Human impacts on marine environments (QCE Marine Science Unit 4)
“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”
Human activities degrade marine environments through nutrient and sediment runoff, chemical and plastic pollution, dredging, coastal development and the crown-of-thorns starfish outbreaks that runoff helps trigger. Each works through a clear mechanism: runoff fuels algae and starfish, sediment smothers coral, plastics and chemicals poison organisms, and dredging and development destroy habitat directly.
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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.
An exam question may ask you to explain how nitrogen runoff degrades a reef. A full-mark answer traces the mechanism step by step:
- Fertiliser runoff adds dissolved nitrogen to the reef lagoon.
- Extra nitrogen fuels phytoplankton blooms, which feed crown-of-thorns larvae and raise their survival, leading to starfish outbreaks that eat coral.
- The same nutrients also fuel macroalgae that overgrow and outcompete coral for light and space.
- Reduced water clarity from associated sediment cuts the light zooxanthellae need.
The result is coral loss through both predation and competition. Always give the mechanism, not just "runoff is bad for reefs."
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.
- Listing impacts without mechanisms
- QCAA marks the process: how nutrients cause eutrophication, how sediment smothers coral, how plastics enter the food web.
- Calling crown-of-thorns starfish an invasive species
- They are native; the problem is outbreaks driven partly by nutrient runoff.
- Confusing eutrophication with acidification
- Eutrophication is nutrient enrichment causing algal blooms; acidification (a separate climate impact) is falling pH from dissolved carbon dioxide.
- Ignoring cumulative effects
- Impacts combine, so a stressed reef recovers slowly from disturbances it could otherwise survive.
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.
Exam-style questions
Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice3 marksCoral cores from the Great Barrier Reef show luminescence (a proxy for river flood discharge) rising sharply after about 1860. Infer how land use changes since 1860 have affected the health of the Great Barrier Reef near this river.Show worked answer →
For 3 marks, connect post-1860 land clearing to run-off and then to reef decline.
Land use change. Since about 1860, European settlement cleared catchment vegetation for agriculture and grazing. This increased soil erosion, so floods carried far more sediment, nutrients (fertiliser) and freshwater to the reef - shown by the rise in coral luminescence.
Process of degradation. The extra sediment reduces light reaching corals and smothers them; the extra nutrients fuel algal growth and have been linked to crown-of-thorns starfish outbreaks; reduced salinity from floodwater also stresses corals.
Effect on reef health. These combined pressures have lowered water quality and reduced coral cover and reef health near the river mouth since 1860, making the reef less resilient to other disturbances.
Practice5 marksA reef was affected from 2018 to 2023 by water quality (poor to very poor, improving to moderate), crown-of-thorns starfish (COTS) outbreaks and a cyclone, and its coral cover collapsed. Infer how each factor contributed to the reef reaching its ecological tipping point.Show worked answer →
For 5 marks, explain the mechanism for each factor and how they combined to push the reef past recovery.
Water quality. Poor to very poor water quality (high sediment and nutrients from run-off) reduced light, smothered corals and favoured algae, weakening coral health and slowing recovery in the early years.
Crown-of-thorns starfish. Repeated COTS outbreaks (the table shows outbreaks across several years) directly consumed large areas of living coral, sharply cutting coral cover.
Cyclone. The cyclone physically broke and removed coral structure in one year, causing a further sudden drop in cover.
Cumulative effect / tipping point. Each disturbance arrived before the reef could recover from the last, so the stresses were additive. Once coral cover and resilience fell low enough, the reef crossed an ecological tipping point and shifted towards an algae-dominated state from which it cannot easily recover. Award marks for linking each factor to loss of coral and for the combined-stress reasoning.