Why get to know your pond's smallest residents?

A healthy garden pond is a busy place, and most of the activity happens at a scale that's easy to miss. Spend ten minutes with a white plastic tray, a cheap magnifying glass and a jam jar, and you'll start meeting creatures that tell you a great deal about how well your pond is working. Different invertebrates tolerate different conditions, so what you find is a rough report card on water quality, planting and shelter.

The good news for beginners is that you don't need to identify everything to species. Getting to know eight or ten common groups is plenty, and it turns a muddy scoop into a genuine bit of wildlife watching.

How to look without causing trouble

Check your pond gently, ideally between April and September when most creatures are active. Take a small sample from the shallows, near the edges among the plants, rather than plunging into the middle.

• Use a kitchen sieve or a small net, and sweep slowly through the water rather than dragging it along the bottom.
• Tip your catch into a shallow white tray with a little pond water. Pale trays make dark creatures far easier to see.
• Work quickly and keep the tray in the shade. Return everything to the pond within twenty minutes, close to where you found it.
• Never move amphibians, fish or plants between ponds. You can carry disease and invasive species with them.
• Wash and dry your net and trays after use, especially if you visit another pond.

The surface crowd: pond skaters and water boatmen

These two are the ones most people meet first, and they're easy to tell apart once you know what to look for.

Pond skaters sit on the surface film and skate across it in short, jerky bursts. They have six long legs, a slim brown body, and they're hunting for insects that have fallen in. Look for the dimples their feet make on the surface. They're harmless to you and completely unable to bite through skin, despite appearances.

Water boatmen swim below the surface, rowing along with long hind legs that look like oars. They are pale brown with a keel-shaped back, and they swim the right way up. If you spot something similar swimming on its back, pale-bellied and rather larger, that's a backswimmer. Both are predators of tadpoles and small invertebrates, and both can give a nip if handled carelessly, so use a spoon or let them swim onto your hand rather than grabbing them.

A third surface oddity worth knowing: whirligig beetles, which spin in frantic circles in groups. Their eyes are split into two halves, one looking up and one down.

Creepers and clingers: snails, leeches and flatworms

Snails are the easiest group to start with because they're slow and easy to see. Ramshorn snails have a flat, coiled shell like a ram's horn and are usually found on plant leaves. Great pond snails are bigger with a tall, pointed shell. Freshwater limpets look like tiny grey cones stuck firmly to stones.

Less appealing to some, but fascinating: leeches move with a looping, measuring-worm gait and can contract into a blob. The common garden pond species feed on worms and snails rather than people, and they're a good sign of a pond with plenty of organic matter. Flatworms are flatter still, glide smoothly rather than looping, and often have a distinctive triangular or arrow-shaped head with two eyespots. A flatworm gliding across the bottom of your tray is a lovely thing to watch under a lens.

Also common: water lice (Asellus), small grey relatives of woodlice that scuttle rather than swim, and water mites, which look like tiny red or brown beads walking about.

Larvae and nymphs: the young of flying insects

Many of the creatures in your net are the aquatic young of insects you'll see on the wing in summer. These are the most rewarding group to learn.

• Caddisfly larvae are unmistakable. Many build portable cases from twigs, sand grains or bits of leaf and drag them along, poking out a dark head and legs. Others spin silken nets. Their presence suggests clean, well-oxygenated water.
• Damselfly nymphs are slim and elegant, with three flat, leaf-shaped tails at the rear. They stalk prey through the weed with a hinged lower jaw that shoots out to grab it.
• Dragonfly nymphs are stouter, mottled brown, and have no long tails. They tend to sit and wait rather than hunt actively.
• Mayfly nymphs also carry three tails, but have gills along the sides of the abdomen rather than just at the tip. They're sensitive to pollution, so a good find.
• Bloodworms are small red threads, actually midge larvae. They're tolerant of low oxygen and thrive in ponds with leaf litter.

A beginner's rule of thumb

One or two groups tells you very little, but the overall mix is informative. A pond with mayfly nymphs, caddis larvae, damselfly nymphs and a good range of snails is usually in fine condition. A pond dominated by bloodworms, water lice and nothing else often has too much rotting vegetation and too little oxygen.

If that sounds like yours, don't panic. Add oxygenating plants, thin out blanket weed by hand in summer, and leave the leaves out of the water where you can. Then check again next spring, and enjoy watching the residents change.