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Submarine groundwater discharge (SGD) supplies substantial quantities of nutrients from land to oceans. However, SGD and associated nutrient fluxes have long been ignored in Bohai Bay, which is subjected to the serious environmental problem. Here, we investigated the concentrations of radon (222Rn) and nutrients in groundwater and surface water in the western Bohai Bay during May 2017. The flushing time in the bay was estimated to be 38.8–58.3 days based on tidal prism model. The SGD flux was estimated to be 7.3 ± 4.8 cm d−1 based on 222Rn mass balance model. The SGD associated nutrient fluxes were estimated to be (6.3 ± 4.1) × 107 mol d−1 for dissolved inorganic nitrogen (DIN), (1.2 ± 0.8) × 106 mol d−1 for dissolved inorganic phosphorus (DIP) and (7.5 ± 4.9) × 107 mol d−1 for dissolved inorganic silicon (DSi). By establishing nutrient budgets, we found that SGD was a major source, contributing 80.8% of all source for DIN, 90.7% of all source for DIP and 78.4% of all source for DSi into the western Bohai Bay. This study shows that SGD associated nutrient fluxes may have significant impact on nutrient budgets in the western Bohai Bay. • SGD was estimated using a 222Rn mass balance model in the western Bohai Bay. • The nutrient fluxes from SGD accounted for 78.4–90.7% of total nutrients inputs. • Nutrients from SGD may have significant impact on nutrient budgets in Bohai Bay. [ABSTRACT FROM AUTHOR]

[Display omitted] • Seasonal sampling of heavy metals in the groundwater was conducted in a subtropical mangrove wetland. • The enrichment trend of heavy metals exhibited seasonal variations along the intertidal transect. • COVID-19-induced attenuation of anthropogenic activity reduced terrestrial input of heavy metals to the sea. • The intertidal aquifer served as a buffer zone for the transport of heavy metals to the sea. Mangrove wetlands connecting the land and sea are critical and dynamic ecosystems which act as biogeochemical reactors that influence the concentrations and distributions of heavy metals in the groundwater and adjacent seawater. In this paper, a subtropical mangrove wetland in China was selected to investigate the spatiotemporal patterns of groundwater heavy metals and the driving mechanism along an intertidal transect with distinct undulated topography and vegetation zonation. Along the transect, five groundwater observation wells (W1-5) were set to collect groundwater samples at three different sediment depths (0.5, 1.2, and 2.0 m) and the advective flux between groundwater and surface water were calculated during four seasonal sampling campaigns from 2019 to 2020. The results showed that the seaward W1-3 acted as the groundwater discharge area while the landward W4-5 served as the surface water recharge area. The results of single-factor contamination index showed that a potential pollution risk of Hg existed at the study sites. The distribution of other heavy metals in the mangrove groundwater exhibited a seasonal variability along the transect during the sampling campaigns. For example, the relative high concentrations of Fe, Pb, Cu, Cr, and Zn focused in the landward zones in the dry seasons (autumn and winter 2019), whereas they changed to the seaward zones in the wet seasons (summer and autumn 2020). Moreover, the intertidal aquifer can act as a sink to enrich heavy metals in winter 2019 and autumn 2020 but become a source to discharge heavy metals to the sea in summer 2020. The above migration patterns of heavy metals were ascribed to the multiple complex factors including the seawater-groundwater interactions, vegetation zonation, biogeochemical reactions, and anthropogenic activities. This study contributed to the better understanding of heavy metal behaviors in mangrove groundwater so as to improve the sustainable management of mangrove wetlands under the pressure of anthropogenic activities and climate change. [ABSTRACT FROM AUTHOR]

[Display omitted] • Seasonal sampling of heavy metals in the groundwater was conducted in a subtropical mangrove wetland. • The enrichment trend of heavy metals exhibited seasonal variations along the intertidal transect. • COVID-19-induced attenuation of anthropogenic activity reduced terrestrial input of heavy metals to the sea. • The intertidal aquifer served as a buffer zone for the transport of heavy metals to the sea. Mangrove wetlands connecting the land and sea are critical and dynamic ecosystems which act as biogeochemical reactors that influence the concentrations and distributions of heavy metals in the groundwater and adjacent seawater. In this paper, a subtropical mangrove wetland in China was selected to investigate the spatiotemporal patterns of groundwater heavy metals and the driving mechanism along an intertidal transect with distinct undulated topography and vegetation zonation. Along the transect, five groundwater observation wells (W1-5) were set to collect groundwater samples at three different sediment depths (0.5, 1.2, and 2.0 m) and the advective flux between groundwater and surface water were calculated during four seasonal sampling campaigns from 2019 to 2020. The results showed that the seaward W1-3 acted as the groundwater discharge area while the landward W4-5 served as the surface water recharge area. The results of single-factor contamination index showed that a potential pollution risk of Hg existed at the study sites. The distribution of other heavy metals in the mangrove groundwater exhibited a seasonal variability along the transect during the sampling campaigns. For example, the relative high concentrations of Fe, Pb, Cu, Cr, and Zn focused in the landward zones in the dry seasons (autumn and winter 2019), whereas they changed to the seaward zones in the wet seasons (summer and autumn 2020). Moreover, the intertidal aquifer can act as a sink to enrich heavy metals in winter 2019 and autumn 2020 but become a source to discharge heavy metals to the sea in summer 2020. The above migration patterns of heavy metals were ascribed to the multiple complex factors including the seawater-groundwater interactions, vegetation zonation, biogeochemical reactions, and anthropogenic activities. This study contributed to the better understanding of heavy metal behaviors in mangrove groundwater so as to improve the sustainable management of mangrove wetlands under the pressure of anthropogenic activities and climate change. [ABSTRACT FROM AUTHOR]