Water Pollution

In this investigation, pupils use pond nets to take samples of invertebrates living in three micro-habitats, or three unique habitats (e.g. pond, river, canal). After identifying the animals they have found, pupils calculate a pollution score using the animals as bioindicators.

Pollution is the addition to the environment of something which causes harm. We can measure the amount of pollution in water by calculating a Biotic Index, which works by giving each type of animal a score. Animals which are sensitive to pollution have a high score, while animals that are tolerant of pollution have a low score.

Learning outcomes

Through this activity pupils will:

use sampling to compare and contrast invertebrates living in three contrasting habitats
calculate a pollution score for each habitat

Resources to download

Background information for teachers

Adaptations

Animals that live in water have different ways of obtaining oxygen

Across the body: oxygen in the water goes through the skin and into the body e.g. flatworms, leeches, snails, water mite, midge larvae
Gills,   e.g. water flea, mayfly nymphs, caddis fly larvae, damselfly nymphs, dragonfly larvae, freshwater shrimps
Snorkels  : some animals have a tube which gets oxygen from the surface e.g. water stick insects and water scorpions
Scuba divers  : some animals have hairy bodies which hold bubbles of air collected from the surface. They take the air down with them and breathe through spiracles,  e.g. water beetles and water boatmen

Characteristics of different freshwater animals

Leeches (Phylum Annelida, Class Hirundinea)

Relatively few leech species are parasitic and blood-sucking.
Many are scavengers or predators.
They use their suckers to attach themselves to the substrate, and move by looping their body with a series of muscular contractions.

Water fleas (Class Crustacea)

Water fleas feed on algae.
They float in the water of ponds and streams; their bristles slow them down.

Freshwater shrimps (Class Crustacea)

Shrimps can be found in vast numbers. They are detritivores.
Shrimps are flattened in shape and swim on their sides. This slows them down in moving water and allows them to hide underneath stones.
Shrimps have a moderate tolerance to water pollution.

Freshwater Hoglouse (Class Crustacea)

Hoglice are detritivores that eat dead or decaying animal and plant material.
They do not have particularly streamlined bodies, so are less common in moving water than in ponds.
They are more tolerant of pollution than freshwater shrimps.

Mayflies (Class Insecta, Order Ephemeroptera)

Mayflies are the only insect where the final nymph stage can fly (called ‘duns’ by anglers). The duns fly out of the water and turn into adults. Adults are short-lived (sometimes only one day) and do not eat.
Mayfly nymphs are flattened in shape for hiding under stones, and are flat enough to remain in the boundary layer around the stone where the current is slower.
The nymphs have one hook at the end of each leg appendage and use these for clinging to the substrate
Nymphs feed mainly on dead vegetation.
Mayflies are normally found in clean, well-oxygenated streams, and are sensitive to pollution, although one type of swimming mayfly is commonly found in ponds.

Dragonflies and Damselflies (Class Insecta, Order Odonata)

Dragonflies are generally the larger. Adult dragonflies rest with their wings outstretched; damselfly wings are folded.
Nymphs have an ovipositor that they pierce aquatic plants, logs and wet mud etc. with and lay the eggs. Eggs are laid singly. The nymphs clamber among stems and are quite inconspicuous.
The nymphs are voracious carnivores. To catch prey they have an elongated labium hinged in the middle and folded back under the thorax. It has grappling hooks and spines at the tip. The muscles are capable of extending it with lightning speed. It is thrown forward and opened by a single movement; it closes on the victim and is withdrawn. The gills are upon the tip of the abdomen.

Caddis flies (Class Insecta, Order Trichoptera)

Caddis flies have larvae instead of nymphs. There are two types. Cased caddis fly larvae live in cases made of twigs, leaves and small stones. The cased caddis fly larva case provides a weight to keep it in the stream bed. The case also provides protection from damage by current. Caseless caddis fly larvae do not make cases. Instead they spin a net of silk which is attached to stones. Invertebrates are washed downstream into it and are consumed. This relies on moderate current flow.
Larvae have filamentous gills, usually along the abdomen.
Cased caddis fly larvae are herbivores; they graze algae. Some caseless caddis fly larvae are carnivores.

Beetles (Class Insecta, Order Coleoptera)

Beetles are a large group found in all habitats except polluted water.
Diving beetles are carnivores. They traps air beneath their wings and renew it by visiting the surface. They have paddle-like legs that are often fringed by hairs to allow them to move swiftly through the water.

Flies (Class Insecta, Order Diptera)

Fly larvae can survive in most conditions, even in heavily polluted water.
Phantom midge larvae, rat-tailed maggots and cranefly larvae are all carnivores.
Non-biting midge larvae are detritivores. They live at the bottom, where there is little oxygen, so they contain haemoglobin and are bright red in colour (often known as bloodworms).They can live in heavily polluted water.
Rat-tailed maggots live in mud in ponds and ditches. They have a telescopic breathing tube which allows them to take in air from the water surface even if the water level changes.