Hydrosphere Contents

5. Aquatic Habitats

Pollution levels in rivers can be monitored by studying the micro-organisms living in the water

Many living organisms cannot tolerate polluted conditions and must either move to cleaner waters or they will die when a stretch of river becomes polluted. Some living organisms can withstand much greater levels of pollution, with some able to persist even in highly polluted waters. Aquatic macro-invertebrates (all invertebrate species able to be seen with the naked eye) are excellent for assessing the level of pollution in a water body - they tend to be ubiquitous and abundant in most waters and they are not extremely mobile - and thus can act as biological indicators.

There are many different indices which can be used for the biological assessment of water quality in the field, including the Trend Biotic Index, the River Invertebrate Prediction and Classification System (RIVPACS) and the BMWP (Biological monitoring working party) index. The principle of these assessment indices is that the number of each organism in a sample of water is counted, and the total scores for each are then calculated. A high overall score indicates a low pollution level in the water, and a low overall score indicates a high level. A downward trend in water quality and in the condition of the biological communities is a warning of a potential lack of sustainability.

Micro-organism Index Score
Mayfly, Stonefly, Caddisfly larvae 10 (intolerant to pollution)
Crayfish, Dragonflies 8
Caddisflies *  
Stoneflies * 7
Caddisflies *  
Snails, Mussels, Shrimps, Dragonflies Caddisflies * 6
Water bugs, Water beetles 5
Caddisflies *, Craneflies, Blackflies, Flatworms Alderflies, Leeches 4
Cockles, Hoglouse 3
Midges 2
Worms 1 (tolerant to pollution)
   
* different families  

The BMWP index for Biological Assessment

The lake or river water is sampled, and the indicator species are counted

By totalling the BMWP index for each organism, an overall score can be calculated for the water.

A low score indicates a high pollution level in the water.

A high score indicates a low pollution level in the water.

How does the Freshwater Indicator indexing system determine water quality?