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pHlight of the mayfly

December 10, 2001

Ecologists at Cardiff University are using genetic techniques alongside classical ecology to help explain why Welsh streams are not recovering from the biological damage done by acid rain. Speaking at the British Ecological Society’s Winter Meeting, to be held at the University of Warwick on 18–20 December 2001, Ceri Williams will explain how genetically analysing mayflies is helping to solve the mystery of why wildlife is not returning to Welsh rivers, despite them becoming less acid.

The importance of the work is in showing that we need to understand the ecological processes involved in rivers recovering from acid rain, rather than only the chemistry as is often assumed. At least half of the 25,000 km of Welsh rivers has been affected by acid rain. This is because of high rainfall and pollution in upland mid and north Wales, together with soils unable to neutralise the acid. According to Williams: “Acid rain has major consequences for biodiversity, because the number of species in streams declines in line with its pH.”

However, a recent survey found that although the pH is rising in many Welsh streams – ie, they were becoming less acid – populations of invertebrates and birds in and around the streams are failing to recover. “We urgently need to understand why Welsh streams are failing to recover biologically from acidification, especially because large amounts of money are being spent to cut emissions of sulphur and nitrous oxides – the cause of acid rain – across Europe,” Williams says. Upland streams in Wales are also an important environmental resource for water supply, conservation, and freshwater fisheries.

Williams is analysing the DNA of the mayfly species complex Baetis rhodani, to find out why they are failing to recolonise streams that have recovered chemically. “B. rhodani is widespread, abundant and among the most acid-sensitive invertebrates in Welsh streams, so the presence and diversity of mayfly larvae has been widely used to indicate water quality. The larvae have long cylindrical bodies, and long tails fringed with hairs, so they are agile swimmers and usually rapidly recolonise disturbed streams, but the isolated nature of many streams affected by acid rain means that recovery can occur only through flight,” Williams says. This species could, therefore, provide an important model of dispersal patterns across headwater streams in acid-sensitive regions of Wales. Mayflies are also an important food source for birds, dragonflies, frogs, bats, and shrews.

Ecologists think that the mayflies are either not reaching the deacidifying streams, or are doing so but then failing to survive for other reasons. Because Williams cannot physically track mayfly movements from one stream to another, she uses genetic markers to tell whether or not recolonisation is occurring. “Because studying insect movements in the field is often difficult, we are measuring the consequences of movement, rather than the movement itself, through DNA,” she says.

Williams has developed some of the first aquatic invertebrate microsatellites for B. rhodani. These genetic markers are useful for detecting genetic variation at small scales, even within a single stream. She hopes the genetic variation between geographically-separate groups of mayflies will reveal how far mayflies can travel between Welsh streams: if the populations are genetically similar, it will be evidence of high levels of dispersal. However, genetically different populations will be evidence of isolation.

Ceri Williams will present her full findings at 12.50 on Tuesday 18 December 2001.

British Ecological Society (BES)