Hot Running Water? Climate Change Effects on Stream Ecology
15 May, 2007 12:36 pm
Recently, we reported on a 25-year study that has shown just how far-reaching climatic effects on streams might be. Writing in ?Global Change Biology?, we analysed data on the temperature, flow pattern and invertebrates of streams in central Wales between 1981 and 2005. Over this period, average winter temperatures in these streams varied between years by over 3 ?C, but warmed overall by 1.4-1.7 ?C.There have been marked ecological effects.
If this were not enough to raise concern, streams are also globally important ecologically. Most obviously, they connect terrestrial landscapes with other water bodies downstream, to which they transfer water, energy and nutrients. Their combined length is huge, and the United Kingdom alone has over 200,000 km of streams, or around 75% of our total river length. These habitats not only support many freshwater species – including migrant fishes such as Salmon Salmo salar - but they are also critical in providing energy to riparian organisms such as birds, bats and other mammals.
Against this background, the worldwide scarcity of evidence about climate change effects on stream ecology is potentially alarming. So far, only a handful of studies have indicated what effects might arise in streams from climate change, with long-term research particularly scarce.
Recently, we reported on a 25-year study that has shown just how far-reaching climatic effects on streams might be. Writing in ‘Global Change Biology’, we analysed data on the temperature, flow pattern and invertebrates of streams in central Wales between 1981 and 2005. Over this period, average winter temperatures in these streams varied between years by over 3 °C, but warmed overall by 1.4-1.7 °C. These changes were matched from other studies in the Alps, in central France, in Scotland and western England, and were bigger than could be explained by weather variations such as the North Atlantic Oscillation. More uniquely, we showed how warming was matched at our sites by two major ecological responses. First, the spring abundance of invertebrates fell as temperatures rose. The reasons are still unclear, but might reflect earlier insect emergence or increased fish predation at higher temperature. Secondly, the blend of species present in the streams has fluctuated, so that typical cool-water species have been lost from the streams while a smaller number of warmer-water species has occupied the sites. If maintained at rates observed over 25 years of our study, these effects would see abundances fall by around one fifth for every 1 °C rise in temperature. Around 4-10 invertebrate species, or up to 25% of the typical annual number of species found in the most affected streams, are at risk of local extinction. These are mostly rarer species, whereas the more general reduction in abundance could have knock-on effects for key predators such as salmonids or river birds such as the Dipper Cinclus cinclus.
Pressing needs now emerge. One of the most critical is to understand what drives these trends, and how stream management might be adapted to reduce any negative effects. Providing shade using stream-side woodland offers one possibility, for example in minimising higher summer temperatures. There is also a need to minimise those other pressures that will make climate-change effects worse – notably pollution and abstraction. We also need to understand better how these apparent temperature effects on stream organisms generalise to other locations, and how they might be exacerbated by the droughts and floods that are widely predicted to become major features of river systems in our warmer future.
I. Durance, S. J. Ormerod. 2007 Climate change effects on upland stream invertebrates over a 25 year period. Global Change Biology, 13, 942-957