Insects are thriving in England's rivers after fall in metal pollution

The diversity of insect and other invertebrate life in England’s rivers is better than at any time over the past 30 years, according to an analysis of Environment Agency monitoring data. This improvement seems to be linked to a reduction in copper and zinc levels in the water.

Andrew Johnson at the UK Centre for Ecology & Hydrology and his colleagues analysed data from samples collected using nets at 1515 sites around England between 1989 and 2018. Each macroinvertebrate – any animal without a backbone that can be seen without a microscope – was identified by what family it belongs to, rather than a particular species.

The results show that the diversity of invertebrate families in English rivers has steadily increased over the past 30 years in both urban and rural rivers. “You could argue our rivers are our greatest environmental success story since the [second world] war,” says Johnson.

The researchers looked at 45 different variables, including chemical levels and physical factors such as temperature, and used a model to determine which had the biggest influence on insect diversity. Their preliminary analysis found that reductions in zinc and copper levels were the trends most frequently linked with increases in invertebrate diversity.

Metals, including zinc and copper, can affect the growth of insects and disrupt their ability to reproduce. Zinc and copper can accumulate in wastewater from a range of sources, such as soaps, meat and shellfish, says Johnson, who reported the findings at the annual meeting of the Society of Environmental Toxicology and Chemistry Europe in Dublin, Ireland, on 1 May.

He says the improvement in zinc and copper levels in rivers coincided with the privatisation of water firms in England and the introduction of European Union regulations regarding wastewater, which required companies to remove more contaminants before releasing wastewater into rivers.

Copper and zinc levels also fell dramatically following the decline of coal burning in England, says Johnson. “This also stopped acid rain, which can mobilise metals and push them into rivers,” he says.

However, chemical pollution in general does seem to be an issue in England’s rivers, and data shows that some animals are declining, including salmon and eels, says Johnson.

Michelle Jackson at the University of Oxford says that she has seen an improvement in invertebrate diversity in the Thames catchment area in London in research that is currently being peer-reviewed.

Tom Oliver at the University of Reading, UK, notes that a study published last year, which he was involved in, found that the number of freshwater invertebrates in England had increased in recent years. “But there is also a substantial variation across locations, whereby species trends differ markedly between rivers and regions,” he says. “This means that high-resolution monitoring, across both space and time, is essential in order to track the responses of our freshwater biodiversity.”

Improvements in water quality are probably the reason for the positive trend, but the rate of improvement has slowed in the past four years and it is unclear what effect this has had on invertebrate biodiversity, he says. “This warrants further investigation into how these recent changes in water quality are impacting freshwater species.”