Biological indicators of the quality of the environment

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In this book, various types of pollution have been described which affect the quality of the air, land and water. The pollutants can usually be measured by analytical chemistry and, in Chapter 12, different methods of chemical analysis are described. However, chemical analysis tells you only the amount of the pollutant present and nothing about its effect on the environment. For this, we need biological methods because living organisms are exposed to pollutants and react according to the length of the exposure and their sensitivity to the pollutant. However, before we can assess whether the organisms are affected by a pollutant, we need first of all to look at an unaffected community. This is an essential part of any scientific investigation, whether it be testing a new medicine or a weedkiller: you have to know what is the state of the unaffected environment before you can draw any conclusions about the impact of the pollutant.

When studying pollution of the environment, the usual technique is to look at those species most sensitive to pollution. For example, for assessing air pollution, there are numerous plants and trees that can tolerate a variety of pollutants at quite high levels, as is shown by the numbers of trees and flowers in city centres where the air is of poor quality because of traffic fumes. However, the lichens that grow on stonework are much more sensitive and there are few, if any, in polluted city centres, although they are numerous in clean air areas. The lichens are known as indicator organisms because a study of the number and variety present gives an indication of the quality of the air.

When assessing water pollution, there are various aquatic organisms that can be studied - fish, water weeds, plankton and invertebrates. The variety of fish life, however, is limited and they can often swim away from pollution when it occurs. The water weeds cannot move away to avoid pollution but they usually die off in winter so can be studied only during summer months. The study of plankton is a very specialist subject so they are not considered to be suitable indicator organisms. The invertebrate life is very diverse, they have limited movement and so are exposed to changes in the water quality. Additionally different species have different tolerances to pollutants. In all, they make ideal indicator organisms. They are easy to catch and be identified, and there is a great deal of information available about the effects of various pollutants on them.

The major part of this book has been describing various aspects of water pollution. This is because it is the sector of the environment that is most affected by pollutants. Even when we considered the effects of emissions of acidifying gases into the atmosphere, it was the rivers and lakes which showed the adverse effects. Likewise in the section on solid waste, the main concern is the pollution of water by substances being washed from the waste into drainage water. The agencies responsible for protecting the environment from pollution spend most of their time and effort on water issues and we also will be looking largely at methods of investigating water pollution.

For a biological assessment of the effect of pollution on aquatic life we must first look at an unpolluted environment. All life on earth depends ultimately on the energy from the sun. This energy is absorbed by plants and used, by the process of photosynthesis, to convert carbon dioxide and water into carbohydrates as shown by the familiar equation:

sunlight

This process is known as primary productivity. The organisms that do this are known as autotrophs: they are the primary producers of food in the world and the great majority of all other organisms depend upon them. They convert solar energy into chemical energy. This chemical energy is released when the organism that feeds on the autotrophs breaks down the carbohydrates and uses the energy stored in it for its life processes.

In the aquatic environment, the autotrophs are the water plants, algae and some bacteria but an important part of the primary productivity comes from the vegetation at the edge of rivers such as the trees, bushes and tall plants. The plants that grow at the edge of water are known as riparian plants and the 'bits' that fall off them into the water (particularly in autumn at leaf fall) adds to the productivity of the water. This addition of organic material to the water is called detritus and is an important food source for a variety of aquatic life on which other organisms prey.

The detritus is broken down and consumed by bacteria, fungi and various invertebrates (known as detritivores). The bacteria are essential for the health of the river community because they provide food for other organisms. The bacteria are consumed by protozoa and these in turn are eaten by various invertebrates such as flat worms, crustacea and molluscs (snails). The crustacea and worms are consumed by insect larvae and fish, whilst the fish also eat water weeds and insect larvae. Finally, the fish are preyed on by birds such as herons and kingfishers, as well as by anglers. This whole system of interconnected producers and consumers is known as the food web and, in an unpolluted environment, there is a balance between all the components that make up the web, as shown in Figure

One important factor which determines the types of organisms present in a stream is the habitat, in other words the place where they live. The water, mud, weeds and the surfaces and undersides of stones are all quite

Trophic level 3

(b) Stickleback

(c) Small fish fry

(d) Dipper

(e) Stonefly nymph

(f) Freshwater shrimp

Trophic level 4

Trophic level 2

(g) Blackfly larva

(h) Mayfly nymphs

(i) Blood worm

Trophic level 1 (k) Algae (I) Waterplants (m) Diatoms

Trophic level 3

(b) Stickleback

(c) Small fish fry

(d) Dipper

(e) Stonefly nymph

(f) Freshwater shrimp

Trophic level 2

(g) Blackfly larva

(h) Mayfly nymphs

(i) Blood worm

Stone Fish Food Chain
Figure 38. An example of an aquatic food web

different habitats which are occupied by various species. If we consider a stone sitting on the river bed, there are variations in the speed of currents of water flowing over it as shown in Figure 39.

