Distribution of Geochemical Species of P, Fe and Mn in Surface Sediments in the Eutrophic Estuary, Northern Taiwan

The Danshuei River Estuary (DRE) in northern Taiwan is a seriously eutrophic estuary due to the domestic effluent discharge. Surface sediment samples were collected from the DRE to study the concentrations and spatial distributions of different fractions of phosphorus through the five-step sequential extraction method which chemically divides the sedimentary P into five fractions: P_SORB, P_CDB, P_CFA, P_DET, and P_ORG. The Fe and Mn contents in the extracted solution were also determined. The total organic carbon (TOC) and grain size in sediment samples were analyzed as well. The sedimentary total P (TP) concentrations ranged within 537–1310 mg/kg and mostly exceeded 800 mg/kg, suggesting that the DRE sediments were moderately polluted by phosphorus. The P_CDB was the dominant fraction of P, averagely contributing 58% of TP, followed by P_DET 31%. The contributions of the P_SORB and P_CFA fractions to the TP were relatively minor. Two fractions, Fe_CDB and Fe_ORG, of sedimentary Fe equally shared approximately 70% of total Fe, followed by Fe_DET with 22%. The contribution of different fractions of sedimentary Mn followed the sequence: Mn_CDB (36%) > Mn_CFA (29%) > Mn_ORG (14.7%) > Mn_DET (14.5%) > Mn_SORB (5.3%). The sedimentary P, Fe, and Mn within the DRE are easily mobilized because they were mainly present in the reducible fraction. The concentrations of sedimentary TP positively correlated with the TOC contents and inversely negatively correlated with grain size, suggesting that the TOC and grain size play the crucial roles in influencing the distribution of sedimentary P within the DRE. Finally, the Fe(III) (hydro)oxides seems to play an important carriers to adsorb dissolved P because P_CDB positively correlated with Fe_CDB.