Ask anyone to name an animal you would find living in a Scottish river or loch and they’d probably say some sort of fish, maybe a frog or toad, possibly an otter. Very few people would give much thought to the hundreds of tiny water beasties that live, hidden for the most part, beneath the surface. But it is these water beasties, or invertebrates as they are known, that play an essential role in maintaining our aquatic systems and help us to protect the water environment.

A huge variety of water invertebrates live in our burns, rivers, lochs, canals and ponds; from the more easily recognisable snails, worms and freshwater shrimps, to the slightly alien looking mayfly and stonefly larvae. All are fascinating in their own right. Some spend all their lives in the water, while others are only there as juveniles or adults. Some are voracious predators hunting smaller water beasties, while others have adapted their own unique ways to catch food like the caddisfly larvae, which spins a net to trap food as it floats past. Some are common in rivers across the UK, while others are of international conservation importance like the endangered freshwater pearl mussel, with half of the worldwide breeding population found in Scottish waters.

Regardless of shape or size, what they eat, how long they live in the water environment and whether they are rare or not, all invertebrates are important in maintaining aquatic ecosystems. Plants and algae grow in the river and some invertebrates graze directly on these. For others, coarse materials like leaves and wood fall into rivers providing a vital food source. Some invertebrates are able to shred and eat this material and in the process break it down into smaller bits. Other animals can then eat these smaller particles using specialised mouthparts or, for the really tiny particles, filter them from the water. This process means nutrients entering rivers and other water bodies are able to be used within the system to support large numbers of invertebrates. These in turn are food for other invertebrates and larger animals such as fish, birds and even bats, which hunt for insects that have emerged from the water. Without water invertebrates the scope for life in our rivers and lochs would be limited.

Not only are water invertebrates essential in maintaining life in our water environment but they can help us to protect it too. Invertebrates provide us with valuable information about how healthy our rivers and lochs are and we can use them as a means of assessing the quality of our freshwaters. Some species, such as the stonefly, are extremely sensitive to pollution and can only live in clean water. Others, like worms, can tolerate highly polluted conditions. In between these two extremes there are a range of species with different degrees of sensitivity. Some prefer rivers with clean gravel and hardly any silt, while others thrive in more sluggish, silty conditions where silt has accumulated. This could be due to excess run off from the land or as a result of low flow rates that are unable to flush the silt downstream.

By taking a sample of the invertebrates living in a river and identifying what species are present we can get a quick and accurate picture of the river’s health. It also helps us to decide whether other changes, such as siltation, are affecting the life in the river. If we find pollution sensitive species and those that like silt free conditions, we can be confident that the water quality is good. If, on the other hand, all we find are those species that are tolerant of pollution or like silty conditions, we can assume that the water environment has become polluted or affected in some way.

Sampling and monitoring the invertebrates in our rivers and lochs has formed the basis for assessing the quality of freshwater in the UK and worldwide for many years. We have a network of hundreds of sites across Scotland that our ecologists regularly visit to take samples. Along with other information recorded at the site, such as the colour and clarity of the water and the condition of the banks, we can judge the quality of the river against a ‘reference level’ - the ideal state of a river where there is no human impact other than what you would expect to find. We can then grade the river, bad, poor, moderate, good or high, based on how close it is to the reference condition. This valuable information then helps us target our action to protect and improve Scotland’s water environment.

So the next time you look at a river or loch, give a thought to the water beasties living beneath the surface – there’s more to them than meets the eye.

Read more about SEPA's ecologists and the work they do in Scotland's rivers.