The powerhouses – freshwater invertebrates

Estimates from studies in Wales suggest that there are well over 250,000 invertebrates for every square metre of river bed – and this figure doesn’t include the phenomenal number of micro-organisms that also occur.

Although we hardly notice them, these are the powerhouse of our rivers, lakes and ponds, turning matter such as living plants and dead leaves into food for fish and frogs, otters and birds.

What kinds of invertebrates?

Because there is little calcium in freshwater in Wales, insects are our most common freshwater invertebrate groups, while molluscs – creatures with shells – for example, are less diverse here than in other parts of lowland Britain. In Wales, insects that need clean water with high levels of oxygen thrive - groups such as dragonflies, stoneflies, mayflies and caddisflies do well.

Special species

Specialities of Welsh rivers include the stonefly Isogenus nubecula, which only occurs in the River Dee and is under threat there.  The beautiful mayfly Potamanthus luteus lives in sandy reaches of the River Wye, which is also home to the native white-clawed crayfish Austropotamobius pallipes.  A few rivers still support small populations of the freshwater pearl-mussel Margaritifera margaritifera, although this is sadly now on the verge of extinction.

Mountain lakes contain rare species, such as the pea-mussel Pisidium conventus and the diving beetle Dytiscus lapponicus that are more typical of high-altitude lakes in northern England and Scotland. In lowland pools, the medicinal leech Hirudo medicinalis and the curious fairy shrimp Chirocephalus diaphanus occur.

Even ditch systems, like those of the Gwent Levels, contain important invertebrates, including the great silver water beetle Hydrophilus piceus, Britain’s largest insect.

Conservation

Freshwater invertebrates react quickly to changes in their habitat and to pollution events, so are good indicators of the health of water quality. From our studies we know that many species are still declining, despite overall improvements in quality. We can only be content with water quality when we know that the invertebrates that live in the water are thriving.