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Oceans are repositories of radionuclides. Radionuclides are transferred through the food chain and cause ionizing radiation hazards for marine organisms. In this study, the transfer characteristics of 226Ra, 40K, 14C, 3H, 137Cs and 90Sr in organisms at different trophic levels in the eastern coast of Yantai city were investigated. The risk of ionizing radiation to organisms was assessed using the ERICA Tool 2.0. The results show no significant changes in the concentration of any of the nuclides in the coastal area compared to the preoperation period of the nuclear power plant. The transfer factor of 137Cs, 40K, 226Ra, 14C, 90Sr and 3H at the different trophic levels of marine organisms were 2.09, 1.29, 1.17, 1.15, 1.06 and 0.74, respectively. The dose rates of ionizing radiation to organisms from six radionuclides ranged from 32.02 nGy·h-1 to 195.49 nGy·h-1 and had a mean value of 102.86 ± 57.30 nGy·h-1. The main artificial radionuclides (14C, 3H, 90Sr, 137Cs) released by nuclear power plants in the study area produced negligible radiation doses to marine organisms. However, other artificial radionuclides present in the effluents of nuclear power plants (99Tc, 110mAg and 131I) as well as other natural radionuclides (includes 210Po, 210Pb, etc) were not included, and further evaluation of these is recommended. [ABSTRACT FROM AUTHOR]

The export of dissolved organic matter (DOM) from rivers is essential for linking terrestrial and marine carbon reservoirs in the global carbon cycle. However, there is limited knowledge regarding how the molecular composition of riverine DOM changes under different hydrological conditions, especially during extreme rainfall events. Moreover, the factors beyond hydrology that impact DOM composition have not been well defined. To address these gaps, samples were collected from a human‐impacted medium‐sized subtropical monsoonal river across various hydrological conditions throughout a complete hydrological cycle. Utilizing high‐resolution mass spectrometry, it was discovered that the solid‐phase extractable DOM (SPE‐DOM) during the high‐flow (1 < runoff (Q): annual mean runoff (Qm) < 3) and extreme‐rain (Q:Qm > 3) periods exhibited a higher number of molecular formulae, lower H/C, higher O/C, and a higher proportion of carboxylic‐rich alicyclic molecules compared to the low‐flow period (LFP) (Q:Qm < 1). These alterations were attributed to input from more diverse sources, particularly a greater input from soil organic matter with higher oxidation degrees. Additionally, the P‐containing formulae were more enriched during the extreme‐rain period, likely from agricultural lands and sediment release. Conversely, the fraction of S‐containing formulae was significantly higher during the LFP, possibly due to the amplified influence of anthropogenic input. Furthermore, the DOM aromaticity did not fluctuate with runoff but was significantly associated with temperature. In summary, the study indicated that the composition of DOM varied significantly under different hydrological conditions, with temperature and anthropogenic activities identified as crucial factors influencing riverine DOM export. Plain Language Summary: Rivers play a crucial role in the global carbon cycle by transporting dissolved organic matter (DOM) from land to the ocean. With the increasing frequency of extreme rainfall events attributed to climate change, there is a growing interest among researchers in understanding how hydrological conditions influence the seasonal variation of DOM. In this study, we investigated a subtropical river in China to explore how the molecular composition of DOM changes under different hydrological conditions and the various factors that contribute to shaping DOM composition. Our findings revealed that the composition of DOM during periods of high discharge and extreme discharge were similar. Still, extreme water discharge, including rainfall events, led to an increased export of heteroatom‐containing DOM. In addition to riverine discharge, factors such as temperature and human activities, including agriculture and sewage discharge, influenced DOM composition from different perspectives. For instance, sewage discharge predominantly introduced S‐containing compounds, while agricultural activities were associated with abundant P‐containing compounds. This study highlights the importance of considering not only changes in sources related to hydrology but also the influences of temperature and human activities when examining how riverine DOM responds to climate pressures. Key Points: The S‐containing formulae were more enriched during the low‐flow periodDissolved organic matter (DOM) exported during both high‐flow and extreme‐rain event periods were higher in O/C and lower in H/CDOM exported during the extreme‐rain event was more diverse, and enriched in P‐containing formulae [ABSTRACT FROM AUTHOR]

