The reactivity of dissolved and suspended particulate phosphorus pools decreases with distance downstream in the Yellow River

The Yellow River is a potentially important source of terrestrially derived phosphorus to the phosphorus limited Bohai Sea. However, seasonal variation in concentrations, partitioning and bioavailability of dissolved and particulate phosphorus along the length of the Yellow River are poorly constrained. Here, we measure dissolved and suspended particulate phosphorus at 72 stations from the source to the estuary of the Yellow River during the rainy season in 2020 and dry season in 2021. Mean concentrations of total phosphorus, total dissolved phosphorus and dissolved reactive phosphorus were higher in the rainy season than the dry season. Analysis with sequential fractionation indicated that generally phosphorus associated with calcium carbonate dominated the suspended particulate pool. However, phosphorus content and the relative contribution of iron-bound phosphorus in suspended particles increased during the dry season, suggesting seasonal variations in bioavailability. Reactivity of the phosphorus pools decreased from source to estuary, suggesting low export potential of bioavailable phosphorus to the Bohai Sea. The composition and reactivity of dissolved and suspended particulate phosphorus pools varies both along the length of the Yellow River and between the rainy and dry seasons, according to 31P-NMR analysis of water and suspended particulate samples from 72 stations along the Yellow River