Key Terms: Marine Ecosystems (AQA A Level Geography): Revision Note

Coral reef ecosystems - key terms

Algae (zooxanthellae) – Microscopic algae that live in symbiosis with coral polyps, providing them with nutrients via photosynthesis.

Atoll – A circular or horseshoe-shaped coral reef that surrounds a central lagoon, typically formed from a submerged volcanic island.

Barrier reef – A type of coral reef that runs parallel to the shore but is separated from it by a lagoon.

Calcium carbonate – A substance secreted by coral polyps to form a hard protective skeleton; the main material of coral reef structures.

Canopy (reef) – The top layer of living coral and other organisms that form the surface of a reef ecosystem.

Carbon store – Coral reefs act as carbon sinks by storing carbon in the form of calcium carbonate skeletons.

Cold water coral reef – Found in deeper, nutrient-rich waters (4°C–12°C), typically in the North Atlantic, and not reliant on sunlight.

Coral bleaching – The loss of zooxanthellae from coral tissue due to stress (usually from temperature rise), turning corals white and potentially killing them.

Coral larvae – Free-swimming early life stage of corals that attach to hard surfaces to begin reef formation.

Coral polyp – A small, soft-bodied organism related to jellyfish; lives in colonies and secretes calcium carbonate to form reefs.

Emergent reef – The uppermost part of a coral reef, which is sometimes exposed during low tide.

Fringing reef – A coral reef that forms directly along a coastline with no lagoon separating it from land.

Net Primary Productivity (NPP) – High in coral reefs due to efficient nutrient cycling and high rates of photosynthesis.

Nutrient cycling – A closed system in which coral polyps and zooxanthellae recycle nitrogen and phosphorus efficiently in nutrient-poor waters.

Soft coral – Flexible, plant-like corals with wood-like cores and fleshy outer layers; they do not contribute to reef building.

Temperature (requirement) – Optimal growth occurs between 23°C and 29°C, though some species can tolerate up to 40°C briefly.

Warm water coral reef – Coral ecosystems found in shallow tropical seas with warm temperatures and strong light availability.

Threats to coral reef ecosystems - key terms

Acidification – The lowering of ocean pH due to CO₂ absorption, which reduces coral calcification and weakens reef structures.

Anthropogenic threats – Human-induced dangers to reefs, including pollution, overfishing, and climate change.

Blast fishing – A destructive practice using explosives to catch fish, which destroys entire sections of coral reef.

Climate change – Leads to warmer seas, rising sea levels, and stronger storms, all of which contribute to reef degradation.

Coral mining – The extraction of coral for construction or decoration, which physically removes reef structures.

Crown-of-thorns starfish – A coral predator whose population outbreaks can devastate large sections of reef.

Desalination – The discharge of salty brine and chemicals from freshwater plants into the sea, altering salinity and harming reefs.

Dredging – The removal of seabed material for shipping or construction, increasing sediment in the water and damaging coral.

Eutrophication – The enrichment of water with nutrients (e.g. from fertilisers), promoting algal blooms that block light and suffocate coral.

Freshwater input – Excessive freshwater from storms, runoff, or river discharge dilutes seawater and harms coral health.

Sedimentation – Suspended particles from land use (e.g. deforestation) reduce water clarity and block sunlight, stunting coral growth.

Thermal stress – Caused by abnormally warm water, leading to coral bleaching and mortality.

Tidal change – Extended high or low tides can expose corals to light and air or reduce light availability underwater, causing bleaching.