Marine Ecosystems: Coral Reefs (AQA A-Level Geography): Revision Notes

Marine Ecosystems: Coral Reefs

Introduction to marine ecosystems

Marine ecosystems represent the planet's most extensive ecosystem type, with oceans and seas covering approximately 70% of Earth's surface. This vast aquatic environment encompasses numerous smaller ecosystems, including:

• Rocky shores and coastlines
• Salt marshes and estuaries
• Submarine canyons
• Hydrothermal vents
• Deep ocean basins
• Seamounts
• Coral reefs

What are coral reefs?

Coral reefs are underwater structures made of calcium carbonate (limestone) that is extracted from seawater by colonies of coral polyps and coralline algae.

The visible reef structure consists of living coral polyps building upon foundations of dead coral skeletons. Over time, successive generations create layers of limestone, gradually forming massive reef structures. For example, Australia's Great Barrier Reef stretches 2,000 km and took approximately 5 million years to develop.

Types of coral reefs

Corals can be divided into two broad categories based on their environmental requirements and locations.

Deep-sea cold-water coral reefs

These reefs represent a relatively recent scientific discovery, found primarily around Europe's margins. Key characteristics include:

Location and environment:

• Situated in deeper, nutrient-rich waters at high latitudes
• Found where strong ocean currents accelerate water movement and prevent sediment accumulation
• Located along northwest European continental margins, within Scandinavian fjords, and in some Mediterranean areas

Species and growth:

• Dominated by colonial stony corals such as Lophelia pertusa and Madrepora oculata
• Grow very slowly at rates of 5-25 mm per year
• Despite slow growth, they form extensive reef structures over time
• The largest discovered cold-water reef sits off Norway's Røst Island, measuring 40 km long and 2-3 km wide

Tropical coral reefs

Tropical reefs are the most well-known and biodiverse coral ecosystems. They occur between 30° north and 30° south of the Equator, where surface water temperatures remain above 16°C year-round.

Scale and distribution:

• Total global coverage of approximately 284,300 km²
• Concentrated in the Indo-Pacific region, Caribbean Sea, and around tropical islands
• Can grow upwards at rates of 1-100 cm per year
• Form the largest and oldest living structures on Earth

The structure of tropical coral reefs

Coral composition and foundation

Tropical coral reefs have a distinctive layered structure. The underlying foundation comprises dead coral skeletons accumulated over thousands or millions of years. Living coral polyps colonise the top of this structure, continuously secreting new limestone to build the reef higher and wider.

The coral-algae partnership

Tropical coral polyps host tiny symbiotic algae called zooxanthellae that live inside their tissues. This mutually beneficial relationship works as follows:

What the algae receive:

• Protection and shelter within the coral polyp
• Nutrients from plankton captured by the coral

What the coral receives:

• Food in the form of nutrients produced through photosynthesis by the algae
• The algae also provide the coral's vibrant colours

Coral bleaching

Coral bleaching occurs when corals become stressed (typically from elevated water temperatures) and expel their symbiotic algae. This exposes the white calcium carbonate skeleton beneath, giving the coral a 'bleached' appearance.

Bleached corals are not immediately dead but are severely weakened and more vulnerable to disease and death if stress continues.

Categories of tropical coral reefs

Tropical reefs develop into three main structural types:

Fringing reefs

• Grow directly outward from the shoreline
• No area of open water (lagoon) separates the reef from land
• The most common reef type
• Particularly vulnerable to pollution and sedimentation from terrestrial sources because they lack a protective lagoon buffer between land and reef

Barrier reefs

• Form extensive linear complexes running parallel to the coastline
• Separated from the shore by a lagoon of varying depth
• The lagoon provides a wide band of water between land and reef

Atolls

• Roughly circular or oval reef systems
• Surround a central lagoon
• Typically form around volcanic islands that have subsided over time

Case study: Belize Barrier Reef Reserve System

Threats to coral reef ecosystems

Coral reefs face numerous pressures that place them under significant stress:

• Pollution from coastal development and agricultural runoff
• Sedimentation from rivers and coastal erosion, which smothers corals and blocks sunlight
• Rising water temperatures linked to climate change, causing widespread coral bleaching
• Ocean acidification reducing the availability of calcium carbonate needed for reef building
• Overfishing and destructive fishing practices
• Physical damage from boats, anchors, and tourism

Ecosystem management and resilience

The way we manage ecosystems, including coral reefs, has profound consequences for both environmental and human well-being.

The diagram illustrates two contrasting pathways:

Non ecosystem-based adaptation (negative feedback loop)

Poor biodiversity and ecosystem protection creates a destructive cycle:

1. Ecosystem degradation occurs
2. Loss of ecosystem services follows
3. Human well-being declines
4. Social disruption increases
5. Greater pressure placed on already degraded ecosystems
6. Further population increase and climate change impacts worsen the cycle

Ecosystem-based adaptation (positive feedback loop)

Good biodiversity and ecosystem protection creates a beneficial cycle:

1. Ecosystem protection and restoration occurs
2. Ecosystem resilience builds, reducing vulnerability
3. Secure ecosystem services maintained
4. Social well-being improves
5. Sustainable economies develop
6. Climate change mitigation enhanced
7. Positive cycle continues supporting both ecosystems and communities

Remember!