Accumulation rates and chronologies from depth profiles of 210Pbex and 137Cs in sediments of northern Beibu Gulf, South China sea.

Beibu Gulf is a highly dynamic and complex coastal environment that is currently experiencing one of the largest rates of development and urbanization in west China. Little is known about the effects of this increased human activity on coastal sedimentation processes and on the rates of sediment accumulation and the variation of organic materials to the coast. In this study, four sediment cores were collected and applied the 210Pb dating method to reconstruct sedimentation rates and historical changes of materials to the northern Beibu Gulf over the past century. Depth profiles of excess 210Pb (210Pbex) showed highest activity values at the surface (28.4-104.0 Bq kg-1) followed by a linear or exponential decay with depth for all but one study site. 137Cs activity ranged between 0.236 and 2.034 Bq kg-1, and a distinct peak activity - representing the 1963 fallout maximum - was observed at all but one site. Sediment chronologies were determined using the Constant Rate of Supply (CRS) model. Calculated accumulation rates in the studied sites were the lowest in the late 1920s and early 1930s (mass accumulation rate (MAR): 0.06 ± 0.01 g cm-2 y-1; sediment accumulation rate (SAR): 0.08 ± 0.01 cm y-1) and increased gradually until reaching maximum values in the 2010s (MAR: 0.22 ± 0.09 g cm-2 y-1; SAR: 0.46 ± 0.32 cm y-1). Current accumulation rates are up to 800% higher than rates observed in the 1920s, with most of the increase happening after 1970, coinciding with the increasing rate of urbanization and development in the region. The highest increase in SAR over the last century (+877%) was observed in Sanniang Bay, with the lowest rate of increase (+283%) observed in Lianzhou Bay. TOC content in these sediments has also increased over the last 100 years. Current values (0.98-1.28%) are about 170% higher than historical concentrations (before 1970). The positive correlations between TOC and population density and GDP growth in major cities surrounding the gulf, provide further indication that human activities have significantly altered the sedimentary environment in recent decades along the northern Beibu Gulf coast.

Spatiotemporal distribution of nitrogen biogeochemical processes in the coastal regions of northern Beibu Gulf, south China sea.

Nitrogen biogeochemistry occupies a central role in nitrogen cycles and exerts a significant influence on primary productivity and global carbon cycles. In order to better understand the nitrogen biogeochemistry in coastal regions, spatiotemporal nitrogen fixation, denitrification and anammox were investigated in the coastal regions of northern Beibu Gulf (NBG), South China Sea (SCS). Nitrogen fixation was mainly detected in the water column of outer bays, attributed to the low nitrate concentration and low N/P (N/P < 16). Comparisons of the nitrogen fixation rates between unicellular (<10 mm) and the filamentous diazotrophs (>10 mm) indicated that the contribution of unicellular diazotrophs was more important than filamentous diazotrophs. Besides, field investigation revealed that Richelia was the dominant species in filamentous diazotrophs. On the other hand, both denitrification and anammox were found in the surface sediment and denitrification dominated the nitrogen loss process. Denitrification was mainly related to the nitrate concentration in pore water and organic matter in the sediment, while anammox was mainly regulated by the concentration of nitrate and nitrite in pore water. Additionally, temperature, dissolved oxygen (DO) and salinity also had an impact on denitrification and anammox. The net areal yield of nitrogen biogeochemical processes was estimated to be -1079t/a, as an important pathway of nitrogen removal. This study adds to the knowledge of nitrogen biogeochemistry in the nutrient-replete coastal region and highlights its significance in such an environment.

Spatiotemporal distributions, source apportionment and potential risks of 15 pharmaceuticals and personal care products (PPCPs) in Qinzhou Bay, South China.

In the present study, we investigated the spatiotemporal distributions and risks of 15 non-antibiotic pharmaceuticals and personal care products (PPCPs) in surface water collected from Qinzhou Bay, China during the summer and winter of 2017. Our results showed that 14 out of the 15 target PPCPs were detected, and they had higher concentrations in the winter than the summer, except for N, N-diethyl-m-toluamide and indomethacin. Furthermore, N, N-diethyl-m-toluamide and caffeine respectively were the dominant contaminants in the summer and winter, with their highest concentrations up to 33.7 ng/L and 26.8 ng/L. Generally, concentrations of PPCPs were higher in the inner bay than outer bays, with untreated sewage and effluents of sewage treatment plants (STPs) as main pollution sources. The results of the risk assessment indicate that diclofenac may pose risks to aquatic organisms. Overall, our results provided occurrence and important insights into the risk management of PPCPs in Qinzhou Bay.