Desorptive behavior of phosphate in the subtropical Miyara River, Ishigaki Island, Japan

The desorption of phosphate-phosphorus (PO₄) from riverine particles with a salinity increase in Miyara River water, Ishigaki Island, Japan, was examined in laboratory experiments involving mixing of unfiltered riverine water with coastal seawater filtered through a 0.4-μm Nuclepore filter and the addition of sodium chloride (NaCl) into unfiltered riverine water. The mixing experiment yielded a convex pattern in the dissolved PO₄-salinity plot, and the dissolved riverine PO₄ concentration increased with the NaCl addition. These results suggested PO₄ desorption from riverine particles with a salinity increase. The pattern of desorption could vary according to the sample conditions, such as the concentrations of dissolved PO₄ and suspended solids. However, the obtained results seemed to simulate the convex distribution pattern in the dissolved PO₄-salinity plot observed at the mouth of the Miyara River in general.