Water quality changes may be driven by changes in hydrological flowpaths in a watershed that are associated with changes in patterns of precipitation and evapotranspiration and changes in total flow in streams and rivers or in water level or duration of ice cover in temperate lakes. In regions such as the Precambrian shield, where watersheds are predicted to become drier in spring and summer, concentrations of dissolved organic material reaching lakes and streams from their catchments will decrease, increasing water clarity and changing physical and thermal regimes by increasing average thermocline depths in small, stratified lakes, for example (Snucins and Gunn, 1995; Schindler et al., 1996; Perez-Fuentetaja et al., 1999). In contrast, in the Great Lakes and other large lakes where dissolved organic carbon (DOC) concentrations are low, thermocline depths are determined by area or wind fetch and are not affected by DOC (Fee et al., 1996). Models for the Great Lakes indicate that rapid spring warming may cause shallower and steeper thermoclines (reviewed by Magnuson et al., 1997). For lakes and streams receiving flow from deep and shallow groundwater sources, drier watersheds could cause the major ion chemistry to be dominated more by the deep baseflow water sources (Webster et al., 1996).
In several regions where warmer temperatures and longer growing seasons are expected, changes in water quality will be driven by increases in primary production, organic matter decomposition, and nutrient cycling within lake or stream ecosystems (e.g., Mulholland et al., 1997). In the Great Lakes region, warming over the past 60 years already has moved forward by an average of 3 weeks the time of ice-cover breakup (ice-out), which has moved ahead the spring bloom of algal growth and changed the seasonal dynamics of nutrient utilization and production and decomposition of organic matter (Magnuson et al., 1997).
Where streamflows and lake levels decline, water quality deterioration is likely as concentrations of nutrients and contaminants from wastewater treatment, agricultural and urban runoff, and direct industrial discharge increase in reduced volumes of carrying waters. The extent of water quality deterioration will depend on adaptations in land use, population, and water use under changing climate. Warmer water temperatures may have further direct impacts on water qualityfor example, by reducing dissolved oxygen concentrations. In the southeast, intensification of the summer temperature-dissolved oxygen squeeze (simultaneous high water temperatures and low dissolved oxygen concentrations) in many rivers and reservoirs is likely to cause a loss in habitat for coolwater fish species (Mulholland et al., 1997).
Changes in the characteristics of precipitation events also may affect water quality, in complex ways. For example, increased incidence of heavy precipitation events as predicted for the southeast may result in increased leaching and sediment transport, causing greater sediment and nonpoint-source pollutant loadings to watercourses (Mulholland et al., 1997). Because these high-flow events are episodic, the associated increase in nutrient and contaminant dilution during the event is not likely to offset the deleterious effects on water quality from low summer flows.
Warmer temperatures will increase the salinity of surface waters, especially lakes and reservoirs with high residence times, by increasing evaporative water losses. Higher initial salinities in reservoir water will then exacerbate salinity problems in irrigation return flow and degrade water quality in downstream habitats.
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