Figure 39. Variations in the speed of currents over a stone in a stream

Those organisms that live on the surface of the stone have to be able to cling on in the strong current. The mayfly larvae do this by having flattened streamlined bodies and sharp claws, whilst the buffalo gnat larvae spin a web over the stone surface and 'glue' themselves to it with a basal pad of many claws. The undersurface of the stone has quieter water and is occupied by water hog lice and shrimps which crawl around the stream bed and eat detritus. Another habitat is the muddy bed of the river in the slower moving stretches. Here, the invertebrates most suited to this environment are those that burrow into the mud, such as freshwater mussels and midge larvae.

In the balanced community, if one component of the food web increases then it is brought under control by an increase in the number of organisms that feed on that component. Eventually the number of consumers exceeds the food supply, so they die off either because they run out of food or because there is an increase in the number of their predators. For example, in autumn there is an increase in the supply of detritus, so the numbers of bacteria increase which in turn improves the productivity overall in the community. The food supply is reduced when the leaves have been broken down and this then results in an overall decline in the numbers of organisms.

Figure 39. Variations in the speed of currents over a stone in a stream

The balance of the community can be markedly upset by pollution. Pollution was defined in Chapter 1 and included in that definition was 'The discharge... of substances or energy... the results of which are such as to cause. harm to aquatic life.' The discharged substances can be harmful by being toxic or causing an imbalance in the aquatic community. Poisonous discharges will kill the organisms according to their susceptibility to the poison or their ability to escape from the polluted water. There are many toxic substances which can enter streams, such as pesticides, toxic metals like copper and lead, and acids. Most often, though, pollution of water is caused by excessive organic matter. This can originate from farm waste, inadequately treated sewage or industrial waste from, say, a brewery or food factory.

The large input of organic matter provides an abundance of food for bacteria and they multiply at a great rate. The food web is upset and the variety of organisms react to this upset. The large number of bacteria break down the waste and, at the same time, utilize the oxygen dissolved in the water. This depletion of the dissolved oxygen is counteracted by more oxygen dissolving in the water from the atmosphere. This process is helped if the river is fast flowing or there are plenty of waterfalls. However, if the river flow is sluggish or the demand for oxygen by the bacteria exceeds the input from the atmosphere, the dissolved oxygen will be in short supply. Those organisms sensitive to depleted oxygen levels will move away by either swimming (fish) or drifting (invertebrates), whilst some will be killed. Other organisms though have adapted to fluctuating oxygen levels. In particular, there are some invertebrates which have haemoglobin in their bodies and are able to store oxygen for use in times of shortage. Examples of these are the midge larvae (also known as blood worms because of their red colour) and tubificid worms.

So, in rivers that are badly polluted with organic waste, the invertebrate population is dominated by masses of worms and little else, whilst a clean river has a wide variety of organisms with no one species dominating. In between these two extremes there is a gradation of organisms that have different sensitivities to oxygen depletion. In a river which is only mildly polluted and which is fairly fast flowing, the input of extra organic matter increases the biomass (the total number of organisms). Those invertebrates that consume organic matter thrive in the favourable conditions because of the extra food supply, so in these waters there are plenty of shrimps and water hog lice as well as the worms.

Freshwater biologists use this variation to assess the quality of water.

