3.5.5 Biological indicators
A great many biological species and individuals occur in normal streams. These often differ markedly in their sensitivity to environmental factors, and likewise the tolerances of various species to different types of pollution vary considerably. The major groups of organisms that have been used as indicators of environmental pollution include bacteria, fungi, protozoa, algae, higher plants, macroinvertebrates and fish. The benthic 'bottom living' macroinvertebrates are particularly suitable as ecological indicators because their habitat preference and relatively low mobility cause them to be directly affected by substances that enter their environment.
When a clean river is polluted there are several events which can occur. There can be:
a decrease in the number of species of organisms;
a change in the type of species present;
a change in the number of individuals of each species present.
These changes are due to the death or the moving away of organisms which cannot tolerate the pollution and an increase in number of those organisms which thrive on it.
The Biological Monitoring Working Party (BMWP) score can be used as a measure of the pollution at a particular site. A visual examination is made of a river sediment sample from the site and a total score is calculated from the organisms found. The more sensitive organisms (e.g. the stonefly) have high scores while the more tolerant species (e.g. the worm) have low scores. The total score gives an indication of the degree of pollution at that site. Table 3 gives typical scores developed for different organisms. In calculating the total score, each family can only score once no matter how many organisms of that family are present, and only one needs to be present to score. Figure 16 shows examples from the groups of organisms listed in Table 3.
Table 3 Simplified BMWP scores
|Mayfly nymphs (e.g. Ephemeridae, Ecdyonuridae); stonefly nymphs (all families)||10|
|Damselfly and dragonfly (all families); freshwater crayfish (Astacidae)||8|
|Mayfly nymphs (Caenidae only); cased caddis larvae (all families); caseless caddis larvae (e.g. Rhyacophilidae)||7|
|Freshwater shrimp (Gammaridae); freshwater limpet (Ancylidae)||6|
|Water bugs (all families); water beetles (all families); caseless caddis larvae (Hydropsychidae only); fly larvae (Simuliidae, Tipulidae); flatworm (all families)||5|
|Mayfly nymphs (Baetidae only); alderfly larvae (Sialidae)||4|
|Snails (e.g. Lymnaedae, Planorbidae, Physidae); freshwater bivalves (Sphaeriidae); leeches (all families); water hog louse (Asellidae); water mites (all families)||3|
|Fly larvae (Chironomidae only)||2|
|True worms (all families)||1|
|Fly larvae (e.g. Culicidae, Tabanidae, Chaoboridae)|
A benthic sample taken from a river contained the following:
mayfly nymphs from the Ephemeridae and Baetidae families
3 stonefly nymphs from different families
2 cased caddis larvae from different families
freshwater shrimps (Gammaridae)
freshwater limpet (Ancylidae)
alderfly larvae (Sialidae)
3 snails from different families
What is the BMWP score of the above sample?
|Stonefly nymphs||3 families||10 × 3 = 30|
|Cased caddis larvae||2 families||7 × 2 = 14|
|Snails||3 families||3 × 3 = 9|
The relatively high score and the presence of clean water indicators (i.e. sensitive species) shows that the river is clean.
The BMWP score can be used both as an instantaneous measure of pollution and as a gauge over a period of time to ascertain how the situation at a particular site is changing. Comparisons can also be made with sites in other parts of the country, providing the same scoring system is used.
The main advantages of biological indicators over chemical methods of assessing water quality are:
The organisms show a lasting response to intermittent pollution events which may be missed during sampling for chemical analysis.
The biological community of a stretch of river responds to all pollutants both known and unknown, including those which may not have been considered in routine chemical analysis.
One disadvantage in the interpretation of all such scores is the assumption that they reflect changes attributable to pollution and not to other factors. Yet there are various environmental factors other than pollution that can have a considerable effect on the composition of communities of aquatic invertebrates, the chief ones being the nature of the stream bed and the velocity of the water flow.
The invertebrate community present in fast-flowing sections is very different both qualitatively and quantitatively from that present in slow-flowing sections. It is possible to overcome this problem to some extent by sampling in stretches of river that are closely similar in terms of the substratum, flow rate and other variables not connected with pollution.