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Climate Change Impacts:
Lakes, Streams, and Fish
Numerous Indiana streams and rivers offer a wide variety of recreational opportunities and tourist attractions. Aquatic ecosystems, and species that depend on them, will be impacted by a warmer climate, land-use changess, and the introduction of invasive species.

Native aquatic plant and animal species will differ widely in their responses to changing water temperature and hydrology. Some will respond by adapting to warmer temperatures, or expanding their ranges northward, or seeking refuge in areas where temperatures and flow patterns remain suitable. Others will decline to extinction. Among the potential impacts of climate change with implications for aquatic ecosystems in Indiana are:

• A Change in the Distribution of Fish Species
  Individual fish actively select and rapidly change living areas based on suitable temperatures, oxygen concentrations, and food availability. As water temperatures in the region increase, cold-water species, such as lake trout and brook trout, and even some cool-water species, such as northern pike and walleye, may decline dramatically, potentially to the point where they disappear from the region. Warm-water fish, such as bluegill and smallmouth bass, are expected to expand their ranges northward in the region. As a result of these changes, anglers may lose their preferred catch. To compensate for range and population changes, current stocking methods may need to be reevaluated.
  
  
• An Increase in Summer Stratification
  Stratification of lakes occurs when a warm surface layer of water develops over cooler, deeper water. A warming climate increases the duration of summer stratification in the deep lakes. This, in turn, makes frequent and larger "dead zones"—areas of water depleted of oxygen and unable to support life—more likely to occur. Persistent dead zones can result in toxic algal blooms, foul-smelling, musty-tasting drinking water, damage to fisheries, and massive fish kills—known as "summerkill".
  
  
• Loss of Winter Ice
  Declines in the duration of winter ice on lakes are expected to continue. The loss of winter ice may be a mixed blessing for fish, reducing winterkill from oxygen deficits in shallow lakes but also jeopardizing reproduction of whitefish in the Great Lakes bays, where ice cover protects the eggs from winter storm disturbance.
  
  
• The Release of Nutrients and Contaminants
  Lower oxygen and warmer temperatures promote greater microbial decomposition and subsequent release of nutrients and contaminants from bottom sediments. Phosphorus release would be enhanced, and mercury release and uptake by biota would also likely increase—exposing humans to higher mercury levels via fish consumption.
  
  
• Changes to Stream Flow and Habitats
  Stream ecosystems will be impacted by a the region's changing precipitation patterns of an increase in precipitation in the winter and spring and a decrease in the summer and fall. Stream responses to these changes will vary according to the relative contribution of groundwater versus surface water in their flow patterns. In general, dryer summers will lead to lower summer stream flow and less stream habitat due to drier conditions. Other potential impacts include disruption in fish and insect life cycles due to earlier ice out; increased floodplain habitat as a result of more precipitation in the winter and spring; and decreases in fish and invertebrate production as a result of more frequent heavy rainfall events.