The literature on ecological effects of recent climate change in fresh waters has been reviewed, with particular reference to freshwater conservation in the UK. Least emphasis is given to predictive models of future change, because of considerable uncertainties even in the climate models, let alone their biological implications. Climate change effects on fresh waters have been superimposed on existing large human impacts, which make separation of climatic effects particularly difficult.
Research in fresh waters has concentrated on communities and processes and there is less emphasis on individual charismatic species than in terrestrial systems. This approach lends itself to space-for-time studies on climate effects. There has been a modest amount of experimentation, particularly in mesocosms, and analysis of long-term biological data sets, the most extensive from lakes. The most detailed information on ecological effects comes from lake plankton.
No species is yet known to have been lost from the UK as a result of climate change but there is extensive evidence of changes in phenology and distribution, and in processes in the plankton. It is likely that temperature effects per se will be less important than effects of changed hydrology and that idiosyncratic behaviour of each species will lead to many indirect effects through biological interactions in communities. Experimental studies suggest major likely changes in plant, fish and invertebrate communities with a several degree increase in temperature and associated hydrological changes expected in the 21st century. Freshwater organisms, however, are well adapted to disturbance and through invasion, redistribution, adaptation and microevolution will re-form functioning communities, though with likely different biodiversity than at present. Some invasive species may come to dominate the new communities. There will be important consequences for the estimation of ecological quality, which will inconvenience statutory obligations under the Water Framework Directive, and symptoms of eutrophication will be exacerbated. Some coastal lakes may revert to estuaries.
Much more important, however, may be the consequences of climate change for the important part of the carbon cycle that is focussed on fresh waters, particularly if the ratio of community respiration to gross primary production increases with rising temperature. Several studies suggest large increases in this ratio with temperature rises of up to 4 °C. A much more radical approach to conservation, involving re-establishment of entire, connected catchment systems rather than the present piecemeal attention to biodiversity issues is likely to be needed if a comfortable human future is to be guaranteed.