Human-induced eutrophication Alters mercury accumulation and speciation in sediments: A comparative analysis of Dianchi and Fuxian Lake, Southwestern China

Abstract

Lake sediments have served as essential reservoirs and sinks for mercury (Hg), providing critical historical records of Hg deposition. However, insufficient research on the accumulation patterns of sedimentary Hg in lakes with diverse trophic states and depositional environments has posed a significant barrier to advancing our understanding of long-term environmental impacts of Hg deposition. This study examined historical trends in Hg concentrations and changes in Hg speciation within sediment cores from two ecologically contrasting lakes on the Yunnan-Guizhou Plateau, China: Dianchi Lake (shallow, hypereutrophic) and Fuxian Lake (deep, oligotrophic). The results showed that Hg concentrations and accumulation rates in all four sediment cores have accelerated since the 1980s, with average Hg concentrations in Dianchi Lake (583 ppb in DC1 and 211 ppb in DC4) significantly higher than those in Fuxian Lake (132 ppb in FX1 and 96 ppb in FX2). Mercury speciation analysis revealed that higher primary productivity in Dianchi Lake favored Hg-organic matter (Hg-OM) complexes, while prolonged anaerobic conditions and lower primary productivity in Fuxian Lake promoted the formation of Hg sulfide (HgS). Strong positive correlations between Hg, total organic carbon (TOC), and total nitrogen (TN) highlighted the pivotal role of organic matter in Hg accumulation and retention. Additionally, C/N and δ¹³C analyses indicated that organic matter in Dianchi Lake originated from a mix of endogenous, exogenous, and anthropogenic sources, whereas it remained primarily endogenous in Fuxian Lake. The synchronous increase in both Hg concentrations and Hg/TOC ratios observed in the DC1 sediment core demonstrated the significant influence of anthropogenic eutrophication on Hg enrichment. In contrast, this effect was less pronounced in the DC4 core, where Hg speciation analysis revealed HgS as the dominant species in surface sediments. These findings highlight the critical role of lake eutrophication and its induced environmental changes in governing Hg speciation and accumulation in lacustrine sediments. This study emphasizes the need to incorporate lake trophic status evaluation and sediment matrix characterization as essential methodological prerequisites when interpreting lacustrine sediment records for reconstructing historical Hg contamination patterns and predicting bioavailability-related ecological risks.