Seawater induced release and transformation of organic and inorganic phosphorus from river sediments

This paper reports an investigation of the release of organic and inorganic phosphorus from a riverine sediment subjected to salinity conditions typical of estuarine mixing. Freshwater sediment was mixed with filtered river water in a thermostatted reactor, and allowed to equilibrate under aerobic conditions for 3 days. Salinity was then increased in a stepwise manner by addition of filtered low-nutrient seawater over a period of 4 days. A control experiment was performed in a second reactor by substituting ultrapure water for seawater. Using a flow injection analysis method for measurement of filterable reactive phosphorous (FRP, the so-called inorganic fraction) and filterable organic phosphorous (FOP), it was found that rapid releases of both FOP and FRP occurred at salinities of 10‰. Over the 4-day experimental period, sediment release increased the filterable P concentration by approx. 50 μg L−1, and of this, nearly half was initially present as FOP, which subsequently underwent rapid mineralisation to FRP. The observed behaviour may be explained by a combination of salinity induced plasmolysis of sediment bacteria and ion exchange by suspended sediment particles.