Phosphorus sources and quantities

Phosphorus occurs in streamwater as orthophosphate (PO4 ) dissolved in water and attached to inorganic particles in suspension, as dissolved organic molecules and in particulate organic form mainly in bacteria and detrital particles (Table 11.1). Total phosphorus (TP) is determined by analyzing unfiltered samples and includes all forms of P, including those present in organisms, detritus, and adsorbed to inorganic complexes such as clays and carbonate (Wetzel 2001). The various P fractions are analyzed using filtration and digestion to separate its forms, followed by measurement using colorimetry and additional reactions. Total dissolved P is determined from water passing through a 0.45 ^m filter and includes organic (colloids, esters) and inorganic (orthophosphate and polyphosphates) forms. An operational category known as soluble reactive phosphorus (SRP) based on the reaction of soluble P with molybdate is commonly used as a measure of orthophosphate (PO3). However, there is evidence that the SRP fraction can also include polyphosphates and therefore may overestimate orthophosphate concentrations (Dodds 2003). In common usage, orthophosphate, phosphate, SRP, and dissolved inorganic P are interchangeable terms that refer to the form of P available for organisms to take up.

Both SRP and TP are widely used as indicators of trophic status, and there is some controversy over which is the preferred measure. SRP usually is considered the best indicator of what is immediately available for uptake, but because P cycles rapidly among its various states, TP may be a better measure of overall availability of P. The usefulness of SRP versus TP in predicting algal productivity likely varies with the residence time of P in the ecosystem, because a longer residence time allows for more efficient use of P by the biota (Edwards et al. 2000). Thus SRP may be most effective in predicting algal production when water residence time is short, as in small streams. However, for broad comparisons of trophic status and nutrient limitation across stream ecosystems, TP may prove to be more useful as a measure of overall P availability (Dodds 2003).

According to Meybeck (1982), average natural levels of dissolved P worldwide are very low, around 0.01 mg P L 1 for PO3", and 0.025 mg P L 1 for total dissolved P (Table 11.2). In a compilation of over 1,000 temperate zone sites including both minimally altered and disturbed streams, the lowest one third had TP concentrations below 0.025 mg L"1 and the highest one third of sites had TP concentrations above 0.075 mg L"1. The median TP concentration for these sites (0.045mgL"1) exceeded the estimated median TP concentration corrected for human influence (0.023 mg L"1) by less than a factor of 2, indicating that anthropogenic influence on P concentrations is less severe than for N (Smith et al. 2003). A compilation of over 600 streams of high altitude catchments in the UK found that SRP concentrations were below 0.01 mg P L"1 at half of the sites and below 0.03 mg PL"1 in most (Mainstone and Parr

2002). Large rivers of the Neotropics, with areas of extensive undisturbed forest underlain by crystalline rock, can have very low P concentrations. In undisturbed clear and blackwater tributaries of the Orinoco Basin, Venezuela, SRP concentrations were below 0.004 mg P L_1, and the upper range of TP was below 0.01 mg L 1 (Castillo et al. 2004). In contrast, whitewater rivers, where suspended sediment concentrations are greater, can exhibit higher P concentrations. In the Apure, a moderately disturbed white water tributary of the Orinoco, TP concentration averaged 0.19mg L 1 (Lewis and Saunders 1990).

In contrast to N, which is abundant in the atmosphere, the principal reservoir for P is rocks and sediments. It is released slowly by weathering, and in unpolluted waters often is in short supply relative to metabolic demand. Phosphorus levels are generally higher in regions

TABLE 11.2 Phosphorus and nitrogen concentrations in various rivers. Values indicate average or range (|g L 1). ''nd'' indicates that phosphorus was nondetectable, which corresponds approximately to values at or below 1 |g L_1.

River

Nitrate

Total N

0 0

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