The Transformation of P, Fe, and Mn Fractions in Sediment Cores of a Highly Eutrophic Estuary in Northern Taiwan

The Danshuei River Estuary (DRE), located in northern Taiwan, is severely polluted by dissolved N (ammonium exceeding 500 μM) and P (dissolved phosphate exceeding 10 μM) due to the domestic wastewater discharging into the river system. However, the knowledge of nutrient pollution in the estuarine sediment is very limited. Three sediment cores (SCs) were collected from the DRE to study P pollution status, and five fractions of sedimentary P, Fe, and Mn were analyzed by Ruttenberg’s sequential extraction method. The analyzed results indicate that the total P concentrations and P fractions in the sediment core of the upper estuary differed significantly from those of the lower estuary. The total p-values in the SC of the upper estuary generally exceeded 2000 mg/kg and were dominated by ferric iron-bound P (PCDB), accounting for 80% of total P pools. This result indicates that the mobilization of the PCDB fraction in the SC was significant and dominated the geochemical P cycle rather than the remineralization of organic P. Meanwhile, the corresponding values decreased to 650 mg/kg and 50% in the SC of the lower estuary. The dominance of the PCDB fraction decreased seaward. Meanwhile, the other four fractions, especially for the PDET fraction, increased seaward. The transformations of the P fractions differed across the different depth intervals in the three sediment cores. The sequence of the dominant fractions of sedimentary P and Fe was similar but differed from the Mn fractions in the three sediment cores.