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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