Acid Rain

25 years later, UK rivers are still too acidic for many sensitive species
3 December 2008

Mayflies, freshwater snails and other invertebrates in Welsh rivers are recovering slower than expected, even though the cause of their decline - acid rain - is no longer the problem it once was. Mayflies are extremely sensitive to acid levels in rivers.
The research confirms that conditions in upland British streams are improving, but ecological recovery is marginal and varies with land use.

The 25-year study of 14 rivers around Llyn Brianne, Carmarthenshire, found just four new species of insect - two types each of mayfly and caddisfly - had recolonised the waterways, compared with an expected 29 species. The findings, published today in the Journal of Applied Ecology, indicate that climate is hampering recovery. It seems recent wet winters increase acidity in moorlands and forest streams, offsetting previous gains.
'Rivers are heading in the right direction.' Professor Steve Ormerod, Cardiff University
Lead author Professor Steve Ormerod from Cardiff University says, 'Streams have been acidified enough to cancel out up to 40 per cent of the last 25 years' improvements: climatic effects have clearly worked against our best efforts.'

Ormerod, who has led the project since it began in the early 1980s, adds, 'Since the 1970's, there have been huge efforts to cleanup sources of acid rain, and our research shows that rivers are heading in the right direction.' Insects and invertebrates are extremely sensitive to acid levels. Some of the most sensitive groups are mayflies, molluscs, such as freshwater snails, and caddisflies. Acid rain reached a peak in the seventies and eighties and affected 12,000 kilometres of rivers in Wales. The effect on invertebrates was devastating with species numbers falling in half.

Heavy rainfalls raise acid levels
UK emissions are now 15 per cent of their peak. But soil acidity levels remain high, having fallen just 50 per cent. The capacity of soil to absorb acidity has been depleted through many years of acidification. Heavy winter rainfalls wash sulfur into rivers, quickly raising acidity levels and preventing invertebrate recovery. The researchers assessed the number and variety of stream insects each year, measuring concentrations of acid and other aspects of stream chemistry, and documenting climatic variation such as warmer, wetter winters. The results support the theory that rainstorms still kill sensitive animals. With average acidity in rivers falling, ecologists thought 29 insect species should have recolonised Welsh streams. Among them should be sensitive mayflies and other groups often eaten by trout and salmon. The findings, funded by Defra, the Natural Environment Research Council and others, however, showed a large short-fall in biological recovery. Co-author Dr Isabelle Durance says, 'More and more evidence now shows that some of the worst effects of climate change on natural habitats come from interactions with existing stressors - in this case acid rain. A wider suggestion from our research is that by reducing these other nvironmental problems, we can minimise at least some climate change impacts.'

Artificial solutions have 'few long-term benefits'
The researchers also note that adding lime (calcium carbonate) to soils to reduce acidity had few long-term benefits compared with natural recovery. The findings have implications for conservation practices in the UK. Ormerod says, 'It is very seldom experiments on this scale go ahead. But the findings demonstrate the timescales you need to see the effects of climate change.'

Durance is funded through the Daphne Jackson Trust and the Natural Environment Research Council.