Multiphase coupling of phosphorus and organic matter across water-particle interaction in aquaculture ponds

The roles of suspended particulate matter in regulating phosphorus (P) and organic matter (OM) cycling remain unclear. We investigated 21 catfish aquaculture ponds across four seasons in 2024-2025 to quantify how seasonality and particle size regulate P-OM interactions. Water samples were collected and then separated into five suspended particle-size fractions (>1000, 1000-450, 450-100, 100-50, and 50-1 nm) and the truly dissolved phase (<1 nm). We integrated Hedley's sequential extraction approach, ³¹P nuclear magnetic resonance spectroscopy, and fluorescence excitation-emission matrix-parallel factor modeling (EEM-PARAFAC) to characterize size-dependent P pools, molecular P signatures, and dissolved- vs. particle-associated OM interactions. Particle mass showed strong seasonal variability, with summer exhibiting substantially higher particle loads, whereas fall showed one-order of magnitude lower particle abundance. Hedley's extraction results showed NaOH-extractable P dominated particulate P pool, while nanoparticles were enriched in exchangeable P fractions (water- and NaHCO₃-extractable P). These results indicate decoupling between particle mass and P reactivity, with nanoparticles serving as highly reactive intermediates for rapid dissolved-particulate P exchange. ³¹P NMR detected orthophosphate across all size fractions, whereas organic monoester P was consistently enriched in larger submicron particles and became weak or undetectable in < 100 nm size fraction. EEM-PARAFAC further linked elevated dissolved P availability with protein-like and labile OM signatures, suggesting enhanced microbial processing and OM-associated nutrient recycling under high-P conditions. Our findings suggest that nanoparticles act as reactive intermediates for dissolved-particulate P exchange; meanwhile, OM composition and microbial transformation processes further regulate the bioavailability of particle-bound P.