The hydrological regime and particulate size control phosphorus form in the suspended solid fraction in the dammed Huanghe (Yellow River)

The Huanghe River (Yellow River) had been the second largest river in the world in terms of sediment load to the sea; however, the river water discharge and sediment flux to the sea and their seasonal variability have been significantly altered by the dam activities and recent water-sediment regulation. These changes are believed to have important impacts on the flux of phosphorus that is generally transported in particulate form. In this article, the samples of suspended particulate matter (SPM) were collected at the Lijin Station during two high-discharge events in 2005 and were separated by particle size. Sequential extractions were applied to determine the forms of P in different particle size fractions and to assess the potential bio-availability of particulate phosphorus (PP). Based on the in-laboratory measurement, the impacts of different hydrological regimes on the source of PP and its bio-availability were also analyzed. The results indicate that exchangeable, organic, authigenic, and refractory P were preferentially associated with clay, very fine, and fine silt fractions. Detrital P was mainly associated with the medium and coarse silt fractions. Detrital P and authigenic P (two forms of calcium bound phosphorus) were the dominant fractions in all samples. Thus, the potential bio-available PP (exchangeable P and organic P) was mainly associated with the finer particles, such as clay. Higher content of exchangeable, organic, authigenic, and residual P and lower content of detrital P were found during the period of rainstorm compared to that of WSR. P forms and partitioning of P forms among different particle size fractions were assumed to depend on the sources of SPM. It is likely that the pathways and fates of PP forms were controlled by damming and by the related changes of hydrological regime. Therefore, anthropogenic changes of hydrological regime and particle size dominanted the amount and distribution pattern of bio-available P transportation to the estuary and the adjacent sea, which will have profound impacts on the marine ecosystems at the Huanghe River Estuary and even the Bohai Sea.