IS ALL PHOSPHORUS CREATED EQUAL? CONSIDERING BIOAVAILABILITY WHEN DEVELOPING P TMDLS

Phosphorus (P) is recognized as the nutrient limiting algal growth in most fresh waters. As a result, water quality managers focus on reducing total P (TP) loads and resultant P concentrations in water bodies in an effort to curtail eutrophication. This practice generally assumes that there is a direct relationship between total P and algal biomass. However, not all phosphorus forms support algal growth. While dissolved P has generally been shown to be 100% bioavailable, the bioavailability of particulate P (PP) is variable, depending on the source of the particulate. Reasearch has been completed using algal bioassays to determine the bioavailablity of particulate bound P present in study tributaries. Chemical fractionation techniques were also used to characterize the particulate bound P collected from these tributaries and the lakes they feed. For the tributaries, it was possible to determine a relationship between the bioassay and fractionation results that allows the bioavailability of PP to be estimated without conducting a time consuming, labor intensive, algal bioassay. With the relatively simple fractionation technique available to water quality managers, it is a significantly less arduous task to consider bioavailablity when developing phosphorus TMDLs. This is important for two reasons. By focusing management efforts on bioavailable P sources, the goal of controlling eutrophication will be reached more efficiently. In addition, when pollution trading is used, if non-bioavailable P credits are purchased to meet a waste load allocation for a source of bioavailable P, then it may not be possible to achieve the desired water quality results. Applying the chemical fractionation technique to lake seston allows PP to be characterized in terms of whether the P is associated with organic (algae and detritus) or inorganic (silts and clays) material. This may be useful information when setting a phosphorus water quality standard (WQS) for a water body. Water quality managers will be able to determine how much of the total P in the water body is associated with algae, which will give a more accurate measure of the trophic state. Then a phosphorus WQS may be established that is more clearly tied to use impairment.