Recovery of acidified surface waters in response to reduced atmospheric pollution: 35+ years of continuous biological monitoring (1988–2025)

Since the industrial revolution the combustion of fossil fuels has been releasing oxides of sulphur and nitrogen into the atmosphere.  These react to form strong acids which return to the earth’s surface as “acid rain”.  As a result, many waters in areas with low acid neutralising capacity had become severely acidified resulting in severe degradation of affected ecosystems.  Concerted international efforts have dramatically reduced the emission of acidifying gases to the atmosphere.  The UK Upland Waters Monitoring Network (UWMN) established a network of stream and lake sites from sensitive geologies across the United Kingdom, which have been monitored continuously since 1988 to assess the chemical and biological response of acidified lakes and streams to these reductions in atmospheric pollution.  The UK UWMN consists of an ongoing, 35+ year time series of matched environmental data that documents the trajectory of hydrochemical and biological (macroinvertebrate, diatom and macrophyte, plus fish to 2015) recovery at 23 sites, the majority of which are acidified.  International legislation has led to large reductions in acid deposition, which in turn has driven major reductions in water acidity accompanied by increases in dissolved organic matter. As such, the UK UWMN provides a national-scale natural experiment to examine how these upland aquatic ecosystems are responding to reductions in this long-range, transboundary pressure. There have been major changes in the biological communities of the majority of UWMN sites, driven mainly by a turnover of taxa rather than an increase in richness. There is strong agreement between the sites showing positive trends in biological indicators of water pH and the sites that are undergoing the clearest chemical recovery from acidification. However, there are also indications that other drivers of change, such as nitrogen enrichment and climate change, may be causing recovery trajectories to deviate from paths taken during acidification as, for example, indicated from palaeoecological data.  The UK UWMN has proven, and is continuing to prove, to be a highly effective source of evidence for assessing the efficacy of emissions reduction policy in restoring damaged upland freshwater ecosystems within the context of other regional pressures.