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by phosphorus accumulation in soils, with high levels of phosphorus runoff. In developed countries annual storage peaked around 1975 and is now at about the same annual rate as in 1961. In developing countries, however, storage went from negative values in 1961 to about 5 teragrams per year in 1996.

Nutrient management can be undermined by the loss of wetlands that assimilate nutrients and some pollutants

Excessive nutrient loading can cause algal blooms, decreased drinking water quality, eutrophication of freshwater ecosystems and coastal zones, and hypoxia in coastal waters. In Lake Chivero, Zimbabwe, agricultural runoff is seen as responsible for algal blooms, infestations of water hyacinth, and fish declines as a result of high levels of ammonia and low oxygen levels (UNEP 2002). In Australia extensive algal blooms in coastal inlets and estuaries, inland lakes, and rivers have been attributed to increased nutrient runoff from agricultural fields (Lukatelich and McComb 1986; Falconer 2001). Diffuse runoff of nu- trients from agricultural land is held to be largely responsible for increased eutrophication of coastal waters in the United States as well as for the periodic development, often varying from year to year, of anoxic conditions in coastal water in many parts of the world, such as the Baltic and Adriatic Seas and the Gulf of Mexico (Hall 2002).

Nutrient management can be undermined by the loss of wetlands that assimilate nutrients (nitrogen, phosphorous, organic material) and some pollutants. Extensive evi- dence shows that up to 80% of the global incident nitrogen loading can be retained within wetlands (Green and others 2004; Galloway and others 2004). However, the ability of such ecosystems to cleanse nutrient-enriched water varies and is not unlimited (Alexander, Smith, and Schwarz 2000; Wollheim and others 2001). Verhoeven and others (2006) point out that many wetlands in agricultural catchments receive excessively high loadings of nutrients, with detrimental effects on biodiversity. Wetlands and lakes risk switching from a state in which they retain nutrients to one in which they release nutrients or emit

box 6.7

Regime shifts from excessive nutrient loads

There are reported cases of regime shifts occurring in lakes because of increased nutrient loading, resulting in the loss of ecosystem services such as sheries and tourism (Folke and others 2004). Some temperate lakes have experienced shifts between a turbid water and a clear water state, with the shift often attributed to an increase in phosphorous loading (Carpenter and others 2001). Some tropical lakes have shifted from a dominance of free-oating plants to submerged plants, with nutrient enrichment seemingly reducing the resilience of the submerged plants, possibly through shading and changes in underwater light (Scheffer and others 2001). Other wetlands and coastal habitats have also experienced similar shifts. In the United States nutrient enrichment caused a shift in emergent vegetation in the Everglades and a shift from clear water to murky water with algal blooms in Florida Bay (Gunderson 2001).

Other evidence comes from lakes subject to inlling and nutrient enrichment. In Lake Hornborga in Sweden emergent macrophytic vegetation proliferated after initial inlling of the lake margins and increased runoff of nutrients. The situation was reversed only after massive mechanical intervention and investment (Hertzman and Larsson 1999). In Australia agricultural runoff has resulted in shifts in vegetation dominance as a consequence of nutrient enrichment, increased inundation and saliniza- tion (Davis and others 2003; Strehlow and others 2005).

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