Climate change is responsible for the limited recovery of rivers which have suffered from acid rain, biologists have said.
Research carried out over the last 25 years by Cardiff University biologists suggests average acidity has fallen due to improvements in the levels of acid rain.
Fourteen streams around Llyn Brianne in mid-Wales were analysed. Up to 29 insect species like sensitive mayflies and other groups often eaten by trout and salmon should have re-colonised them.
Instead they found only four new species, a development blamed on high acid levels seen during periods of intense rainfall.
Professor Steve Ormerod said acidified upland streams were enough to cancel out up to 40 per cent of improvements in the last 25 years.
"Since the 1970s, there have been huge efforts to clean-up sources of acid rain, and our research shows that rivers are heading in the right direction," Prof Ormerod ? who has led the research since the early 1980s ? said.
"However, our results support the theory that acid conditions during rainstorms kill sensitive animals."
The study is published online in today's Journal of Applied Ecology
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