated bleaching events reduce long-term reef recovery?
Corals don’t have time to regrow tissues, rebuild zooxanthellae populations, or restore energy reserves, leading to chronic decline.
How do multiple stressors create a feedback loop that leads to phase shifts?
Stressors weaken coral, allowing algae dominance, which further blocks coral recruitment, locking the reef in a degraded state.
How does long-term monitoring data help predict future reef trajectories?
It reveals patterns, trends, and links between temperature, bleaching, COTS, and coral cover, allowing prediction of whether reefs recover or continue declining.