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Agriculture, water, and ecosystems: avoiding the costs of going too far


the greenhouse gas nitrous oxide. Regime shifts are often rapid, but they have likely fol- lowed a slower and difficult to detect change in ecosystem resilience. It is generally difficult to monitor changes in resilience before a system hits the threshold and changes from one state to another (box 6.7; Carpenter, Westley, and Turner 2005).

Agrochemical contamination. Pollution and contamination from agricultural chemi- cals have been well documented since the publication of the seminal book Silent Spring (Carson 1962). Bioaccumulation as a consequence of the wide use of agrochemicals has had dire outcomes for many species that reside in or feed predominantly in wetlands or lakes that have accumulated residues from pesticides [well established]. e decline in the breeding success of raptors was a turning point in developing awareness about the dangers of using pesticides (Carson 1962).

An increasing amount of analytical and ecotoxicological data has become available for aquatic communities, and more recent research has also focused on risk assessments and the development of diagnostic tests that can guide management decisions about the use of such chemicals (van den Brink and others 2003). Taylor, Baird, and Soares (2002) have highlighted the high levels of pesticide use and low levels of environmental risk assessment in developing countries. ey have promoted an integrated approach to evaluating envi- ronmental risks from pesticides that incorporates stakeholder consultation, chemical risk assessment, and ecotoxicological testing for ecological effects, also taking into account the potential effects on human health.

Vörösmarty, Lévêque, and Revenga (2005) report that water contamination by pes- ticides has increased rapidly since the 1970s despite increased regulation of the use of xenobiotic substances, especially in developed countries. However, bans on the use of these chemicals have generally been imposed only two to three decades after their first commer- cial use, as with DDT and atrazine. Many of these substances are highly persistent in the environment, but because of the generally poor monitoring of their long-term effects the global and long-term implications of their use cannot be fully assessed. Policy responses to contamination may lag far behind the event, as shown in the well documented case of agricultural pesticide bioaccumulation of DDT in the Zambezi Basin (Berg, Kilbus, and Kautsky 1992).

Siltation of rivers. In many parts of the world extensive sheet wash and gully erosion due to land management practices have devastated large areas, reduced the productivity of wide tracts of land, led to rapid siltation of reservoirs and threatened their longevit , and increased sediment loads in many rivers (see chapter 15 on land). On a regional scale some reservoirs in Southern Africa are at risk of losing more than a quarter of their storage capac- ity within 20–25 years (Magadza 1995). While many Australian and Southern African wa- ters are naturally silt , many have experienced increased silt loads as a result of agricultural practices (Davies and Day 1998). Zimbabwe’s more than 8,000 small to medium-size dams, for example, are threatened by sedimentation from soil erosion, while the Save River, an in- ternational river shared with Mozambique, has been reduced from a perennial to a seasonal river system due in large part to increased siltation as a result of soil erosion.

Globally, rivers discharge nearly 38,000 cubic kilometers of freshwater to the oceans and carry roughly 70% of the sediment input, though rivers draining only 10% of the

Water contamination by pesticides has increased rapidly since the 1970s despite increased regulation, especially in developed countries


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