Phosphorus availability of fluvial sediments determined by algal assays

Alternative methods to define the bioavailability of phosphorus in soils and sediments to algae were compared, using Selenastrum capricornutum as the test algae in bioassays. Two algal assay techniques were used; one involved the incubation of the alga, with the soil or sediment providing the sole source of phosphorus, the resultant cell numbers being compared with those from cultures using a medium with known concentrations of phosphorus pentoxide; the other method again used soil or sediment as the sole phosphorus source, but assessed algal assimilation by the decrease of remnant phosphorus with time. Two chemical extraction techniques for the assessment of available soil phosphorus were also used, one employing sodium hydroxide and the other nitrilotriacetic acid, and their results were compared with those from the bioassays. Uncertainties in the interpretation of the determinations by each method are discussed extensively. In the absence of a trustworthy chemical extraction technique, it is concluded that algal assays must be preferred.