Research and analysis

An analysis of national macroinvertebrate trends for England: 1991–2019 - summary

Published 22 October 2021

Applies to England

1. Chief Scientist’s Group report summary

This report presents national-scale trends in the macroinvertebrate communities of English rivers over 29 years (1991–2019).

Macroinvertebrates are monitored routinely as they are very good indicators of environmental quality. Understanding national invertebrate trends helps the Environment Agency to evaluate the effectiveness of management actions including changes to legislation, improved river quality standards and river restoration measures, against the impact of changing environmental pressures such agriculture, climate change, water abstraction and new chemicals.

2. Approach

River invertebrate trends were analysed at approximately 3,500 locations across England focussing on i) the number of invertebrate families present in a standard sample (the richness) and ii) the range of families present (using a score in which higher values indicate the presence of pollution-sensitive families such as mayflies and stoneflies). Bias caused by the under-representation of some types of water course in the dataset, such as headwaters, was reduced by calibrating against the River Habitat Survey Baseline of England (2007–2008) which sampled more evenly across the full river network. The invertebrate trends were examined in relation to several environmental factors such as water chemistry and whether catchments were urban or rural.

3. Results

The main results were:

1. The average number of invertebrate families present increased by ~11%. This increase was restricted to the first half of the period investigated: the increase was rapid in the early 1990s, but slowed in the second half of the decade, with no overall increase after about 2005.
2. The prevalence of pollution-sensitive invertebrates increased from 1991 to 2019, with observed fluctuations around this trend likely to reflect variation in climate.
3. Rivers draining urban catchments improved most, but still do not match the quality of more rural areas. Increases in the richness of urban rivers stalled after ~2010 (later than the overall national trend).

4. Conclusions

The changes in the invertebrate communities are consistent with ongoing improvements in water quality, although this study does not establish a causal relationship. The reason for the increase in richness stopping is unclear: this could be a natural mechanism or indicate some form of environmental pressure.

The improving trends are the opposite of what was expected because of climate warming, suggesting that improvements in water quality have continued to offset some of the impacts of climate change.

Whilst overall trends indicate national-scale improvements, they may mask local declines in quality.

These findings will inform management actions and future investigations undertaken by the Environment Agency and its partners to target management actions and measures to adapt to climate change.

5. Project details

This summary relates to information from the following project:

• Title: An analysis of national macroinvertebrate trends for England, 1991–2019.
• Project manager: John Murray-Bligh, Ecology and Ecosystems
• Research contractor: Emma Pharaoh, Steve Ormerod and Ian Vaughan, Water Research Institute and School of Biosciences, Cardiff University

This report is published by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

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