Phosphorus is considered the primary nutrient limiting eutrophication. The release of phosphorus from sediments not only provides hydrophytes with essential nutrients, but can also produce entrophication in lakes. In this study, the sediment characteristics, phosphorus speciation, and environmental factors affecting phosphorus release from sediments from Taihu Lake, Tien Lake, and East Lake were investigated. The results indicated that phosphorus levels of sediments ranged from 647 ± 67 to 2343 ± 122 mg/kg. Inorganic phosphorus comprised the majority of total phosphorus. The main phosphorus speciation of sediments obtained from Taihu Lake and Tien Lake was Residual-P, and the main phosphorus speciation of sediment obtained from East Lake was NaOH-P. Phosphorus release occurred in acidic and alkaline conditions but more frequently under alkaline conditions. The amount of phosphorus released from the sediments increased as the disturbing intensity rose. In addition, less phosphorus was released in natural than anaerobic conditions. The amount of phosphorus released was elevated as temperature increased.

Aquaculture wastewater is characterized by large discharge volumes and variable nitrogen concentrations, posing challenges for stable and efficient treatment. This study investigated biochar-amended bioretention systems (BBSs) under varying temperatures (8.0–26.0 °C), influent TN levels, and operation modes (intermittent and continuous flow). In intermittent runs, the 20% biochar system (BBS20) achieved 72.4% TN removal at low influent TN (9.55 mg/L) and 80.4% at high TN (29.96 mg/L), significantly outperforming the control (CBS). In continuous runs, BBS20 reduced effluent TN to 1.75 mg/L within 72 h, yielding higher average HRT, HLR, and ELR than CBS. Mechanistic analyses showed that biochar addition enhanced extracellular polymeric substance (EPS) secretion, stimulated electron transport system activity (ETSA), and increased the relative abundance of denitrifying genera and functional genes (e.g., nirS, narG). These synergistic effects optimized nitrification–denitrification coupling, particularly under low-temperature conditions. The findings demonstrate that biochar amendment is a practical and effective strategy for improving nitrogen removal from aquaculture wastewater.