24. Pollution of water: eutrophication

20.5 Pollution of water:

eutrophication

OBJECTIVES

To recall why water is important to living organisms

To understand that water supplies oxygen to living organisms

To know how excess nutrients in water lead to depletion of oxygen levels

To recall other aspects of water pollution

The causes of oxygen depletion

All living organisms depend on a supply of water, as we saw on page 256. Many organisms actually live in water. Most of these aquatic organisms respire aerobically and so require oxygen from their environment. Any change that alters the amount of oxygen in the water can seriously affect the suitability of the water as a habitat. The two pollutants that most often reduce oxygen in water are:

Fertilisers-nitrates and phosphates are added to soil by farmers (see page 266). Some of the fertiliser is washed from the soil by rain into the nearest pond, lake or river. This process is called leaching.

Sewage - this contains an excellent source of organic food for bacteria, and also contains phosphates from detergents (see page 58).

Sea water can also be polluted!

Marine environments are affected by human activities:

spillages of oil which can seriously reduce oxygen levels on the seabed as well as reducing

waterproofing properties of seabirds' feathers

radioactive compounds from cooling of nuclear power stations

temperature changes caused by global warming.

How fertilisers and sewage affect the oxygen concentration

Water that contains few nutrients is rich in oxygen and supports a wide variety of living organisms. The oxygen enters the water from the atmosphere by diffusion and from photosynthesising aquatic plants. Simpler forms of life, such as algae and bacteria, are controlled because the low concentration of nutrients such as nitrate is a limiting factor for their growth. If more nutrients are made available, from fertiliser runoff or from sewage, then:

Algae and other surface plants grow very rapidly, and block out light to plants rooted on the bottom of the river or pond.

The rooted plants die, and their bodies provide even more nutrients.

The population of bacteria increases rapidly. As they multiply, the bacteria consume oxygen for aerobic respiration. There is now a biological oxygen demand (or BOD) in the water because of oxygen consumed by these microbes.

■Other living creatures cannot obtain enough oxygen. They must leave the area, if they can, or they will die. Their bodies provide even more food for bacteria, and the situation becomes even worse. This is an example of positive feedback - the change from ideal conditions causes an even greater change from ideal conditions.

The lower diagram opposite shows what happens if a pond or river receives too many nutrients. The process is called eutrophication. The pond or river soon becomes depleted of living organisms. Only a few animals, such as Tubifex (sewage worms), can respire at the very low oxygen concentrations that are available. The solution to this problem is straightforward - do not allow excess nutrients into the water.

A well-balanced natural pond or river

Populations of algae and bacteria are limited by low levels of nutrients.

Light penetrates to rooted plants which photosynthesise, producing oxygen.

Invertebrates and fish can obsain enough oxygen for aerobic nespiration.

A nutrient-enriched pond or river

Farmyard runoff

Soluble nitrate and phosphate

Leaching of inorgan fertilisers from farm

Input of raw sewage

Algae and green protoctists use nutrients to multiply rapidly-algal bloom.

The

Aerobic organisms (fish and invertebrates) die from lack of oxygen.

Positive feedback

Large quantities of organic material

Aerobic decomposers.

(mainly bacteria) multiply and consume oxygen.

Biological oxygen demand (BOD) is the mass of oxygen consumed by microorganisms in a sample of water. It is determined by measuring oxygen concentration with an oxygen electrode before and after a period of microbial respiration. It shows the oxygen that is not available to more advanced organisms.

Rodation of light for Dodom-growing plants

The

Many garden ponds become eutrophic because of leaves falling into them in autumn. To improve conditions for fish, ponds should be sited well away from overhanging trees

Causes of eutrophication

Unnaturally high levels of nutrients:

from leaching of fertilisers

from input of raw sewage

from liquid manure (slurry)

washed out of farmyards.

Effects

Depleted oxygen levels in water cause death of fish and most invertebrates. High nitrate levels can be dangerous to human babies

Solutions

Treat sewage before it enters rivers (see page 282).

Prevent farmyard drainage entering rivers and ponds. Control use of fertilisers:

- apply only when crops are growing never apply to bare fields

- do not apply when rain is forecast

- do not dispose of waste fertiliser into rivers and ponds. Bubble a stream of air through badly polluted ponds