The Long Hard Reign of Acid Rain
24 Oct, 2007 11:59 am
For many environmentalists, acid rain was the cause c?l?bre of the 1980s, but how far have we come in solving the problem? Recent work shows how continuing acid episodes can explain the weak biological recovery of acidified rivers.
Now, 30 years on, the clean-up of power stations and industry across Europe has reduced the emissions of SO2 by up to 80%. NOx emissions are also falling. Sulphur deposition has declined in many places by around a half. In combination, these measures were expected widely to bring recovery to affected ecosystems.
Intriguingly, however, emerging trends now suggest a marked mismatch between the chemical and biological response of surface waters, particularly in rivers. Mean pH in acidified streams has increased by around 0.3-0.4 pH units every 10 years, but only small proportions of the species lost during acidification have re-colonised. Possible explanations include problems in species’ dispersal, or competitive exclusion where organisms able to tolerate low pH now occupy the niches once filled by acid-sensitive species.
Recently, writing in Global Change Biology, we advanced and tested an alternative hypothesis which appears to be supported by the available evidence (Kowalik et al. 2007). Put simply, episodic acidification during periods of high flow is still sufficient in many locations to exclude acid-sensitive species. More significantly, these effects are most pronounced where acid episodes are driven by acid anions of anthropogenic origin, in other words sulphate from acid rain.
The work, carried out in 89 streams spread across acid-sensitive areas of Wales, Galloway (SW Scotland) and the Scottish Highlands, involved two distinct components. First, by sampling each of the streams chemically at both high and low-flow, we determined not only how much pH declined at high flow, but also the chemical processes responsible. While some streams were acidified moderately at high flow by the dilution of the carbonate compounds that normally provide buffering, acidification in others was caused by dilute sulphuric and nitric acid. pH fell more in these streams than elsewhere (often to pH<5), while concentrations of toxic aluminium also increased. Next, we compared invertebrates in streams with acid episodes of different severity. Here, again, the evidence was clear: many acid-sensitive species were absent only where there were pronounced acid episodes caused by ‘excess’ sulphate addition (i.e. dilute sulphuric acid). In parallel work, we showed that acid-sensitive mayflies could survive only for a few days such acid episodes in Welsh streams (Kowalik & Ormerod 2006).
So what do these results tell us about recovery from acidification? First, they show that headwater acidification is still a widespread and enduring problem in Britain. This is particularly clear when judged on biological evidence, and rivers important to conservation, such as the Welsh River Wye, are still affected (Lewis et al. 2007). Second, continuing episodic acidification is sufficient to explain why biological recovery has been so patchy. Although the exact causes differ, our own evidence is that this situation is similar in other regions of Europe where acid episodes still occur (Lepori & Ormerod 2005). This result has wider ramifications, and shows that monitoring recovery from large-scale stressors using average conditions is less effective than assessing the extremes that determine species occurrence and survival. In integrating the effects of both long-term change and episodic events, organisms will be the ultimate indicators that tell us when recovery from acidification finally occurs.
Kowalik, R. A., D.M. Cooper, C. M. Evans & S. J. Ormerod (2007) Acid episodes retard the biological recovery of upland British streams from acidification. Global Change Biology, 13, 2439–2452
Kowalik, R. A. and S. J. Ormerod (2006) Intensive sampling and transplantation experiments reveal continued effects of episodic acidification effects on sensitive stream invertebrates. Freshwater Biology, 51, 180-191
Lepori, F. and S. J. Ormerod (2005) Population data and in-situ toxicity tests reveal consistent effects of spring acid episodes on macroinvertebrates. Freshwater Biology, 50, 1568-1577
Lewis, B. R., I. Jüttner, B. Reynolds & S. J. Ormerod (2007) Comparative assessment of stream acidity using diatoms and macroinvertebrates: implications for river management and conservation. Aquatic Conservation: Marine and Freshwater Ecosystems, 17, 502-519