Phosphorus speciation and availability in intertidal sediments of a temperate estuary: relation to eutrophication and annual P-fluxes

For a better understanding of the phosphorus dynamics and bioavailability in temperate climates, sequential chemical extraction techniques were used to study sediment P-pools distribution and relative importance in a eutrophicated estuary. Results indicate a shift in importance from the iron-bound P fraction to the CaCO3-bound P fraction with increasing salinity, which suggests a decrease in bioavailable P from the most inner part of the estuary seaward, as the CaCO3 associated P is believed to be less available for organisms. Salt marsh plants also influence sediment P speciation, through higher organic P fractions (mainly humic acid associated P) when compared to non-vegetated sediments. The ratio of iron-bound P to iron-oxyhydroxides may be used as an indicator of P availability, since it reflects the number of available P sorption sites in the metal oxides. This ratio was lowest in the intermediate zone of the estuary (13.7) when compared to the outer site (29.6) and the upper site (26.1), suggesting saturation of sorption sites. Co-incident with this, the intermediate site is characterized by the occurrence of macroalgal blooms during summer. In the present case, superficial sediments in the Mondego estuary do not show a clear seasonal trend, either in total amounts of exchangeable P or P speciation. Pools of exchangeable P in the surface sediment were w30 times higher than the annual net-export of P (14 ton year 1) from the estuary. The annual P release from the sediment appears diminutive in comparison with the pool-size. Management measures to reduce nutrient input into coastal water systems should therefore consider that phosphorus availability may persist as a result of sediment supply, even after reducing the external point sources.