Sedimentary Records of Organic Matter and Organochlorine Pesticides in a Eutrophic Shallow Plateau Lake: Implications for Eutrophication-Driven Pollutant Remobilization

Lake eutrophication may promote the remobilization of historically buried organochlorine pesticides (OCPs), yet the underlying mechanisms remain unclear. This study reconstructed an 80-year sedimentary record from Lake Qilu to elucidate the linkages between organic matter evolution and OCP dynamics. Sedimentary proxies reveal a transition from oligo-mesotrophic conditions before the 1980s to eutrophic states since the 1990s, with significant increases in total organic carbon (TOC), dissolved organic matter (DOM), and hydrophilic organic matter (HIM), alongside δ15N enrichment from +2.0‰ to +11.3‰ and a positive δ13C shift from −21.7‰ to −14.5‰. The OCP record exhibits three distinct phases: accumulation during the 1940s–1980s, decline following regulatory bans during the 1990s–2000s, and anomalous resurgence during the 2010s–2020s, with ∑Drin (sum of aldrin, dieldrin, and endrin) reaching a historical peak of 10.54 μg/kg in the 2020s. Significant positive correlations were observed between both individual OCP congeners and total OCP classes and total nitrogen (TN), DOM and HIM (p < 0.01 or p < 0.05), indicating that eutrophication-driven algal proliferation releases hydrophilic organic matter, which forms soluble complexes with hydrophobic OCPs, facilitating their upward migration. This study provides direct field evidence for eutrophication-driven OCP remobilization, with important implications for risk assessment of pollutants in eutrophic lakes worldwide.