Phosphorus speciation in coastal sediment of Osaka Bay: Relation to anthropogenic phosphorus loading

This study assessed the effect of increased anthropogenic phosphorus (P) loading on P speciation in coastal sediments in Osaka Bay, an urbanized enclosed bay, and one of Japan's most industrialized regions. The study analyzed Total P (TP) and determined the accumulations of five different P species Exchangeable P (Ex-P), Iron-bound P (Fe-P), Calcium-bound P (Ca-P), Detrital P (Det-P) and Organic P (Org-P)in the sediment using the sequential extraction method. We also estimated the historical P loading into Osaka Bay from major river catchments (the Yodo and Yamato Rivers) using the Soil and Water Assessment Tool. This study identified peak P loading values for the Yodo River in 1965 and the Yamato River in 1968, with the Yodo River being the primary anthropogenic source of P inflow for Osaka Bay. The concentration of TP ranges from 290.5 to 353.1 mg/Kg respectively, and the order of concentrations of the different P species is Org-P > Det-P > Ca-P > Fe-P > Ex-P. The results indicate a positive relationship between total P loading and Ca-P during the rapid growth period. We examined two potential mechanisms (adsorption-precipitation process and sink switching) for Ca-P formation and hypothesized that the adsorption-precipitation process is the dominant process, indicating that high anthropogenic P loading causes the accumulation of immobile Ca-P species in coastal sediments and serves as a permanent sink for P, as well as for water quality restoration. This result also suggests the possibility of reconstructing historical human-induced P loadings based on Ca-P in coastal sediments.