Marine pollution: plastics, oil and nutrients (QCE Marine Science Unit 4)
“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”
Marine pollution comes mainly from the land. Plastics and microplastics entangle and are eaten by marine animals and persist for centuries. Oil spills coat animals and smother shores. Excess nutrients from fertiliser and sewage trigger eutrophication, where algal blooms grow, die, and their decay strips oxygen from the water, creating dead zones. Sediment from land clearing smothers seagrass and coral and blocks the light they need. On the Great Barrier Reef, nutrient and sediment runoff from catchments is a major and manageable threat.
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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.
- Excess nutrients (nitrogen and phosphorus) wash into the water from fertiliser, sewage or runoff.
- The nutrients fuel rapid growth of algae and phytoplankton, an algal bloom.
- The bloom blocks light from reaching seagrass and coral below, and the algae soon die.
- Bacteria decompose the dead algae, and this decomposition consumes large amounts of dissolved oxygen.
- Oxygen levels crash (hypoxia or anoxia), killing fish and other animals and creating a dead zone.
The fatal step in eutrophication is not the algae themselves but the oxygen crash that follows. When the algal bloom dies, bacteria decomposing it use up the dissolved oxygen, suffocating fish and bottom-dwelling animals. So a nutrient pollution problem becomes an oxygen problem. This is why nutrient runoff, which sounds like fertiliser feeding the sea, actually devastates it.
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.
- Saying algae directly kill fish in eutrophication
- The killer is the oxygen crash when bacteria decompose the dead algal bloom.
- Treating nutrients as good for the reef
- Reef corals need low-nutrient water; extra nutrients favour seaweed and crown-of-thorns starfish.
- Calling all pollution oceanic in origin
- Most marine pollution comes from the land via rivers and runoff.
- Forgetting sediment is pollution
- Sediment from land clearing smothers coral and seagrass and blocks light, a major reef threat.
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.
Practice5 marksThe graph shows the aragonite saturation state on a fringing reef monitored before and after land clearing in nearby river catchment areas (land clearing occurred from 2016 to 2020), for shallow water (less than 20 m) and deep water (20 m or more). [In the exam graph, the saturation state fell at both depths during and after the clearing, and the fall was larger in shallow water.] a) Compare the effect of land clearing on the aragonite saturation state in shallow and deep water. b) Predict an effect of changing aragonite concentrations on corals and the nearby fringing reef ecosystem.Show worked answer →
a) (3 marks) Give a similarity, a difference and its significance, using the graph.
- Similarity. In both shallow and deep water the aragonite saturation state falls during and after land clearing (2016 to 2020).
- Difference. The fall is larger in shallow water (it drops towards or below saturation) than in deeper water, where the decline is smaller. Quote the saturation values.
- Significance. Shallow fringing-reef corals are most exposed to the catchment run-off, so they suffer the greatest reduction in aragonite availability.
b) (2 marks) Prediction. Lower aragonite saturation means less carbonate is available, so corals calcify more slowly and their skeletons may begin to dissolve. Coral growth and reef-building decline, the reef structure weakens, and habitat and biodiversity on the fringing reef fall. Land clearing drives this by washing sediment, nutrients and organic matter into the water, which lowers the aragonite saturation state nearshore.