Dissolved and particulate phosphorus turnover in a river dominated marginal sea traced by cosmogenic ³²P and ³³P

Given the excessive nitrogen inputs and phosphorus (P) limitation in estuarine and coastal environments, P turnover rate is critical for regulating ecological dynamics. In this study, the naturally cosmogenic radioisotopes ³²P and ³³P were measured in the total dissolved P pool and two particulate size fractions (0.45-20 μm and >20 μm), and ³³P/³²P activity ratios were used to estimate P turnover times in the Changjiang Estuary (CJE) and adjacent area of East China Sea during October 2021 (autumn) and March 2022 (spring). Results revealed that atmospheric wet deposition was a primary source governing the spatiotemporal variation of ³²P and ³³P. The ³³P/³²P activity ratios were consistently higher in particulate phases than dissolved P, and increased with particle size, indicating a directional transfer of P from dissolved to particulate phases and prolonged retention in larger particles. Dissolved P residence times were estimated ranged from 4.6 to 13.7 d, with shorter values in spring and nearshore waters, correlating with higher bio-available P fractions and chlorophyll-a levels. Particulate P exhibited longer residence times (5.3-14.4 d), with larger particles having slower turnover, except in the estuary where small particles P exhibited the longest residence time since resuspended activities in turbidity zones. Dissolved P turnover rates ranged from 54.4 ± 2.8 to 506.6 ± 21.9 μmol m^-3 d^-1, with higher values observed in spring and nearshore water. The rapid dissolved P turnover rate facilitated efficient nutrient recycling especially under P-limited conditions in spring, sustaining high primary productivity and potentially exacerbating algal bloom risks. This study highlights that the crucial regulatory role of P turnover rate, rather than concentration alone, in driving coastal eutrophication dynamics, providing essential insights for ecological management in the CJE and similar systems globally.