Release pattern of internal nutrients after sediment dredging driven by particulate particles deposition in eutrophic...

Persistent deposition of particulate particles (PP) in eutrophic lakes accelerates sediment nitrogen (N) and phosphorus (P) release after dredging, undermining the long-term efficacy of post-dredging remediation. However, the influence of PP across varied quantities and compositions on dredging effects still lacks quantitative study. Herein, we quantified dredging efficacy under laboratory simulations of PP deposition with different cumulative thicknesses and organic matter (OM) contents. The results showed that PP with higher quantity and OM content significantly increased pore water N, P concentrations and their fluxes at the sediment-water interface (SWI), alongside decreasing sediment N, P contents (p < 0.001). Notably, accelerated loss of dredging efficacy manifested specifically in sediment N management, with PP quantity demonstrating dose-dependent effects on N dissolution and release (revealed by two-way ANOVA). In contrast, P release exhibited a "delayed pattern", remaining constrained until the O₂ concentration at SWI declines below critical thresholds. For instance, the reduction in P flux by dredging was maintained at 41 % under 12 mm high-OM PP deposition at 60d (p < 0.001), significantly higher than the 13 % reduction rate of N flux achieved by dredging in the same condition (p > 0.05). The differential variation patterns of N and P potentially arise from the mechanistic differences in their responses to sediment oxygen uptake (SOU), which are dominated by OM mineralization and coupled OM mineralization-iron oxide dissolution, respectively. Thus, prioritizing the suppression of N release is critical for sustainable dredging management. Additionally, our data also demonstrated a risk threshold for annual PP deposition and associated OM enrichment in PP within dredging zones (conservatively defined as both ≥8 mm cumulative thickness and ≥3.3 % TOC), where cautious evaluation prior to dredging operations should be conducted.