The hidden legacy of acid rain

Thankfully, emissions of the pollutants that cause acid rain have plummeted since the 1970s - by 98% for sulphur dioxide and 78% for nitrogen oxides in the UK. When these emissions enter lakes and rivers in rainfall they turn the waters toxic to many species.  

But freshwater ecosystems have been slow to improve in response to this dramatic clean up. Some remain deeply damaged today. 

Alan Hildrew (Emeritus Professor of Ecology, School of Biological and Behavioural Sciences) has studied damaged freshwater ecosystems since the 1970s.  

Hildrew helped establish the UK Acid Water Monitoring Network, a valued resource for environmental policymakers and managers since 1988. Its data reveal how nature responds - or not - to changes in pollution.  

Today, Queen Mary’s River Communities Group, led by Professor Iwan Jones, expands on the acidification work that Hildrew began in the ‘70s, enriching the unique dataset that supports a brighter future for biodiversity. 

Research

Alan Hildrew co-established the UK Acid Water Monitoring Network in 1988, now called the UK Upland Waters Monitoring Network and coordinated by UKCEH. Its consortium of biologists, chemists and environmental managers from Queen Mary, CEH, UCL and Marine Scotland investigate the chemical and ecological impact of acid deposition across 23 sensitive lakes and streams.

Iwan Jones’s research team has led Queen Mary’s contribution since 2020. Using established methods, the group samples and identifies a wide spectrum of species, including insect, algae and plant species, annually at the sites, expanding the programme’s datasets yet further.  

Effective air pollution targets

The UK Upland Waters Monitoring Network provides policymakers with evidence to help evaluate the impacts of emissions reductions policy on freshwater ecosystems and set new targets. The Environment Agency describes its datasets as of “the highest quality” for this purpose.  

Historically, Defra used the consortium’s research, based on the Network’s data, to argue for tighter limits on acidifying pollutants under the Gothenburg Protocol. This landmark UN Treaty and major environmental success story was subsequently revised in 2012. The researchers’ discovery that acidic waters trigger hard-to-reverse damage in food webs strengthened Defra’s call for even more ambitious targets. 

Since 2020, the Environment Agency has contracted Jones’s River Communities Group to conduct ecological monitoring for the Network. The UK Government uses the data to determine whether current pollution limits under the National Emission Ceilings Regulations are safe. The research indicates that they are, indeed, appropriate as they do not damage the Network’s sensitive ecosystems.  

Nature's recovery

Today, acid rain is probably at its lowest levels since the early stages of the Industrial Revolution. This is thanks to the ambitious policies set by the UN, the EU and individual countries, including the UK, and which are supported by the UK Upland Waters Monitoring Network’s data.  

This dramatic change is making a difference to nature. In 2019, the Network reported that  19 of its 23 sites have become far less acidic. Among these, 14 have much healthier biodiversity. In general, the greater the recovery of the lake or stream, the greater the biodiversity. 

Other scientists have reported similar improvements in North America and north-western Europe. These changes “serve as a demonstration of what can be achieved where there is a common international resolve to mitigate an environmental problem.” (UK Upland Waters Monitoring Network, 2019).  

How else can we support lakes and rivers? 

Not all of the UK Upland Waters Monitoring Network’s 23 sites are in a healthy state. Several barriers may be hindering straightforward recovery. Among these is climate change.   

The Network, thus, has policy value beyond air pollution. As the effects of acidification decline, the Network’s data help paint a picture of freshwater biodiversity in the face of a changing climate.  The researchers have installed temperature loggers and level gauges to investigate climate change’s impacts on the sites, giving the Network increasing pertinence to wider biodiversity policies, climate change policies and Net Zero strategies. 

 Elsewhere, the Network has helped Forest Research, the Forestry Commission’s research agency, understand the impacts of commercial forestry on upland waters and led to more protective practices by the forestry sector.