Zhangjiangkou Mangrove National Nature Reserve is a subtropical wetland ecosystem in southeast coast of China, which is of dense population and rapid development. The concentrations, sources, and pollution assessment of trace metals (Cu, Cd, Pb, Cr, Zn, As, and Hg) in surface sediment from 29 sites and the biota specimen were investigated for better ecological risk assessment and environmental management. The ranges of trace metals in mg/kg sediment were as follows: Cu (10.79–26.66), Cd (0.03–0.19), Pb (36.71–59.86), Cr (9.67–134.51), Zn (119.69–157.84), As (15.65–31.60), and Hg (0.00–0.08). The sequences of the bioaccumulation of studied metals are Zn > Cu > As > Cr > Pb > Cd > Hg with few exceptions. Cluster analysis and principal component analysis revealed that the trace metals in the studied area mainly derived from anthropogenic activities, such as industrial effluents, agricultural waste, and domestic sewage. Pollution load index and geoaccumulation index were calculated for trace metals in surface sediments, which indicated unpolluted status in general except Pb, Cr, and As.

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The South China Sea (SCS) is the largest semienclosed marginal sea in the western Pacific (WP) and connects to the west Pacific through the Luzon Strait (LU). In this study, we use the observation of transient tracer chlorofluorocarbon-12 (CFC-12) to calculate the ventilation time scales of the SCS, LU, and WP. The CFC-12 and oxygen data are used together to identify the sandwiched structure vertically of the flows across the LU. The CFC-12 and oxygen distributions reveal a pronounced decrease westward across the LU and a slight decrease southward in the transport of the SCS. The mean age gradient of the salinity minimum (S min ) water between the WP and the northern SCS could be a consequence of intensive mixing and entrainment of the inflow water from the WP. An expected difference in age between the LU and SSCS is verified to reflect the transit time for the given water layers in the SCS. Thus, a mean transit time of 77 ± 20 years is estimated for the intermediate water in the SCS interior.

CFC‐12 and SF6 data were used in combination to estimate the mean age of water in the northern South China Sea (NSCS), to explore oceanographic processes related to “time,” including the transit time through the Luzon Strait (LS) with a three‐layer circulation structure and the apparent oxygen utilization rates (AOUR). Significant differences in mean ages of water were observed at the same density level in the water columns on both sides of the LS, presented as a westward flow in the upper layer, eastward flow in the intermediate layer, and westward flow in the deep layer with transit times of 8 ± 5, 39 ± 22, and 20 ± 18 yr, respectively. The AOUR was estimated to be 8.4 μmol kg−1 yr−1 at about 100 m and decreased to approximately 0.66 μmol kg−1 yr−1 at 1,500 m in the NSCS. The average organic carbon flux in the depth range of 100–1,500 m was 1.7 mol C m−2 yr−1 in the NSCS and 1.3 mol C m−2 yr−1 in the western Pacific Ocean (WP). The activation energy—derived using the Arrhenius equation—in the NSCS and WP (87.7–154.2 kJ mol−1) are close to those in the northern Pacific Ocean (60.8–133.5 kJ mol−1). These results suggest a conspicuous correlation between temperature and AOUR. The impact of climate change on the ocean and the feedback mechanism between ocean warming and oxygen consumption needs further investigation.

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