phosphorus load from wastes (44 to 50 percent) in water was due to runoff from dairy cattle operations. In other regions, such as the southeastern United States, swine waste is a potentially large source of phosphorus (K. R. Reddy et al., 1978).

In some cases, reduction of sediment phosphorus losses can result in increases of soluble phosphorus loss (Sharpley and Menzel, 1987), so the answer to phosphorus loading problems is not as simple as sediment control and is likely to involve trade-offs.

Forms and Bioavailability of Phosphorus

Phosphorus occurs in many forms in both the solution phase and, in particular, the solid phase. These forms are little understood, even though there are many data in the literature concerning the chemistry of phosphorus in water, soils, and sediments. The relative bioavailability of various forms of phosphorus varies, but there is no standard method of determining this important quantity.

Soluble Phosphorus

Soluble phosphorus is arbitrarily defined as phosphorus that will pass through a 0.45-µm-pore filter. Soluble reactive phosphorus is that fraction of phosphorus that is reactive with molybdate, according to the Murphy-Riley procedure or its variants. This fraction has been assumed to consist of orthophosphate, but there is evidence that some organic phosphorus is included (Rigler, 1968); for this reason, molybdate-reactive phosphorus is usually referred to as soluble reactive phosphorus or dissolved reactive phosphorus rather than orthophosphate.

Not all of the dissolved reactive phosphorus in lake water is completely available for algal growth (Sharpley and Menzel, 1987). The relative difference in dissolved reactive phosphorus and bioavailable phosphorus in water is greater in waters with low levels of phosphorus and is less in solutions with higher dissolved reactive phosphorus concentrations (Sharpley and Menzel, 1987). Even though not all of the phosphorus in water is available to algae, there is often a close relationship between the total amount of phosphorus in water and the standing algal crop (Dillon and Rigler, 1974).

Particulate Phosphorus

Phosphorus is strongly bound to sediments by anion adsorption reactions. These reactions probably account for the rapid removal from



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