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Some freshwater ecosystems have received waste water for many centuries (AlvarezCobelas & Verdugo, 1995), i.e. the Latium wetlands close to Rome or the Thames and the Spree rivers downstream London and Berlin, respectively. The strong development experienced in many areas of the world in the 20th century has resulted in increasing waste water disposal almost everywhere that has increasingly threatened freshwater ecosystems receiving these loadings. The concern of human health and, later, ecosystem health resulted in the implementation of waste water treatment facilities in many developed countries, such as those of Northern and Central Europe, USA, Canada and Japan (Tchobanoglous et al., 2003), which diminished wastewater inputs to freshwater environments. Unfortunately, this has not been the case in many semiarid countries whose economy or, more often, some lack of concern for environmental quality does not enable funding enough for these otherwise expensive facilities. While waste water pollution cannot always be abated, its effects are likely to be diminished if enough water is available to produce both dilution and wash out, as often occurs in cold temperate and tropical environments. Unfortunately, this is not the case in semiarid areas where rainfall is unevenly distributed throughout the year, also showing a strong interannual variability. For example, Fig. 1 depicts the long-term (19452006) annual precipitation falling on a semiarid central Spanish area, which results in an average of 418 ± 128 mm/year, ranging 189-857 mm/year. In fact, traditional approaches to water management in semiarid regions have been based more on the increase of water availability rather than improving the water quality of waste waters to make them feasible for future use. In water shortage scenarios, domestic lifestyle adaptations and optimization of water consumption by both agriculture and industry have been managed to maintain the balance between water supply and demand. However, although this balance could be achieved and the amount of waste water reduced, the characteristic low water flow of semiarid rivers makes impact of waste water discharge in freshwater ecosystems stronger. Streamwater discharge to wetlands and lakes is highly variable over time in semiarid areas. Fig. 2 shows an example of these fluctuating water flows of a semiarid river that drains to a central Spanish wetland (Las Tablas de Daimiel National Park). Semiarid regions of the world are confronted with a largely unpredictable |
