Behaviour of phosphate in estuarine water

Abstract

The phosphate load in many rivers has been increased through soil disturbance and discharge of sewage and other wastes. Understanding how much phosphate may be removed from the waters of rivers and estuaries to the sediment would aid waste disposal planning. Phosphate carried through estuaries to coastal waters can increase growth of phytoplankton and macroalgae, with effects on fisheries and fouling of structures and beaches. Removal of dissolved phosphate to the sediment in estuaries has been predicted from indirect evidence^1–3, but measurements of dissolved phosphate in estuaries have shown various patterns of behaviour^4–7. A constant level of dissolved phosphate over a wide salinity range, observed in some estuaries^6,7, has been attributed to buffering of the phosphate by particulate matter. This implies the release of phosphate from particulate matter in some conditions. Seawater held in contact with large concentrations of sediment in laboratory conditions tends to a steady-state level of dissolved phosphate in the range 3–28 µg P l⁻¹ (refs 8–11). Our laboratory studies, using a simple estuarine mixing model system, show that inorganic removal of phosphate is likely to be less than 10% in estuaries fed by rivers of iron content near the world average¹².