Describe the major types of marine pollution (plastics, oil, nutrients and sediment) and explain their sources, impacts and the mechanism of eutrophication, using Great Barrier Reef examples

What this dot point is asking

QCAA wants you to identify the major types of marine pollution, explain where they come from and how they harm marine life, and explain the mechanism of eutrophication step by step. This is a common IA3 and extended-response area, so you need named sources and clear cause-and-effect chains using Great Barrier Reef examples.

Types of marine pollution

Most marine pollution originates on land and reaches the sea through rivers, runoff, stormwater and the air.

• Plastics and microplastics. Bags, bottles, fishing gear and tiny fragments. Animals are entangled in or eat plastic, blocking their guts; microplastics enter the food web and can carry toxins. Plastic persists for hundreds of years and accumulates in gyres and on shores.
• Oil. From spills, shipping and runoff. Oil coats feathers and fur, destroys insulation and buoyancy, poisons animals, and smothers intertidal and mangrove shores that are slow to recover.
• Nutrients. Nitrogen and phosphorus from fertiliser, sewage and animal waste. In excess they cause eutrophication (below).
• Sediment. Soil washed from cleared land and farms. It clouds the water, blocking light for seagrass and coral, and settles to smother them.
• Other pollutants. Heavy metals, pesticides and warm water from industry can all stress marine life and accumulate up the food chain.

The mechanism of eutrophication

Eutrophication is a favourite QCAA mechanism question because it is a clear chain.

1. Excess nutrients (nitrogen and phosphorus) wash into the water from fertiliser, sewage or runoff.
2. The nutrients fuel rapid growth of algae and phytoplankton, an algal bloom.
3. The bloom blocks light from reaching seagrass and coral below, and the algae soon die.
4. Bacteria decompose the dead algae, and this decomposition consumes large amounts of dissolved oxygen.
5. Oxygen levels crash (hypoxia or anoxia), killing fish and other animals and creating a dead zone.

Impacts on the Great Barrier Reef

The Great Barrier Reef is the central Australian example. Reef corals are adapted to clear, low-nutrient water, so runoff harms them in several linked ways:

• Sediment from land clearing and floods clouds the water and smothers inshore corals and seagrass, reducing the light their algae need.
• Nutrients favour fast-growing seaweed that outcompetes coral, and have been linked to outbreaks of the coral-eating crown-of-thorns starfish, whose larvae survive better when there is more plankton to eat.
• Pesticides from agriculture add chemical stress.

These pressures are concentrated on inshore reefs near river mouths, which is why catchment management is a major plank of reef protection.

Why this matters and how it is managed

Unlike global warming, much marine pollution can be tackled locally and relatively quickly, which makes it a strong management case study. Responses include improved farming practices and vegetation buffers to cut nutrient and sediment runoff (such as Australia's Reef 2050 water-quality targets), banning single-use plastics, upgrading sewage treatment, and tighter shipping controls. Because pollution is land-based and reversible, reducing it builds the reef's resilience to the climate stresses it cannot escape so easily.