Untreated sewage pollution in aquatic ecosystems

Untreated sewage adds organic waste and nutrients to water. Microorganisms break down the organic waste and use dissolved oxygen as they respire. As more waste is decomposed, the concentration of oxygen dissolved in the water can fall.

Fish and other aquatic animals need dissolved oxygen for respiration. If its concentration falls too low, they cannot obtain enough oxygen and may die.

The added nutrients can also promote algal growth. When algae die, their organic material is broken down by microorganisms. These decomposers respire as they break it down, using more dissolved oxygen and potentially reducing its concentration further.

Fertiliser runoff and aquatic eutrophication

Rain can wash excess fertiliser from land into rivers and lakes. This runoff adds nutrients, including nitrates and phosphates, to the water. The extra nutrients can cause algae to grow rapidly, forming a dense layer known as an algal bloom.

From algal growth to oxygen loss

The dense algae reduce the light reaching plants below the water’s surface. With too little light for photosynthesis, these submerged plants may die. The algae also eventually die.

Microorganisms decompose the dead plants and algae. As they do so, they respire, using oxygen dissolved in the water. The dissolved oxygen concentration falls, so fish and other aquatic animals may not have enough oxygen to survive.

This sequence of nutrient enrichment, rapid algal growth and the resulting effects is called eutrophication.

Non-biodegradable plastic pollution

Living organisms do not readily break down non-biodegradable plastics. When these plastics are discarded, they can remain in the environment for a long time and accumulate in both aquatic and terrestrial ecosystems.

Animals in water or on land may become entangled in discarded plastic. The plastic can restrict their movement or cause injuries. Animals may also swallow plastic, which can harm or kill them.

Because plastic does not quickly decompose, these hazards remain in the environment rather than disappearing soon after the plastic is discarded.

Methane and carbon dioxide in the enhanced greenhouse effect

Carbon dioxide and methane are greenhouse gases. They absorb some of the infrared radiation emitted by Earth, reducing the rate at which energy escapes to space. This helps keep Earth warm. When their concentrations in the atmosphere rise, this warming effect becomes stronger.

Sources of carbon dioxide and methane

Burning fossil fuels releases carbon dioxide into the air. Deforestation also increases atmospheric carbon dioxide: carbon can be released when cleared vegetation burns or decays, and fewer trees remain to take up carbon dioxide.

Methane is released by sources including livestock and the decay of organic waste. For example, some livestock produce methane during digestion.

How extra greenhouse gases warm Earth

Earth absorbs energy from the Sun and emits some energy as infrared radiation. Carbon dioxide and methane absorb some of this outgoing radiation and emit infrared radiation in all directions, including back towards Earth’s surface. This reduces the rate at which energy escapes to space.

The natural greenhouse effect warms Earth without the additional greenhouse gases from human activities. Higher concentrations of carbon dioxide and methane strengthen this effect. This is the enhanced greenhouse effect, which raises global temperatures.

The enhanced greenhouse effect drives climate change, including changes in temperature and rainfall patterns.