NO 62 : THE IMPACT OF GLOBAL WARMING :Lakes and Rivers

Lakes and Rivers

Climate change is already beginning to affect plants and animals that live in freshwater lakes and rivers, altering their habitat and bringing life-threatening stress and disease.

Displacement of cold-water species. As air temperatures rise, water temperatures do also—particularly in shallow stretches of rivers and surface waters of lakes. Streams and lakes may become unsuitable for cold-water fish but support species that thrive in warmer waters. Some warm-water species are already moving to waters at higher latitudes and altitudes.

• Dead zones. In a warming climate, a warmer upper layer in deep lakes slows down air exchange—a process that normally adds oxygen to the water. This, in turn, often creates large "dead zones"—areas depleted of oxygen and unable to support life. Persistent dead zones can produce toxic algal blooms, foul-smelling drinking water, and massive fish kills.
• Effects on reproduction. Earlier snow melt, rising amounts of precipitation that falls as rain rather than snow, and more severe and frequent flooding—all linked to global warming—may affect the reproduction of aquatic species. Some salmon populations have declined, for example, as more intense spring floods have washed away salmon eggs laid in stream beds.
• Stress. When stream flow peaks earlier in the spring owing to warmer temperatures, low stream flow begins earlier in the summer and lasts longer in the fall. These changes stress aquatic plants and animals that have adapted to specific low-flow conditions. The survival rates of fish such as salmon and trout are known to diminish when water levels in rivers and streams are dangerously low, for example. That's partly because bears can snag spawning salmon more easily in very shallow water, as the salmon struggle upstream.
• Disease. The more intense precipitation that accompanies a warming world makes river flooding more likely. This flooding—combined with sewer system overflows and other problems stemming from inadequate sanitation infrastructure—can lead to disease outbreaks from water-borne bacteria.

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