This describes the situation where total dissolved ions, and thus salts, are high, often because of human activities. Salinization refers to either the process or the result of the buildup of dissolved salts in fresh waters. The natural range of salinity in inland waters is considerable, but when referring to the result of human activities, we often are concerned with changes from relatively low background concentrations. Using a world average of 120 mg L 1 for reference (Table 4.2), which is thought to be slightly elevated above its natural state due to pollution, a TDS > 250 mg L 1 indicates high salinity. From the human perspective, salinities > 250-500 mg L 1 are undesirable for drinking water, and detrimental effects on crops and industrial uses occur at levels > 0.5 - 1 g L 1 (Jackson and Jobbagy 2005, Williams 2001).

Salinization is a particular problem in arid and semiarid areas due to irrigation and dryland farming. Irrigation concentrates salts by evaporation and also because the remaining, more concentrated solution leaches soil salts. Ultimately this more concentrated water returns to the stream, via surface or subsurface flows. In Australia, where salinization is widespread in semiarid agricultural areas, the lower South Australian Murray River averages ~ 0.5gL-1 (Williams and Williams 1991). In North America, the elevated salinity of the Colorado (> 0.8g L-1 at its delta) is the recent consequence of irrigation and impoundments. Although the biological effects of this are not well known, river water in the lowermost sections is unsuitable for agriculture without expensive desalinization facilities.

In urban areas, the runoff of salts and other deicing compounds applied to roads can significantly elevate the salinity of receiving waters and cause large fluctuations at short time scales. In the United States, some 10-15 million tons of road salt are used each year (Benbow and Merritt 2005), primarily in the Northeast and Midwest, and quantities have increased dramatically since 1950 (Jackson and Jobbagy 2005). Kaushal et al. (2005) report chloride concentrations as high as 25% of seawater in some northeastern US

streams during winter, and the long-term trend is increasing (Figure 4.7).

Whether the salinities that we would consider high in rivers are harmful to the biota is unclear, although faunal changes unquestionably occur in the brackish waters of estuaries. Acute toxicity levels for stream macroinvertebrates exposed to saline water are relatively high, ranging from 2 to 13 g L1 (Benbow and Merritt 2005), but can decrease with increasing temperature. Road salt runoff had no discernible effect on macro-invertebrates of the Au Sable River, Michigan, which Blasius and Merritt (2002) attributed to dilution from snow melt. In two river systems of Australia, excluding estuarine reaches, Williams et al. (1991) found no relationship between macroinvertebrate assemblages and salinity, which exceeded 2 g L_1. Several fishes of the Murray River survived laboratory exposures to salinities up to 30 g L_1, possibly reflecting a relatively recent marine ancestry for these spe cies (Williams and Williams 1991), and most cyprinids (the minnow family) survive up to 14-20 gL1 (Threader and Houston 1983). Where the salinity range is great, such as the Red River of Texas, a prairie stream where salt concentrations range from ~ 200 mg L 1 TDS to approximately full strength seawater, fish assemblages form low-, medium-, and high-salinity groupings and salinity-sensitive species have shown greatest declines in recent years (Higgins and Wilde 2005). In most circumstances, however, although individual freshwater species may be harmed by even moderate salinities, current evidence does not indicate widespread damage to the fauna.

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