Coral Reef Distribution
What type of reef develops directly beside a coastline?
A fringing reef.
What specialised structures allow coral polyps to capture and immobilise prey?
Nematocysts
What is the free-swimming larval stage of a coral called?
A planula.
Why are corals described as habitat formers?
Their three-dimensional skeletons create shelter, feeding areas and breeding sites for many other marine organisms.
What term describes the number of different species present in a community?
Species richness.
Identify the three main zones commonly found across a coral reef cross-section.
Reef slope, reef crest or rim, and lagoon or back reef.
Describe the role of zooxanthellae within a reef-forming coral colony.
Zooxanthellae photosynthesise and provide the coral with energy-rich organic compounds. In return, the coral provides shelter, carbon dioxide and nutrients.
Identify two methods of asexual reproduction used by corals.
Fragmentation and polyp detachment. Asexual budding also expands an established colony.
Explain how herbivorous fish benefit coral reefs.
Herbivorous fish consume algae that would otherwise compete with corals for light and space. This keeps suitable substrate available for coral settlement and recruitment.
Distinguish between species richness and species abundance.
Species richness is the number of different species present. Species abundance is the number of individuals belonging to each species or within the community.
Explain why reef-building corals are generally restricted to warm, shallow and well-lit waters.
Reef-building corals contain photosynthetic zooxanthellae, which require sufficient light. Warm, shallow water supports photosynthesis and provides suitable conditions for coral growth and calcium carbonate deposition.
Explain how calcium and carbonate ions are used to construct a coral’s limestone skeleton.
Coral polyps combine calcium ions and carbonate ions to produce calcium carbonate:
Ca2++ CO32−→ CaCO3
The calcium carbonate is deposited as aragonite, forming the coral’s hard limestone skeleton.
Describe the stages of sexual reproduction from the release of coral gametes to the formation of a new colony.
Adult corals release eggs and sperm. Fertilisation produces a zygote, which develops into a planula larva. The planula disperses, selects a suitable substrate, settles and develops into a coral polyp. The polyp then reproduces through asexual budding to form a colony.
Explain how fish may use mangroves, seagrass meadows and coral reefs during different stages of their life cycle.
Juvenile fish may use mangroves and seagrass meadows as sheltered nursery and feeding habitats. As they mature, they may move to coral reefs to feed, reproduce or establish adult territories.
A reef has high species richness but is dominated by one species. Predict whether its Simpson’s Diversity Index would be relatively high or low and justify your answer.
It would be lower than a reef with similar richness and greater evenness. Simpson’s Diversity Index considers both the number of species and how evenly individuals are distributed among those species.
Explain how changes in sea level over the past 20,000 years have shaped the modern Great Barrier Reef.
Sea level was approximately 125 metres lower during the last glacial period. As sea level rose, coastal plains and continental shelves were flooded, allowing corals to colonise newly submerged areas. Sea level stabilised approximately 6,500 years ago, supporting the development of the modern Great Barrier Reef.
Discriminate between soft corals, reef-forming hard corals and non-reef-forming hard corals.
Soft corals belong to Alcyonacea and do not produce a large, rigid calcium carbonate skeleton. Reef-forming hard corals are hermatypic corals that contain zooxanthellae and contribute substantially to reef construction. Non-reef-forming hard corals are ahermatypic and do not make a major contribution to reef development.
Explain how currents, substrate type, sediment and competition can affect coral larval settlement and recruitment.
Currents influence where larvae are transported. A stable, hard substrate provides a suitable settlement surface, while soft or unstable substrates reduce settlement success. Sediment can bury larvae or prevent attachment, and competition from algae or other organisms can reduce the space and resources available to newly settled corals.
Explain how connectivity between estuaries, mangroves, seagrass meadows and coral reefs supports species replenishment.
Water movement and animal migration connect these habitats. Larvae, juveniles, nutrients and organic matter move between them, allowing populations to be replenished and organisms to access the different habitats required during their life cycles.
Explain how declining water quality could indirectly reduce coral cover by contributing to an increase in crown-of-thorns starfish.
Increased nutrients can stimulate phytoplankton growth, providing more food for crown-of-thorns starfish larvae and increasing their survival. Larger adult populations may then consume more living coral, reducing coral cover.
Two reef sites have similar temperatures and light levels. Site A has clear, saline water and a hard substrate. Site B has high nutrient levels, heavy sediment and a soft substrate. Analyse which site is more suitable for coral reef development.
Site A is more suitable because its clear water allows light to reach zooxanthellae, its salinity is suitable for marine corals, and its hard substrate provides a stable surface for larval settlement. Sediment at Site B could smother coral polyps and block light, while high nutrient levels could encourage algal growth and reduce coral recruitment.
Predict how a sustained decrease in seawater pH would affect coral skeleton formation and reef growth. Justify your prediction using coral chemistry.
Coral calcification and reef growth would decrease. Lower pH reduces the availability of carbonate ions and lowers aragonite saturation, making it more difficult for corals to combine calcium and carbonate ions to form calcium carbonate skeletons. Existing reef structures may also become more vulnerable to erosion and dissolution.
A coral reef receives large numbers of planula larvae but shows very little recruitment. Analyse three environmental or ecological factors that could explain this result.
Predict the consequences for an offshore coral reef if nearby mangrove and seagrass habitats were extensively removed. Explain the ecological connections involved.
Fish recruitment on the reef could decline because many juvenile fish depend on mangroves and seagrass meadows as nursery habitats. Reduced herbivorous fish populations could allow algae to increase and compete with corals. Sediment and nutrients reaching the reef could also increase because mangroves and seagrasses normally trap sediment and absorb nutrients.
Coral cover on a reef decreases from 55% to 18%, while macroalgal cover increases from 12% to 64%. Explain how an ecological tipping point and hysteresis could prevent the reef from returning quickly to its original state.
The reef may have crossed a tipping point from a coral-dominated state to an algal-dominated state. Established macroalgae can occupy settlement surfaces, compete with corals and prevent coral recruitment. Hysteresis means that simply removing the original disturbance may not restore the reef because the new algal-dominated state is maintained by ecological feedback mechanisms.