Climate Change Impacts on Freshwater Ecosystems. First edition. Edited by M. Kernan, R. Battarbee and B. Moss. © 2010 Blackwell Publishing Ltd. 4 Climate Change and the Hydrology and Morphology of Freshwater Ecosystems Piet F.M. Verdonschot, Daniel Hering, John Murphy, Sonja C. Jähnig, Neil L. Rose, Wolfram Graf, Karel Brabec and Leonard Sandin Introduction This chapter focuses on existing knowledge and new data, as well as hypotheses, on how climate–hydromorphological interactions might alter freshwater ecosystems in future. We include projected changes in climate and land use, and their effects at the catchment, reach and habitat scales. The emphasis is on streams and rivers, although some aspects of lake hydromorphology are also considered. We focus on streams and rivers mainly because more knowledge exists on the small-scale effects of climate change on stream and river hydrology and morphology and ultimately on their biological effects, compared with lakes. Finally, we discuss how climate change might affect attempts to restore stream and river ecosystems. Climate affects freshwater ecosystems indirectly through societal and economic systems, such as land management, as well as directly by temperature and precipitation. In many cases, climate change is an additional stressor adding to the impacts of human activity. Freshwater biodiversity, for example, is at present affected by the over-exploitation of natural resources, water pollution, flow modification, habitat degradation and by invasive alien species (Dudgeon et al. 2006). In future, however, the effects of climate change are expected to become more prominent, especially if the magnitude and rate of climate change are at the higher end of the projected range (cf. Solomon et al. 2007; European Environ- mental Agency 2008), which, on current evidence, seems likely. The principal changes expected in Europe are described in Chapter 3. For streams and rivers, changes in precipitation and discharge regimes are as important as changes in temperature. Already there is evidence for an increase in annual river discharge in eastern Europe and a decrease in southern Europe, and annual discharge is also projected to decline strongly in southern and south-eastern Europe in future 9781405179133_4_004.indd 65 9781405179133_4_004.indd 65 7/8/2010 7:16:36 PM 7/8/2010 7:16:36 PM