They collect representative samples of the invertebrate community and examine the number and variety that are present. From this examination they can produce a Biotic Index, which is a number expressing the water quality: the higher the number the cleaner the water. One of the most regularly used Biotic Indexes in the UK was devised by a group of biologists and is known as the Biological Monitoring Working Party (BMWP) score. In this scheme, each invertebrate family is assigned a number depending on its sensitivity to pollution: a stonefly larva requires clean water with a high concentration of dissolved oxygen so has a score of ten, whereas a blood worm will have a score of one. The scores for all the various families present are added to produce the BMWP score. Table 23 gives examples of the invertebrate populations, with their Latin names,

Table 23. Invertebrate families and BMWP scores for four rivers of different levels of pollution

River Stretch BMWP Family Score sampled score

Kittoch Water downstream East Kilbride

27 Baetidae (mayfly larvae) Simulidae (blackfly larvae) Chironomidae (midge larvae) Gammaridae (shrimp) Asellidae (water hog louse) Erpobdellidae (leech) Lymnaeidae (snail) Oligochaeta (worm)

River Kelvin near source

50 Baetidae Simulidae Gammaridae

Hydropsychidae (caddis fly larvae) 5

Lymnaeidae

Tipulidae (crane fly larvae) Limnephilidae (snail) Muscidae

(house fly type larvae) (no score)

Planariidae (flat worm) Hydrobiidae (snail) Physiidae (snail) Sphaeriidae (bivalve snail) Oligochaeta

River Irvine middle reaches 100 Baetidae

Chironomidae

Gammaridae

Lymnaeidae

Table 23 (continued)

River Stretch

BMWP Species Species

sampled

score score

River Irvine (continued)

Tipulidae

5

Planariidae

5

Asellidae

3

Hydrobiidae

3

Physidae

3

Oligochaeta

1

Erpobdellidae

3

Leuctridae (stonefly larvae)

10

Perlodidae (stonefly larvae)

10

Ephemerellidae (mayfly larvae)

10

Heptageniidae (mayfly larvae)

10

Dytiscidae (beetle)

5

Elmidae (beetle)

5

Hydracarina (mite) (no score)

Glossiphoniidae (leech)

3

Ancylidae (limpet)

6

Sphaeriidae (mussel)

3

River Doon middle reaches 132 Baetidae

4

Chironomidae

2

Gammaridae

6

Asellidae

3

Erpobdellidae

3

Tipulidae

5

Limnephilidae

7

Simuliidae

5

Leuctridae

10

Oligochaeta

1

Ephemerellidae

10

Leptoceridae

10

Heptageniidae (mayfly larvae)

10

Caenidae (mayfly larvae)

7

Ryacophilidae (caddis fly larvae)

7

Glossosomatidae

(cased caddis fly larvae)

7

Hydropsychidae (caddis fly larvae)

5

Brachycentridae

(cased caddis fly larvae)

10

Empididae (fly larvae) (no score)

Rhagionidae

(snipe fly larvae) (no score)

Elmidae (beetle)

5

Aphelocheiridae (saucer bug)

10

Hydrobiidae (snail)

3

Ancylidae

6

Sphaeriidae

3

for four rivers in Scotland with different degrees of pollution.

The examination of invertebrate stream life need not be as complicated as this. A great deal of skill and experience are required to identify all the different invertebrate species in order to produce a biotic score. A rough estimate of the water quality can be obtained by collecting a representative sample of the stream bed community and seeing which groups are present and which are absent. You will notice in Table 23 that the higher the biotic score, the greater the variety of invertebrates present.

These results are all for rivers in Scotland where the upper limit for BMWP scores is close to 200. In the chalk rivers of southern England, there is an even greater diversity of invertebrate life and the maximum BMWP scores can be much higher.

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Responses

  • sinit
    What is the trophic level for crane fly nymph?
    8 years ago
  • michael
    What trophic level is a bloodworm?
    8 years ago
  • brigida
    What is a worms trophic level?
    8 years ago
  • dawit
    Why are biological indicators of water quality so important?
    8 years ago
  • nazzareno
    How are hog louse indicators of water quality?
    8 years ago
  • eberardo
    How are gnats part of the aquatic food chain?
    8 years ago
  • adamo
    How are limpets sensitive to pollution biological indicators?
    8 years ago
  • dorotea
    What are indicator organism of Air pollution?
    8 years ago
  • Alfrida
    Are mayfly, water shrimp ,water beetle,water snail and worm biological indicators?
    6 years ago
  • VELI-MATTI
    Are water hog louse indicators of poor water quality?
    6 years ago
  • jorma
    What are the indicators of a badly polluted river?
    6 years ago

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