Coupled polyphosphate storage and DOP mobilization along estuarine gradients in the Pearl River Estuary

River-dominated estuaries increasingly experience phosphorus (P) stress as dissolved inorganic nitrogen rises, elevating the role of dissolved organic P (DOP) in sustaining productivity. We surveyed 23 surface stations across the Pearl River Estuary (PRE) and quantified dissolved and particulate P pools, the enzyme-hydrolyzable share of soluble non-reactive P (SNP; a proxy for bioavailable DOP), size-fractionated extracellular phosphomonoesterase (PMEA) and phosphodiesterase (PDEA) activities, and particulate polyphosphate (polyP). Soluble reactive phosphorus (SRP) declined seaward, whereas SNP dominated offshore; 17.9-92.7 % of SNP was enzyme-hydrolyzable. PMEA and PDEA were concentrated in the 2-200 μm fraction, implicating phytoplankton and particle-attached bacteria. Particulate polyP was abundant and covaried positively with phosphatase activities across stations. A principal-component analysis separated a river-influenced regime (higher SRP, particulate organic P, and diester-hydrolyzable DOP) from a phytoplankton-dominated regime (higher chlorophyll a, polyP, and phosphatase activities). These patterns are consistent with a co-occurring microbial strategy: luxury polyP storage during nutrient pulses coupled with phosphatase-mediated DOP mobilization when SRP is scarce at micro-scales, in particle-rich microhabitats. Management implications include reducing particulate and bioavailable organic P alongside inorganic P to weaken this storage-mobilization coupling in turbid waters.