<p>Suspended particulate matter (SPM) is commonly regarded as a carrier of particulate phosphorus (P), yet adsorption, desorption, mineral transformation, and biogeochemical reactions during transport can alter P partitioning, retention, and release. Source–sink interpretations are often difficult to compare because studies differ in their definitions of SPM, P pools, control volumes, and observation windows. To address this problem, an evidence-bounded SPM–water interface framework is proposed, in which the exchange–transport ratio, Da_P, distinguishes exchange-limited, partially expressed, and near-equilibrium regimes. System boundaries further determine their environmental expression: rivers are generally characterized by incomplete exchange and downstream displacement, lakes and reservoirs by coupled depositional storage and internal release, and estuaries by the combined effects of salinity, particle concentration, and mixing. On this basis, a diagnostic chain is established that links process-window identification, particle-state characterization, timing assessment, and management response. Current prediction remains constrained by unmatched exchange and transport timescales, weak correspondence between operational P fractions and ecological availability, and limited cross-system transferability. The framework helps prevent short-term aqueous-P attenuation from being misinterpreted as permanent removal and provides a basis for tiered monitoring, dynamic modeling, and process-oriented management.</p> Graphical Abstract <p></p>