SEASONAL VARIABILITY AND BIOGEOCHEMISTRY OF PHOSPHORUS IN THE SCHELDT ESTUARY, SOUTH-WEST NETHERLANDS

The geochemistry of phosphate in the highly polluted Scheldt Estuary has been studied during eight cruises in 1987-88. Property-salinity plots show significant seasonal variability in the environmental conditions which trigger phosphate behaviour, such as dissolved oxygen, pH and phytoplankton activity. Consequently, the behaviour of orthophosphate differs from season to season. During autumn and winter, when phytoplankton activity is low or negligible, the orthophosphate profiles show the occurrence of a buffering mechanism, probably caused by desorption of particle-bound phosphorus in response to the increase in pH with increasing salinity. During spring and summer, the geochemistry of orthophosphate is influenced by the presence of anoxic headwaters. High phosphate concentrations are observed in the anoxic river water entering the estuary, probably reflecting release from the sediments. However, phosphate is rapidly removed from solution in the low-salinity zone due to co-precipitation with iron oxyhydroxides, formed by reoxidation in the water column. Seaward from the low-salinity high-turbidity zone, phosphate is removed from solution during phytoplankton blooms, especially during spring. Blooms also affect the suspended matter composition, in that a shift towards biogenic elements (POC, N, P) at the expense of lithogenic elements (Fe, Al) occurs. Moreover, the speciation of particulate phosphorus shifts from iron- and aluminium-bound forms to POC-bound phosphate in zones of high primary productivity. These findings have implications for the future management of the Scheldt Estuary, as nutrient reduction policies are being implemented.