Characterization of the sediment
The sediments comprise a wide range in P bioavailability and P-retention capacity. In preparation for the algal assay, sediments were dried at 25ºC and passed through a 2-mm sieve. Particle size distributions of the sediments were determined by hydrometer method and organic carbon content followed the dichromate oxidation method (Rowell, 1994). Soil pH and electrical conductivity (EC) were measured in a sediment-solution ratio of 1:5 with a pH and conductivity meter, respectively. Calcium carbonate equivalents (CCE) were determined by acid-neutralization method (Rowell, 1994) and the amount of active calcium carbonate equivalence (ACCE) by reaction with neutral ammonium oxalate (Loeppert and Suarez, 1996).
Sediment chemical and physical properties are shown in Table 1. The data was normally distributed according to the Kolmogorov-Smirnov test. The texture of sediments was often loamy sand and sandy loam. Sand (CV = 58.8%) and clay (CV = 54.3%) contents of sediments showed large variations in comparison with silt (CV = 39.3%). Electrical conductivity was low suggesting that the water was fresh (non-saline). Overall, sediment pH was alkaline (7.26-8.02), which might be related to higher calcium carbonate equivalents (CCE) in carbonate-rich sediments in the basin, and pH was quite similar in these samples. The average CCE and ACCE of sediments were 15.03 (2.50-23.75) and 2.08 (0.54-3.86), respectively.
Sediments were extracted using several single extraction methods that are summarized in Table 2. Olsen-P and Colwell-P extraction method have been designed for calcareous soils. Mehlich III-P and Bary II-P have been designed for acidic soil, and both contain HCl acid and ammonium fluoride for dissolution of Fe/Al-P minerals such as Strengite, (FePO4·2H2O) and Variscite, (AlPO4·2H2O). AB-DTPA extraction method is chelating agent for Fe, Al, Ca, and Mg and has been shown to be a good extraction method for acidic and calcareous soils. Morgan-P was designed for soil with a high association of Ca-P pools and usually dissolve soluble calcium phosphate (Ahmad and Jones, 1967). 0.1 M NaOH is for soil and sediment with high Fe/Al-P pools. Chemical analysis of P was done colorimetrically (Murphy and Riley, 1962).