A study in England has found that freshwater macro-invertebrate richness has increased over the past 30 years, indicating a positive trend in biodiversity.

Aquatic macro-invertebrates are insects in their nymph and larval stages, that spend at least part of their lives in water. They play a large role in freshwater ecosystems by recycling nutrients as well as providing food for other species.

According to researchers, the recovery of pollution-sensitive macro-invertebrates, such as Ephemeroptera, Plecoptera, and Trichoptera families, suggests water quality is improving. The study, a three-year £2 million project using data gathered by the Environment Agency, analysed a dataset of over 220,000 macro-invertebrate records from 1989 to 2018 and matched them with factors such as wastewater exposure and land cover.

The study found an improvement in macro-invertebrate diversity and sensitivity across all regions, river types, land covers, and levels of wastewater exposure. However, the rate of improvement has slowed in some cases post-2003.

The increase in macroinvertebrate richness suggests that water quality has improved, likely due to more effective wastewater treatment. This improvement has been linked at least in part to improving water quality associated with more effective wastewater treatment, driven by the European Urban Waste Water Directive.

What are macro-invertebrates?

Macro-invertebrates are a part of virtually every freshwater ecosystem in the world, even those seemingly inhospitable to life. There, they form the base of the aquatic food chain, serving as a significant food source for other animals, including amphibians, birds, reptiles, and fish.

Macro-invertebrates also break down both living and decaying plant material in freshwater ecosystems, providing a critical link in the transformation of plant material into forms of energy that can be consumed by other animals in aquatic ecosystems.

How are they used in environmental monitoring?

Freshwater macro-invertebrates are a valuable measure of human impact on the environment due to their natural biological diversity, multiple positions in the food web, range of tolerances to environmental changes, relative ease of monitoring, and because their lives remain closely tied to a specific location.

It is common practice to survey macroinvertebrates before infrastructure projects, especially those that involve dredging and the construction of power plants or sewage treatment plants.

Does this mean English rivers are clean?

Although this study does not imply that every region, catchment or reach of every river in England is in an ideal state, the findings challenge the perception of continual biodiversity decline in freshwater environments and highlight the potential for recovery when human pressures are addressed.

The study, 'Significant improvement in freshwater invertebrate biodiversity in all types of English rivers over the past 30 years' is available to read in the journal Science of The Total Environment.