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 greater than or equal to 10parts per thousand. Over the 4-day experimental period, sediment release increased the filterable P concentration by approx. 50mug 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. (C) 2003 Elsevier Ltd. All rights reserved.