0610 · 20.3Pollution

Eutrophication and oxygen depletion

0:00Imagine a pond beside a farm. After heavy rain, fertiliser washes into the water, and a thick green layer of algae develops at the surface. At first, more growth might sound like good news for the ecosystem, but later, fish begin to die. How can adding substances that help plants grow end up killing aquatic animals? In this video, we will follow that chain of events and understand the effects of untreated sewage, excess fertiliser, non-biodegradable plastics, carbon dioxide and methane.

0:29These pollutants do not all cause harm in the same way. Some increase the demand for oxygen, some persist and physically harm organisms, and others change Earth’s energy balance. Understanding those differences makes this topic much easier to explain. We need three ideas before we begin. Producers, such as plants and algae, make organic nutrients using light energy through photosynthesis. Decomposers, including bacteria and fungi, break down dead organisms and waste materials.

0:58Aerobic respiration releases energy from nutrients using oxygen, and aquatic organisms such as fish obtain the oxygen they need from oxygen dissolved in the water. Dissolved oxygen is oxygen present in the water, not the oxygen chemically combined with hydrogen in water molecules. Fish take up dissolved oxygen through their gills, while many decomposers also use it for aerobic respiration. The amount available is limited and can change. If organisms use oxygen faster than it is replaced, its concentration falls, and animals may no longer obtain enough to survive.

1:30Now consider fertiliser. Farmers apply fertilisers to supply mineral ions that crops need for growth, including nitrate ions. Plants use nitrates to make amino acids, which are used to make proteins. Fertiliser is useful when crops absorb it, but excess fertiliser can remain in the soil and be carried into nearby streams, rivers or ponds. Rainwater can dissolve these mineral ions and carry them through the soil, a process called leaching.

1:55Water flowing over the land can also carry dissolved fertiliser into an aquatic ecosystem. This increases the availability of nitrate and other ions in the water. If a shortage of mineral ions was limiting producer growth, that shortage is reduced, so algae and aquatic plants can grow more rapidly. The resulting enrichment of water with nutrients, which can lead to excessive producer growth and oxygen depletion, is called eutrophication. Think of the mineral ions as materials needed for building new cells, rather than as food or energy for plants.

2:27The producers still need light for photosynthesis. Adding more building materials allows much more growth when those materials were previously in short supply. Rapid algal growth can form an algal bloom, a dense population of algae in the water. A thick bloom near the surface blocks light from reaching submerged plants. Those plants photosynthesise less, so they make less of the organic nutrients they need, and may die. Eventually, algae also die, leaving a larger amount of dead organic material in the ecosystem.

2:57This is where the crucial oxygen problem develops. More dead producers provide more material for decomposers to break down, so decomposition increases. The decomposers carry out more aerobic respiration and use more dissolved oxygen. As the dissolved oxygen concentration falls, fish and other aquatic organisms that require it may die because they cannot obtain enough oxygen for aerobic respiration. Notice that the fertiliser has not simply poisoned the fish in this explanation.

3:24It has changed the ecosystem by increasing producer growth and, later, the demand for oxygen during decomposition. The surface bloom blocking light helps explain why submerged plants die. The essential final link is increased aerobic respiration by decomposers, which reduces the dissolved oxygen available to other organisms. There is a subtle point here because algae can release oxygen during photosynthesis. You might therefore expect more algae always to mean more oxygen for fish.

3:52But photosynthesis needs light, whereas aerobic respiration uses oxygen, and decomposition increases when large amounts of organic material die. Producing oxygen at one stage does not prevent the ecosystem from becoming oxygen depleted at a later stage.

Sewage and fish deaths

4:08Untreated sewage can cause a similar oxygen shortage, but it can take a shorter route. Sewage contains organic waste, including faeces, which provides material for decomposers to break down. Increased decomposition involves increased aerobic respiration by decomposers, using dissolved oxygen. Fish and other organisms requiring that oxygen may die even without an algal bloom developing first. Sewage can also supply mineral nutrients that encourage producer growth and contribute to eutrophication.

4:36In addition, untreated sewage may contain pathogens, meaning disease-causing organisms, which can contaminate the water. Keep those effects separate in your explanation. Sewage can create a disease risk, but the oxygen shortage is explained by decomposition and aerobic respiration, not by pathogens somehow removing all the oxygen themselves. Let us work through a hypothetical example. A farmer applies excess fertiliser, heavy rain follows, and a nearby pond develops a green surface bloom.

5:06Later, submerged plants die and fish are found dead. To explain the observations, start with what entered the pond, then connect each visible change to the biological process that could have caused it. First, dissolved fertiliser is carried into the pond, increasing nitrate and other mineral ions. These support increased algal growth, explaining the green bloom. Next, the bloom reduces light reaching submerged plants, so their photosynthesis decreases and they may die.

5:34Dead plants and algae then provide more organic material for decomposition, connecting the increased growth to a later increase in decomposer activity. Finally, decomposers use dissolved oxygen in aerobic respiration as they break down that material. The oxygen concentration falls, explaining why fish and other oxygen-requiring organisms may die. This reasoning gives a connected explanation rather than a list of unrelated observations. If the question instead described untreated sewage entering the pond, you could begin with its organic waste supplying material directly for decomposition.

