Climate Change and the Hydrology and Morphology of Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 78

catchment pressures, the small scale of the restoration or a lack of potential
re-colonizers. In general, river restoration schemes have proved similarly
disappointing for these reasons.
One of the most important keys to restoration is understanding the complex
links between physical, chemical and biological components, ultimately at the
catchment scale. Stream restoration is usually focussed on individual stream
reaches, largely ignoring the importance of connectivity, such as channel
movement for terrestrial-aquatic linkages (e.g. stream-floodplain exchanges)
and biotic dispersal (Verdonschot & Nijboer 2002). Catchment-scale processes
largely determine the structure and function of streams and their floodplains.
Thus, the scale of restoration needs to be adjusted to the scale of the dominant
shaping processes. The key themes in stream ecology deal with the four dimensions
(Ward 1989) of hierarchy of physical organization (Frissell et al . 1986), adaptation,
response of species and human disturbance. A catchment approach includes all
these themes. Climate change, and thus hydromorphological change, affects the
stream at the highest hierarchical levels, implying that under changing climate
conditions, either the measures to meet restoration targets must be increased or
the targets must be less ambitious.
Conclusions
Climate change will alter the hydromorphological conditions of lakes, streams
and rivers in Europe. The magnitude of change induced by climate is still relatively
small in comparison with the impact of anthropogenic land use, but in future,
climate change may cause significant change in hydrology and, at the same time,
impose land-use changes in catchments. Changes in stream hydrology are best
characterized in terms of dynamics, particularly associated with increases in
drought as well as in spate frequency. In lakes, hydrological changes are expressed
in terms of more dynamic fluctuations as well as overall changes in water level
and their impact on eutrophication (cf. Chapter 6). In rivers, climate change may
also increase flow variability resulting in higher scouring and siltation rates, and
where rivers flow into lakes, sediment loads may increase, leading to accelerated
SARs. To some extent, biota in streams are adapted to changes in habitat-scale
conditions and will tolerate such dynamics. However, different life stages require
different environmental conditions, and a disturbed timing, for example, of snow
melt and the connected high-flow conditions, can result in a loss of taxa. More
dynamic conditions and a loss of native taxa widen the opportunity for non-
native species to enter ecosystems. Globalized transport systems and hydrological
links between large catchments further enhance this process.
Restoration of streams and rivers is promoted by the EU Water Framework
Directive and other legislations (e.g. EU Habitats Directive, EU Floods Directive,
Natura 2000). Restoration success, however, has been lower then expected. This
failure is mainly due to the scale of the restoration being too small in many cases,
a lack of an ecosystem approach and a lack of understanding of key biological
processes governing dispersal and colonization rates and the role of barriers. Future
climate change is likely to reduce further the chances of restoration success.