Spatiotemporal variations and burial fluxes of methylmercury in the sediments of Jiaozhou Bay.

Methylmercury (MeHg) in coastal sediments poses significant ecological and human-health risks due to its high toxicity, strong bioaccumulation potential, and mobility within benthic environments. This study investigates the spatiotemporal distribution, preservation mechanisms, and historical burial fluxes of MeHg in surface sediments and sediment cores collected from Jiaozhou Bay (JZB), a semi-enclosed and urbanized bay in northern China. Surface sediments exhibited low-to-moderate MeHg concentrations (0.03-0.37 ng/g) with a pronounced east-west concentration gradient. Higher MeHg levels were found in the northeastern estuary and the bay mouth, primarily driven by elevated inorganic Hg inputs (from riverine and wastewater sources) and enhanced methylation potential. Correlation analyses revealed that MeHg concentrations in surface sediments were weakly and positively correlated with total Hg (p < 0.05). Furthermore, the %MeHg were significantly modulated by key environmental parameters, including total organic carbon, nitrogen, sulfur, and grain size (p < 0.05). In the sediment cores, Core 14 exhibited significant inter-annual variability with a gradual long-term increase; meanwhile, Core 20 showed a sharp increase in 2014-2015, and Core 28 recorded elevated values during the 1990s, 2016, and 2018. The sedimentary MeHg burial fluxes across the cores increased steadily over time, reaching a common peak around 2013-2017. This maximum accumulation is likely due to the combination of increased MeHg inputs and favorable methylation conditions. The maximal MeHg accumulation observed in the subsurface layers of this coastal bay raises significant concern regarding the ecological contamination risk posed by sediments acting as a persistent secondary source.