Limited ability of estuarine fine-sized suspended particulate matter to carry bioavailable-phosphorus to mitigate marine phosphorus-limitation

Phosphorus (P) input from major rivers is considered to be vital for marine nutrient balance, supplying bioavailable-P. However, the potential for bioavailable-P carried by suspended particulate matter (SPM) to be transported to the open sea is unclear. This study investigated bioavailable-P distribution and transport on SPM at the Yellow River estuary using P fractionation, Ivanoff method, and ³¹P-NMR spectroscopy. Results indicate that the gravity sedimentation time of SPM is strongly negatively correlated with particle size (p < 0.05, r > 0.8). P fractionation showed NaOH-P and organic P (OP) in fine-sized SPM (settled after 200 min) were 1.84 and 1.69 times higher than in SPM settled at 10 min. However, ³¹P-NMR analysis demonstrated that the OP carried by fine-sized SPM was mainly orthophosphate monoester (low bioavailability and immobile) and pyrophosphate, accounting for over 95 % of biogenic-P (excluding orthophosphate). Ivanoff method found pyrophosphate in SPM would be eluted by estuarine salinity, increasing dissolved non-reactive P to 0.357 mg L⁻¹. Specially, dissolved reactive P adsorption experiments showed fine-sized SPM is a weak source of dissolved reactive P in water column, while coarse-sized SPM is strong. It proved that NaOH-P in fine-sized SPM is more bio-unavailable Si-Al/Fe/Mn-P. Therefore, there is few bioavailable-P that can be carried by fine-sized SPM and transported over long distances to the open sea to alleviate nutrient imbalance in the Bohai Sea.