Geography (9635) | Marine ecosystems

Marine Ecosystems: Coral Reefs Under Stress (9635 Section 3.3.2.5)

Hello Geographers! Welcome to the section on Marine Ecosystems. Specifically, we are diving into the vibrant but fragile world of coral reefs. This chapter is critical because it sits within the broader theme of Ecosystems under stress. You will learn how these incredible habitats function, what natural processes control them, and critically, how human actions are pushing them toward collapse. Let's make sure you know your corals from your clams!

1. The Coral Reef Ecosystem: Characteristics and Distribution

Coral reefs are often called the 'rainforests of the sea' because of the immense biodiversity they support. They are complex marine ecosystems built primarily by tiny animals called coral polyps.

1.1 Key Characteristics and Structure

• Structure: Corals are colonies of polyps that secrete calcium carbonate (limestone) skeletons. Over thousands of years, these skeletons build up into massive reef structures.
• Symbiotic Relationship: The key to a coral's survival is its relationship with microscopic algae called zooxanthellae. The algae live within the coral tissue, providing the coral with up to 90% of its required food through photosynthesis. In return, the coral provides the algae with a protected environment.
• Biodiversity Hotspot: Despite covering less than 0.1% of the ocean floor, coral reefs support around 25% of all marine species, acting as nurseries, feeding grounds, and shelter.

1.2 Environmental Conditions for Reef Development (The Coral "Goldilocks Zone")

Corals are picky! They only thrive in very specific, narrow environmental conditions. Think of these as the inputs required for the system to function effectively.

Key Tip: If any of these conditions change significantly, the ecosystem experiences severe stress.

1. Water Temperature: Must be warm, ideally between 23°C and 29°C. If temperatures rise above 30°C for extended periods, it causes coral bleaching.
2. Light: They need clear, shallow water (less than 50m deep) so the zooxanthellae can photosynthesise.
3. Salinity: They require normal marine salinity (around 32–40 parts per thousand). Too much freshwater (low salinity) from river runoff is toxic.
4. Water Clarity: Clear water is essential. High levels of sediment or turbidity (cloudiness) block light, suffocating the polyps and inhibiting photosynthesis.
5. Nutrients: They thrive in nutrient-poor (oligotrophic) water. High nutrient levels (e.g., nitrates/phosphates from pollution) encourage fast-growing seaweed and algae, which can smother the slower-growing coral.

2. Factors Affecting Reef Health: Natural Stresses

Even without human intervention, coral reefs face natural threats that challenge their survival and health.

2.1 Natural Factors in Survival

• Water Temperature Fluctuation: Natural weather phenomena, such as strong El Niño events, can cause temporary but widespread temperature spikes leading to mass bleaching events.
• Acidity (Natural Variability): While most acidification is human-caused (see Section 3), reefs naturally cope with minor pH fluctuations due to factors like tidal cycles or upwelling of deep, CO2-rich water.
• Salinity Shifts: Heavy rainfall, especially during tropical storms, can drastically lower salinity near the coast, stressing or killing corals accustomed to high salt levels.
• Algal Blooms: Natural explosions in algal populations can sometimes block light or deplete oxygen. However, these are often triggered or worsened by human factors like nutrient pollution.
• Predators: The Crown-of-Thorns Starfish (COTS) is a natural predator that feeds on coral polyps. Outbreaks of COTS can devastate reefs, although these outbreaks are often linked to human activity (like overfishing COTS predators).

3. Human Activity and its Impact: Ecosystems Under Severe Stress

The biggest threats to coral reefs come from growing human populations and economic pressures, which destabilise the delicate environmental conditions required for survival.

3.1 Human Impacts on Reef Health

The impacts of human activity are complex and often interconnected, affecting the four primary environmental conditions (temperature, acidity, salinity, clarity).

A. Climate Change and Global Pressures

• Rising Water Temperature: Caused by global warming (enhanced greenhouse effect). This is the leading cause of mass coral bleaching events globally.
• Ocean Acidification: The ocean absorbs approximately 30% of the CO2 released into the atmosphere. When CO2 dissolves in water, it forms carbonic acid, lowering the ocean's pH.

