Heavy-metal and nutrient enrichment triggered a regime shift in marsh diatom assemblages over the past millennium

The Sanjiang Plain represents the largest contiguous area of freshwater marshes in China. However, these valuable ecosystems have experienced significant degradation due to anthropogenic disturbances associated with rapid economic development. Investigating the historical impacts of human activities on marsh evolution in this region is crucial for future ecosystem management and conservation. We analyzed a 5700-year sediment core from the Dongfanghong marsh using a multi-proxy palaeoecological approach. Diatom assemblages, together with geochemical characterization, including trace metals, nutrient elements, black carbon (BC) and polycyclic aromatic hydrocarbons (PAHs), were integrated with radiocarbon dating to explore the long-term effects of anthropogenic pressure on marsh evolution and to establish natural baseline conditions for the restoration of degraded freshwater marshes. Our results reveal that the Dongfanghong marsh retained a near-natural state until 1300 cal yr BP, as evidenced by low background concentrations across all geochemical proxies. A gradual rise in population during the Han Dynasty initiated small-scale mining and metallurgy, elevating Pb, Zn and Cu enrichment. As Han Chinese agricultural civilization expanded into the region under the Tang Dynasty, population growth, cropland extension, and industrial emissions drove an increase in N, P, Hg, BC and PAHs. These limited activities had not yet perturbed the marsh ecosystem. Nevertheless, a regime shift occurred at 700 cal yr BP, when fertilizer application, biomass and coal burning, and the discharge of industrial and domestic wastes combined to generate a pronounced anthropogenic depositional signal. Progressive nutrient loading, sedimentary heavy metal enrichment, and organic pollutant deposition have deteriorated water quality and driven a shift toward disturbance-tolerant diatom taxa during the last millennium, with this trend accelerating markedly over the past century. Given their sensitivity to water-chemistry changes, diatoms serve as robust bio-indicators for monitoring water quality and guiding adaptive management of freshwater marshes. The stable state of the Dongfanghong marsh prior to 700 cal yr BP thus provides a valuable reference for restoration targets on the Sanjiang Plain.