Antibiotic Pollution Characteristics and Ecological Risk Assessment in the Tidal Reach of the Minjiang River During Low-Water Periods Under Policy Intervention

• • Policy intervention reduced tetracycline concentrations: doxycycline fell from 7.75 ng/L to undetectable, and basin-wide mixed risk quotient (MRQ) dropped from medium to low risk, demonstrating direct efficacy of source control. • • Hidden substitution risk emerged: lincomycin (up to 4.6 ng/L), clarithromycin (1.3 ng/L), and florfenicol (0.6 ng/L) were newly detected, indicating that policy-driven reduction of one antibiotic class can be offset by increased use of alternatives. • • Spatial shift of pollution hotspots: high-concentration zones moved from urban residential areas (2022) to intensive aquaculture zones and upstream reservoir areas (2024), driven by land-use conversion and tidal effects, with sulfamethazine reaching an extreme value of 1112.3 ng/L in aquaculture areas. • • Land-use and policy interaction: agricultural land positively correlated with sulfonamide and macrolide concentrations, while construction land dominated urban sources; effective policy intervention counteracted negative effects of construction land expansion, but aquaculture pond expansion amplified local pollution under tidal conditions.