Increasing anthropogenic loading of phosphorus (P) threatens aquatic ecosystems. The bioavailability of P in sediments for algal growth depends on several physiochemical properties such as silt, clay, mineralogy, pH, electrical conductivity (EC), and carbonate-content. This study was aimed on selecting the best chemical extraction method to characterize P-availability for the algae Selenastrum capricornutum. Principal component analysis (PCA) of the data identified two components that cover 79.3% of the total variation, and these components were dominated by particle size distribution, active calcium carbonate equivalence (ACCE), and EC. Many of the considered extractions were positively correlated with each other, with the exception being Bray-II. The sediments of some rivers had a higher Olsen-extractable P than 20 mg kg-1, that is considered a threshold value above which the aquatic environment may become negatively affected. The average rank order of P extraction by single extractants was: Colwell > Mehlich III > NaOH 0.1 M > Olsen > Morgan > AB-DTPA > Bray II. The Colwell-extractable P concentrations of sediments varied from 1.44 to 88.0 mg kg-1. This extractant significantly correlated with algal growth (r2=0.92, P<0.001) and gave a rough estimate of the amount of bioavailable P in sediments. Therefore, the bioavailable content of P in sediments is quite high in comparison with the soils (agronomic requirement = 15 mg P kg-1) around the Urmia Lake basin.