Many invertebrate animals live in ponds, lakes, streams and rivers. Where they live depends on how polluted the water is and how much dissolved oxygen is present.
You can’t tell how polluted water is by just looking at it, but by sampling and analysing the invertebrate population you can soon get a good idea of the level of pollution through using indicator species. Indicator species are species that are particularly sensitive to a specific environment. By examining the types and abundance of the species present in a body of water, you can get a good idea of how oxygenated and how polluted it is.
Changes in water quality are not always noticeable as they can take place slowly. A stream may look the same from one year to the next but could have lowered oxygen levels. So when studying water pollution, samples of indicator species need to be taken over a period of time. By drawing graphs of the results, you can easily see any trends and monitor improvements or deteriorations over a period of years.
The following 6 invertebrates are pollution and oxygen sensitive.
1. Stonefly Nymph
This prefers to live in fast-moving water in amongst the gravel and stones. It requires high oxygen levels and cannot survive in places with anything more than a trace of pollution. They are usually herbivores or detritivores, feeding on fresh or decaying vegetation. Later in their life cycles, many become omnivorous or even carnivorous.
2. Mayfly Nymph
Like the stonefly nymph, this invertebrate cannot tolerate any pollution and needs water with high oxygen levels. It too feeds on fresh or decaying vegetation, though some larger species are carnivorous. It prefers shallow cool water such as the edges of lakes and streams and burrows into the sediment.
3. Freshwater Shrimp
The freshwater shrimp likes high levels of oxygen, flowing water with pondweed growing on the rocks and stream bed, and can tolerate some pollution. It feeds on fresh or decaying vegetation, including what is termed the biofilm — microbial growths made up of bacteria, fungi, algae. It swims in bursts when disturbed or caught in the current.
4. Water Louse
If the previous species are absent, but you discover water lice living in your pond or stream, then the water is moderately polluted. These are related to woodlice and live and feed on decaying materials on the bottom.
5. Rat-Tailed Maggot
This odd looking maggot is equipped with a breathing tube through which it can obtain the oxygen it needs to survive as it feeds on decaying matter in highly polluted or stagnant water with low oxygen levels.
This worm, as its name suggests, lives in burrows in the sludge that collect at the bottom of heavily polluted water with little oxygen. It is an annelid worm and is related to the more helpful earthworm. They are found in the most stagnant water and can tolerate high concentrations of heavy metals, high and low pH and high salinity.