  Analogy: Imagine trying to build a house when the bricks (calcium carbonate) are constantly dissolving. Acidification makes it harder for corals to grow their skeletons, especially juvenile corals.

B. Local and Regional Pressures

• Pollution:
  • Nutrient Pollution: Runoff from agricultural fertilisers and untreated sewage introduces nitrates and phosphates. This encourages algal growth, which smothers the coral and disrupts the delicate ecosystem balance.
  • Oil and Plastics: Oil spills and microplastic pollution introduce toxic substances and physically block light or cause disease.
• Major Drainage Basin Schemes & Onshore Development:
  • Large river diversion projects or dams reduce the natural flow of freshwater but can increase the concentration of pollutants and sediments entering the sea.
  • Coastal construction (onshore development) and dredging activities (to expand ports or tourist areas) increase the amount of sediment in the water. This dramatically reduces water clarity, starving the zooxanthellae.
• Fishing and Whaling (Indirect Impacts):
  • Overfishing: Removing key herbivores (like parrotfish) means there are no animals left to graze the seaweed. The seaweed then outcompetes and smothers the slow-growing coral.
  • Destructive Fishing: Practices like dynamite fishing or cyanide fishing destroy the physical structure of the reef instantly.
• Tourism: While often managed sustainably, high volumes of tourism can lead to physical damage (anchors, boats, touching coral) and increased sewage/waste discharge.
• Desalination: Desalination plants produce freshwater but discharge a highly concentrated byproduct called brine back into the sea. This brine has very high salinity and often contains trace chemicals, severely impacting local reef areas.

4. Case Study: The Great Barrier Reef (GBR), Australia

To demonstrate these concepts, we must refer to a named, located coral reef. The GBR, located off the coast of Queensland, Australia, is the world’s largest coral reef system and illustrates the impact of both natural and human stresses.

Location and Scale

The GBR is over 2,300 km long, comprising thousands of individual reefs. Its sheer size means it experiences a wide range of pressures, particularly from the densely developed coastline of Queensland.

Factors Affecting the GBR's Health

• Natural Stress: The GBR has suffered four major mass bleaching events since 1998 (1998, 2002, 2016, 2017), driven primarily by El Niño cycles and rising sea surface temperatures.
• Human Stress (Agriculture/Drainage): The biggest regional threat comes from the rivers draining into the reef lagoon, especially the Burdekin River Basin. Agricultural runoff carries vast quantities of sediment (from grazing land erosion) and nutrients (from sugarcane and cattle farms).
  • Impact: Reduced light penetration and increased algal growth, which fuels COTS outbreaks.
• Human Stress (Development/Shipping): Expansion of coal ports along the Queensland coast requires massive dredging, which stirs up sediment and destroys local reef areas. Increased shipping traffic raises the risk of oil spills and anchor damage.

5. Future Prospects for Coral Reefs

The outlook for coral reefs is highly dependent on whether global communities can urgently address climate change (global action) and implement strict local management (regional/local action).

5.1 The Challenge of Resilience

While reefs have historically shown resilience, the rate and scale of current threats are overwhelming their ability to recover. Recovery from a mass bleaching event can take a decade or more, but if bleaching events occur every few years (as they are now), the reefs have no chance to regenerate.

5.2 Management and Mitigation Strategies

Sustainable management involves addressing both the global drivers (CO2 emissions) and the local stressors (pollution/development):

• Coastal Zone Management: Implementing stricter laws to control coastal development, dredging, and land clearing to reduce sediment runoff.
• Improved Water Quality: Working with farmers in drainage basins (like the GBR catchment) to improve fertilisation practices, reduce pesticide use, and restore riverside vegetation buffers to trap sediment.
• Marine Protected Areas (MPAs): Establishing 'No-Take Zones' where fishing and resource extraction are banned. This allows fish populations to recover, restoring the ecological balance (like controlling algal grazers).
• Technological Solutions: Researching coral engineering—growing resilient coral species in labs to transplant onto damaged reefs, or developing methods to shade reefs during heatwaves.

Key Takeaway

Marine ecosystems, particularly coral reefs, are under extreme stress due to the synergistic effect of global warming (bleaching, acidification) and regional human pressures (pollution, sediment runoff). Their future depends on coordinated global action to limit CO2 emissions and disciplined local conservation management.