Quinn and Adams, 1996; Huntington et al., 2003; Cooke et al., 2004; Juanes et al., 2004; Lawson et al., 2004

1.3.45 Changes in rivers

In rivers, water flow can influence water chemistry, habitat, population dynamics, and water temperature (Schindler et al., 2007). Specific information on the effect of climate change on hydrology can be found in Section 1.3.2. Increasing river temperatures have been associated with increased biological demand and decreased dissolved oxygen, without changes in flow (Ozaki et al., 2003). Riverine dissolved organic carbon concentrations have doubled in some cases because of increased carbon release in the catchment as temperature has risen (Worrall et al., 2003).

Abundance, distribution and migration

Climate-related changes in rivers have affected species abundance, distribution and migration patterns. While warmer water temperatures in many rivers have positively influenced the breeding success of fish (Fruget et al., 2001; Grenouillet et al., 2001; Daufresne et al., 2004), the stressful period associated with higher water temperatures for salmonids has lengthened as water temperatures have increased commensurate with air temperatures in some locations (Bartholow, 2005). In the Rhône River there have been significant changes in species composition as southern, thermophilic fish and invertebrate species have progressively replaced cold-water species (Doledec et al., 1996; Daufresne et al., 2004). Correlated with long-term increases in water temperature, the timing of fish migrations in large rivers in North America has advanced by up to 6 weeks in some years (Quinn and Adams, 1996; Huntington et al., 2003; Cooke et al., 2004; Juanes et al., 2004). Increasing air temperatures have been negatively correlated with smolt production (Lawson et al., 2004), and earlier migrations are associated with greater en-route and pre-spawning mortality (up to 90%) (Cooke et al., 2004). Warming in Alpine rivers caused altitudinal habitat shifts upward for brown trout, and there were increased incidences of temperature-dependent kidney disease at the lower-elevational habitat boundary (Hari et al., 2006).

13.4.6 Summary of marine and freshwater biological systems

In marine and freshwater ecosystems, many observed changes in phenology and distribution have been associated with rising water temperatures, as well as changes in salinity, oxygen levels and circulation. While there is increasing evidence for climate change impacts on coral reefs, separating the impacts of climate-related stresses from other stresses (e.g., over-fishing and pollution) is difficult. Globally, freshwater ecosystems are showing changes in organism abundance and productivity, range expansions, and phenological shifts (including earlier fish migrations) that are linked to rising temperatures. Many of these climate-related impacts are now influencing the ways in which marine and freshwater ecosystems function.

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