Distribution and potential provenance of trace elements in a 120-year dated sediment core from west Daya Bay, northeastern South China Sea

Diffusion kinetic processes and release risks of trace metals in plateau lacustrine sediments

The ecological risk posed by trace metals in the plateau lacustrine sediments of China has attracted worldwide attentions. A better understanding of the kinetic diffusion processes and bioavailability of these metals in plateau lakes is needed. Using the diffusive gradient in thin films (DGT) and Rhizon, concentrations of Mn, Mo, Ni, Cr, and Co in the sediments, labile fractions, and interstitial water of Lake Fuxian were comprehensively analyzed. According to the DGT-induced fluxes in sediments (DIFS) model, fully sustained and unsustained resupplies are possible ways in which metals are released from solids to the solution. Moreover, the resupply characteristics of metals varied at different depths in the sediments and at different sites in the lake. Based on the DIFS model, the effective concentrations (CE) of the trace metals were calculated and all except Cr showed good linear relationships with the DGT-labile concentrations, indicating that the CE values were valuable for predicting metal bioavailability. According to the CE values, the metal contamination released from the sediments was relatively low based on the Monte Carlo simulation. This study provides a comprehensive solution for studying the environmental behavior and potential ecological risks of toxic metals in sedimentary environment.

Gut microbiome play a crucial role in geographical and interspecies variations in mercury accumulation by fish

Mercury (Hg) contamination in fish has raised global concerns for decades. The Hg biotransformation can be manipulated by gut microbiome and it is found to have a substantial impact on the speciation and final fate of Hg in fish. However, the contribution of intestinal microbiota in geographical and interspecies variations in fish Hg levels has not been thoroughly understood. The present study compared the Hg levels in wild marine fish captured from two distinct regions in South China sea. We observed a quite "ironic" phenomenon that MeHg levels in carnivorous fish from a region with minimal human impacts (Xisha Islands, 92 ± 7.2 ng g-1 FW) were much higher than those from a region with severe human impacts (Daya Bay, 19 ± 0.41 ng g-1 FW). Furthermore, the results showed that gut microbiome determined Hg biotransformation and played a crucial role in the variances in fish Hg levels across different geographical locations and species. The intestinal methylators, rather than demethylators, were more significant in affecting Hg biotransformation in fish. The carnivorous species in Xisha Islands exhibited a higher abundance of intestinal methylators, leading to higher MeHg accumulation. Besides, the gut microbiome could be shaped in response to the elevated Hg levels in these fish, which may benefit their adaptation to Hg toxicity and overall health preservation. However, anthropogenic activities (particularly overfishing) in Daya Bay have severely affected the fish population, disrupting the reciprocal relationships between fish and intestinal microbiota and rendering them more susceptible to pathogenic microbes. Overall, this study provided a comprehensive understanding of the role of gut microbiome in Hg bioaccumulation in fish and offered valuable insights into the co-evolutionary dynamics between fish and gut microbiome in the presence of Hg exposure.

Submarine groundwater discharge and associated metal elements into an urbanized bay

In this study, geochemical tracers (radium isotopes) and heavy metals (Pb, Zn, Cd, Cr and As) were analyzed to derive the submarine groundwater discharge (SGD) and associated metal fluxes during four seasons in an urbanized bay (Daya Bay, China). Results showed that Pb and Zn were the main pollutants in bay water. SGD was found to exhibit an obvious seasonal trend (autumn > summer > spring > winter). Such seasonal patterns may be related to the hydraulic gradient between groundwater level and sea level, storm surges and tidal range. SGD was a dominant source of marine metal elements, contributing 19 %-51 % of the total inputs of metals into Daya Bay. The bay water was classified as slight pollution to heavy pollution, which could be linked to SGD-derived metal fluxes. This study provides a better understanding of the important role that SGD plays in metal budgets and ecological environments of coastal waters.

Multidecadal heavy metals and microplastic deposition records in an urban lake: the ecological risk assessments and influencing factors

With the development of urbanization and economic growth, the urban lake ecosystem faces many challenges derived from external factors. As pollutants in the aquatic environment, heavy metals and microplastics negatively influence the urban lake ecosystem due to their intrinsic properties. To understand the distribution patterns and multidecadal deposition characteristics of heavy metals and microplastics, six sediment cores were collected in March 2021 from a Chinese urban lake, Xinghu Lake, and the isotopic composition of cesium-137 and lead-210 was analyzed for the chronology of the sediment core. Here, the classifications of comprehensive ecological risk evaluation methods for heavy metals and microplastics were adjusted further. Meanwhile, the correlations among heavy metals, microplastics, sediment grains, and natural and social factors were further analyzed. The results showed that the sediments of Xinghu Lake were mainly fine silt (39%), and the average surface area of sediment was 1.82 ± 0.60 m²/g. The average concentrations of cadmium, chromium, copper, nickel, lead, vanadium, and zinc were 0.268 ± 0.077, 59.91 ± 16.98, 23.29 ± 6.48, 52.16 ± 13.11, 36.83 ± 11.78, 119.57 ± 26.91, and 88.44 ± 29.68 mg/kg, respectively. The average comprehensive potential ecological risk indexes of heavy metals and microplastics in sediment cores were 46.59 ± 9.98 and 105.78 ± 23.32 in Xinghu Lake, and their risks were projected to reach high and very high levels by 2030 and 2050. The annual average temperature was the key natural factor for the abundances of heavy metals and microplastics, and the small sediment grain had a significant correlation with these. Agricultural activities were major pollution sources of heavy metals and microplastics, while the chemical fibers and plastic products were closely related to the abundance of microplastics.

Chronology of trace elements and radionuclides using sediment cores in Golden Horn Estuary, Sea of Marmara

Trace elements and radionuclides are substantial pollutants in marine environment since they are non-biodegradable and can be harmful even in minute concentrations. The Golden Horn estuary, where is an inlet of Bosphorus and two creeks, has been seriously polluted by untreated municipal and industrial dischargers for several decades. Since 1998, a large restoration and rehabilitation efforts have been undertaken in the estuary to mitigate the pollution. In the present study, four sediment cores were taken from the Golden Horn estuary to assess the historical accumulation of trace elements and radionuclides. Radiometric dating was implemented by ²¹⁰Pb and ¹³⁷Cs radionuclides and CRS model. Sedimentation rates were calculated in the range of 0.92-0.97 cm yr^-1 in the estuary. The distribution of radionuclides (⁴⁰K, ²²⁶Ra, and ²²⁸Ra) indicated some slight variations which ascribes to the geological characteristics of sediment along the cores. The concentrations of the anthropogenic elements were relatively higher in the intensive industrialization period. Their concentrations reduced in the latest 15-20 years thanks to the large-scale rehabilitation project in the estuary. The pollution indices, namely EF, Igeo, CF, and PLI showed that the concentrations of Cd, Cr, Cu, Pb, Sb, and Sn were above the world averages. Our results provide an insight on the long-term accumulation trends of trace element in the Golden Horn, which revealed that the estuary remains moderately polluted. We suggest that preventive countermeasures are much more important than post pollution remediation in the case of metallic pollution in the estuaries.