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Weibezahn, 1990, Castillo et al. 2004 Weibezahn, 1990, Castillo et al. 2004 Weibezahn, 1990 Lewis et al., 1995; 1999

Guo et al. 2004 Goolsey et al. 1999 Goolsey et al. 1999 Meybeck et al. 1988 Abdelhamid et al. 1992. Liu et al. 2003 Zhang 1996

draining sedimentary rock deposits, and low in regions of crystalline bedrock. Dillon and Kirchner (1975) reported greater P export from Ontario streams draining watersheds of sedimentary origin compared to those of igneous origin. Sedimentary watersheds with phosphate-bearing limestone contained substantially more phosphate in streamwater in comparison to watersheds where sandstone and shales were the main geologic types present (Thomas and Crutchfield 1974). High SRP concentrations in a Costa Rican stream influenced by geothermal activity indicate that geothermal groundwater can be a significant source of P (Pringle et al. 1993).

Atmospheric inputs, although small, can be significant in areas where P is scarce, like the Caura River, a tributary of the Orinoco (Lewis et al. 1987). The forest canopy also is a source of P because of leaching when rain and cloud water deposition drip from vegetation surfaces. In relatively undisturbed parts of the Upper River Severn in Wales, UK, P concentrations in throughfall and stemflow were markedly higher than observed in rain water, illustrating the strong influence of the vegetation canopy (Neal et al. 2003). Overall, however, P inputs from the atmosphere are minor.

Phosphorus generated from plant breakdown and stored in the soil organic layer is an important input, entering streams by surface runoff and subsurface pathways (McDowell et al. 2001). Because orthophosphate readily adsorbs to charged particles, clays in particular, it is transported with sediment that is eroded during storms, especially where slopes are steep and vegetation cover is minimal. Phosphorus concentrations in surface runoff vary with the amount of P in surface soils (Sharpley et al. 1995, Weld et al. 2001) and the proximity of P-rich soils to the stream channel (Sharpley et al. 1999). However, uptake of P by plant roots and immobilization by soil microbes can reduce concentrations in mineral soil water (Kaiser et al. 2000, Goller et al. 2006). In a montane forest in

Ecuador, the soil organic layer was the main source of organic P while forest canopy drip was the main source of inorganic P. Because P concentrations in streamwater were as low as observed in rainfall, it appears that mineral soil in this system was a major P sink (Goller et al. 2006).

Anthropogenic sources include municipal and industrial wastewater, termed point source pollution because it enters surface waters at a point, usually through a pipe; and fertilizers and manure from farm fields, referred to as nonpoint sources because of their diffuse entry into streams via surface and subsurface runoff (Edwards et al. 2000, Goller et al. 2006). Dissolved inorganic P often is in the range of 0.05-0.10 mg P L"1 in streams receiving agricultural runoff, and can reach 1 mg P L"1 below sewage outlets. The majority of large lowland rivers in the UK have concentrations above 0.3mg P L"1, and some are above 1 mg P L"1 (Mainstone and Parr 2002). Point sources such as sewage treatment plants discharge P in dissolved forms. Because dissolved P is highly bioavailable and the supply is essentially constant throughout the year, including during the growing season when low flows result in less dilution, sewage waste effluent can have a disproportionately large influence on receiving waters. Nonpoint P inputs commonly are transported in particulate form in association with sediments, particularly where erosion is high, although P originating in fertilizer and manure also can be transported in dissolved forms (Hatch et al. 1999, Vanni et al. 2001). This can change temporally because of human activities and hydrology. For example, P loss from fertilized grasslands is likely to be primarily in dissolved forms during baseflow, whereas particulate forms are more likely to be transported during stormflow (Heathwaite and Dils 2000). Because much of the diffuse P supply is associated with sediments and storms, often during the nongrow-ing season, there is a high likelihood that these inputs will be exported downstream (Mainstone and Parr 2002).

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