Human Pressures
This destructive activity removes coral or reef limestone for construction, decoration or trade.
Coral mining
This occurs when stressed coral loses its symbiotic algae or their photosynthetic pigments.
Coral Bleaching
Scientists extract these cylindrical samples to study a coral’s historical growth.
Coral Cores
This chemical system helps seawater resist sudden changes in pH.
Carbonate Buffering System
Name the two main forms in which calcium carbonate is precipitated by marine organisms.
Calcite and Aragonite
This land-based pressure carries sediment, nutrients and pollutants into coastal reef waters after rainfall.
Surface run-off
Use Shelford’s law of tolerance to explain why unusually warm water may cause bleaching.
Coral has a limited temperature tolerance range. When water temperature moves beyond its optimum and approaches or exceeds its upper tolerance limit, physiological stress causes bleaching.
Explain how coral cores can provide a record of previous environmental conditions.
Coral cores contain growth and density bands and chemical signatures that reflect conditions such as temperature, rainfall, water quality and coral growth rates.
What compound forms when atmospheric carbon dioxide dissolves and reacts with seawater?
Carbonic acid, H2CO3H_2CO_3.
Name three groups of marine organisms whose shells or skeletons may be affected by reduced carbonate availability.
Any three of hard corals, coralline algae, molluscs, plankton and crustaceans.
Explain how overfishing herbivorous fish can reduce coral recovery.
Fewer herbivores allow algae to proliferate. The algae compete with coral recruits for light and space, reducing coral settlement and recovery.
A reef remains 2°C above its bleaching threshold for four weeks. Calculate the accumulated thermal stress.
8 degree heating weeks, calculated as
2°C × 4 weeks
A coral core shows progressively narrower annual growth bands during a period of increasing sediment run-off. What does the evidence suggest?
Coral growth declined as sediment run-off increased, suggesting that deteriorating water quality placed the coral under stress. The pattern shows an association but does not alone prove causation.
Explain why increasing dissolved carbon dioxide lowers ocean pH and reduces carbonate-ion availability.
Carbon dioxide forms carbonic acid, which releases hydrogen ions. These hydrogen ions lower pH and bond with carbonate ions to form bicarbonate, leaving fewer carbonate ions available.
A graph shows that shell thickness decreases as ocean pH falls. Explain this relationship using the availability of carbonate ions.
Lower pH means more hydrogen ions are present. These hydrogen ions bond with carbonate ions, leaving fewer carbonate ions available for organisms to build and maintain calcium carbonate shells.
Identify two ways shipping can damage coral reefs.
Shipping may cause oil or chemical spills, introduce organisms through improper ballast-water discharge, physically damage reefs through groundings, or require destructive dredging.
Explain two ways a severe bleaching event may affect reef fish populations.
The loss of live coral reduces food and shelter. If dead coral structures erode, habitat complexity also declines, reducing the abundance and diversity of coral-dependent fish.
Compare the likely reef futures under RCP2.6 and RCP8.5.
RCP2.6 projects less warming and a lower frequency of severe bleaching, giving reefs more opportunity to recover. RCP8.5 projects greater warming, more frequent bleaching and less time for recovery.
Explain why the carbonate compensation depth may be shallower in a region affected by cold, carbon-dioxide-rich upwelling.
Cold, carbon-dioxide-rich water has a lower pH and fewer carbonate ions. Calcium carbonate therefore dissolves at a shallower depth, raising the carbonate compensation depth.
A laboratory experiment finds that shell mass decreases as pH falls. Why should scientists also conduct field-based investigations before predicting ecosystem-wide effects?
Laboratory experiments provide strong control but may not reproduce natural food webs, adaptation, water movement or multiple interacting stressors. Field studies provide greater ecological realism but less control.
After heavy rainfall, a reef experiences low salinity, high turbidity and elevated nitrogen levels. Analyse how these changes could affect the coral.
Low salinity causes physiological stress, while sediment reduces light and may smother coral. Added nitrogen can encourage algal growth, increasing competition and compounding the stress on coral.
Two reefs both decline from 45% to 15% coral cover after bleaching. Five years later, Reef A has recovered to 38%, while Reef B remains at 18%. Run-off and fishing were reduced at Reef A but continued at Reef B. What conclusion can be drawn?
Reef A is more resilient. Reducing local pressures improved coral survival, recruitment and recovery, while continuing pressures restricted recovery at Reef B.
Why are global climate predictions generally more reliable than predictions for an individual reef?
Global trends emerge from large-scale patterns and multiple models. Conditions at an individual reef are also influenced by local currents, depth, water quality, weather and ecosystem resilience, creating greater regional uncertainty.
Compare the capacity of a cold, highly productive ocean region and a warm, low-productivity region to absorb and store carbon dioxide.
The cold region can dissolve more carbon dioxide, while high productivity transfers carbon to deeper water through the biological pump. Warm water absorbs less carbon dioxide, and low productivity provides less biological storage.
Explain how reef management can improve resilience to ocean acidification, even though it cannot directly stop the global decline in ocean pH.
Reducing nutrient run-off, habitat destruction and overfishing removes additional pressures. Healthier coral populations, strong herbivory and better water quality improve growth and recovery, partially offsetting acidification effects in the short term.