Describe the major human impacts on marine environments (pollution, runoff, plastics, dredging, crown-of-thorns outbreaks, coastal development) and explain the processes by which they degrade ecosystems such as the Great Barrier Reef

What this dot point is asking

QCAA wants you to name the main human impacts on marine environments and explain the mechanism by which each one degrades ecosystems, not just list them. The Great Barrier Reef is the key case study, so connect each impact to a real reef process.

Agricultural runoff

Runoff from farmland in Great Barrier Reef catchments carries nutrients (nitrogen and phosphorus), sediment and pesticides into the reef lagoon.

• Nutrients cause eutrophication: they fuel blooms of algae that overgrow and outcompete coral for light and space, shifting reefs from coral-dominated to algae-dominated.
• Sediment reduces water clarity, blocking the light that zooxanthellae need, and settles on coral, smothering polyps.
• Pesticides are toxic to seagrass, larvae and other marine life.

This is why the Queensland and Australian governments fund the Reef Water Quality Improvement Plan to cut runoff at its source.

Crown-of-thorns starfish outbreaks

The crown-of-thorns starfish (Acanthaster planci) is a coral-eating starfish native to the reef. At normal densities it is harmless, but population outbreaks can strip the coral from whole reefs. Outbreaks are linked to nutrient runoff: extra nutrients fuel the phytoplankton that starfish larvae feed on, boosting larval survival. Outbreaks have been a leading cause of coral loss on the Great Barrier Reef over recent decades, alongside cyclones and bleaching, which is why control programs physically remove or inject starfish on high-value reefs.

Plastic and chemical pollution

Plastics break down into microplastics that are eaten by plankton, fish and filter feeders, passing up the food web. Larger plastics entangle turtles, seabirds and marine mammals. Chemical pollutants such as oil spills, heavy metals and antifouling chemicals poison organisms directly and can bioaccumulate up the food chain, reaching dangerous concentrations in top predators. A single oil spill near a reef can kill corals and the organisms that depend on them across a wide area as currents spread it.

Dredging and coastal development

• Dredging (for example to deepen shipping channels and ports along the Queensland coast) stirs up sediment plumes that smother seagrass and coral and reduce water clarity, and it destroys sea-floor habitat directly.
• Coastal development clears mangroves and seagrass for ports, marinas and urban growth, removing nursery habitat and the sediment traps that protect reefs (linking back to connectivity in Unit 3).
• Overfishing is covered in the fisheries dot point but also degrades ecosystems by removing key species and reshaping food webs.

Cumulative and interacting impacts

These impacts rarely act alone. A reef weakened by poor water quality recovers more slowly from a cyclone or a bleaching event, and crown-of-thorns outbreaks, sediment and warming combine to push reefs past tipping points. This is why the 2024 and 2025 mass bleaching events were so damaging on reefs already stressed by runoff.

Why this matters

These impacts are the problems that the management and fisheries dot points aim to solve. Knowing the mechanism behind each one is what lets you evaluate whether a management strategy will actually work, which is the core skill assessed in IA3 and the external exam.