Effects of non-biodegradable plastics

6:07Plastic pollution causes harm in a different way. Non-biodegradable plastics are not broken down by decomposers through biological processes, so they persist and can accumulate in ecosystems. Weathering may break a plastic item into smaller pieces, but that does not mean it has been biodegraded. The material can remain in the environment even when the original object is no longer recognisable. In aquatic ecosystems, animals can become entangled in discarded fishing nets, lines or plastic loops.

6:36Entanglement can restrict movement, cause injury, prevent feeding or make it difficult for an air-breathing animal to reach the surface. Animals may also mistake plastic for food and swallow it. Ingested plastic can obstruct the digestive system or reduce feeding, potentially leading to starvation and death. These effects are not restricted to the sea. In terrestrial ecosystems, animals can also swallow plastic litter or become entangled in it, causing injury, restricted movement or feeding problems.

7:03Plastic waste can cover plants or soil, interfering with plant growth. The important connection is persistence. Because non-biodegradable plastic is not broken down by decomposers, opportunities for harm can continue long after it is discarded.

Greenhouse gases and climate change

7:17Now move from water and land to the atmosphere. Carbon dioxide is released by burning fossil fuels, including coal, oil and natural gas, in activities such as electricity generation and transport. Burning forests also releases carbon dioxide, and removing trees reduces its uptake by photosynthesis. Respiration releases carbon dioxide naturally too, but human activities can increase its atmospheric concentration. Methane has several important sources. It is released during digestion in cattle and other ruminant animals, and when microorganisms break down organic material without oxygen, such as in landfill sites and waterlogged rice fields.

7:56Methane can also escape during the extraction and transport of fossil fuels. Both methane and carbon dioxide are greenhouse gases, so increasing their concentrations affects Earth’s temperature. To understand the greenhouse effect, follow the energy rather than imagining a solid lid around Earth. Sunlight passes through the atmosphere and warms Earth’s surface. The warmed surface emits infrared radiation, which carries energy away. Greenhouse gases absorb some of this outgoing infrared radiation and emit infrared radiation in all directions, including back towards the surface.

8:29The natural greenhouse effect helps keep Earth warm enough for life as we know it. The problem is the enhanced greenhouse effect, caused by increased concentrations of greenhouse gases. At first, more outgoing infrared radiation is absorbed, so less energy escapes to space than otherwise would. Earth’s surface and lower atmosphere warm until the energy entering and leaving become balanced again at a higher temperature. This warming contributes to climate change, meaning long-term changes in temperature and weather patterns.

8:57Effects can include altered rainfall patterns, increased risks of drought or flooding in some regions, and rising sea levels as land ice melts and warmer seawater expands. Ecosystems can change because organisms depend on particular environmental conditions. Some species may shift their distribution, while others may struggle to survive. Do not confuse the enhanced greenhouse effect with damage to the ozone layer. They are different processes, and the greenhouse explanation here concerns outgoing infrared radiation, not increased ultraviolet radiation reaching Earth.

9:28Also, climate change does not mean every place becomes warmer every day. Weather varies over short periods, while climate describes patterns over much longer periods.

Writing linked exam explanations

9:37In an examination, describe means give the relevant features or effects, while explain means show how or why something happens. For example, consider the question, explain how excess fertiliser entering a lake can cause the death of fish. The key is a linked biological sequence, not simply saying that pollution is harmful. A clear answer gives a separate, creditworthy point in each sentence. Excess fertiliser leaches into the lake, increasing the availability of nitrate and other ions.

10:06These ions increase the growth of producers such as algae. The algal bloom blocks light from reaching submerged plants, reducing their photosynthesis and causing them to die. After producers die, decomposition by decomposers increases. Decomposers carry out more aerobic respiration, using more dissolved oxygen. The concentration of dissolved oxygen in the water decreases. Fish and other aquatic organisms requiring dissolved oxygen may die because insufficient oxygen is available for aerobic respiration.

10:36Each sentence supplies a link, so the answer explains the outcome rather than merely naming eutrophication. For a question asking you to describe the effects of non-biodegradable plastics, identify the harm in both aquatic and terrestrial ecosystems. You could describe entanglement causing restricted movement or injury, and ingestion causing digestive obstruction or reduced feeding. For greenhouse questions, connect a named source to carbon dioxide or methane, then explain increased absorption of outgoing infrared radiation.

11:06Avoid replacing these mechanisms with the vague statement that pollution destroys nature.

Pollution routes to harm

11:11Let us finish with the three different routes to harm. Excess fertiliser can increase producer growth, followed by increased decomposition and aerobic respiration, reducing dissolved oxygen. Untreated sewage supplies organic material directly for decomposition and can also contribute nutrients and pathogens. Non-biodegradable plastics persist, creating risks such as entanglement and ingestion on land and in water. Carbon dioxide and methane affect ecosystems by enhancing the greenhouse effect and contributing to climate change.

11:41Their increased concentrations change how outgoing infrared radiation moves through the atmosphere, warming Earth. Across this topic, the most useful question is not simply which pollutant is present, but how it changes the conditions organisms need. Once you can explain that chain of cause and effect, the biology becomes much easier to understand.