The use of 234Th–238U disequilibrium has been widely employed to estimate the sinking flux of particulate organic carbon (POC) from the upper sea and ocean. Here, the deficits of 234Th relative to 238U in the water column and the carbon isotope signature (δ13C) of POC in the East China Sea (ECS) Shelf were measured, which was used to distinguish the fraction of marine and terrestrial POC export fluxes. In the ECS Shelf, very strong deficits of 234Th relative to 238U were observed throughout the water column, with 234Th/238U activity ratios ranging from 0.158 ± 0.045 to 0.904 ± 0.068 (averaging 0.426 ± 0.159). The residence times of particle reactive radionuclide 234Th (τTh–T) in the ECS shelf water varied between 9 and 44 days, which is significantly shorter than that in the continental slope area or the basin area. This phenomenon indicates that there is a more rapid particle scavenging process in the ECS shelf water compared to the continental slope and basin upper water. By applying a two-end-member mixing model based on the δ13C, the fraction of terrestrial POC was estimated to be 0 to 74% (mean: 30 ± 22%) and the fraction of marine POC was in the range of 25% to 100% (mean: 70 ± 22%). Fluxes of marine and terrestrial POC settling to the seafloor exhibited significant spatial differences among different stations, ranging from 11 to 129 mmol C/m2/day and from 2.6 to 38 mmol C/m2/day, respectively. The averaged terrestrial POC fluxes in the southern and northern ECS Shelf were similar (~ 21 to 24 mmol C/m2/day), while the marine POC fluxes in the north (86 ± 37 mmol C/m2/day) were approximately four times higher than those in the south (26 ± 20 mmol C/m2/day). Interestingly, the estimated export flux of both marine and terrestrial POC were approximately one order of magnitude higher than the previously reported burial fluxes of POC (ranging from 1.1 ± 0.1 to 11.4 ± 1.1 mmol C/m2/day) in the underlying bottom sediments, indicating that the majority (> 90%) of both terrestrial and marine POC exported from the upper water column are degraded in the sediments of the ECS Shelf. This "carbon missing" phenomenon can greatly be attributed to rapid decomposition by other processes (including microbial reworking, cross-shelf transport, and possible consumption by benthic organisms). Our findings highlight the dynamic nature of carbon cycling in the continental shelf and the need for further research to understand these processes and improve carbon budget assessments. [ABSTRACT FROM AUTHOR]

The transport and diffusion of substances in seawater are limited by the mixing motion of water bodies, while the main forms of mixing in offshore water bodies are advection and eddy diffusion. The eddy diffusion process of water indicates the possible transport direction of dissolved substances. However, the complex environment in the coastal zone makes it difficult to quantitatively assess the water diffusion process. 222Rn is a useful tool to trace the diffusion process of water bodies. However, studies on the 222Rn distribution and its behavior in the Beibu Gulf are scarce. In this study, the activity distribution characteristics of 222Rn in surface seawater of the Guangxi shelf area of the Beibu Gulf were measured. Based on the one-dimensional, steady-state model, the vorticity diffusion coefficient of 222Rn in the horizontal direction was calculated as (0.42–2.13) × 108 m2/d, and the offshore fluxes of 222Rn under the influence of water mixing were calculated as 2.00 × 1012 Bq/d. Correspondingly, the horizontal transport fluxes of silicate, phosphate, nitrite and nitrate were 6.28 × 10−3 mol/(m2·d), 0.10 × 10−3 mol/(m2·d), 0.20 × 10−3 mol/(m2·d) and 4.15 × 10−3 mol/(m2·d), respectively. These results indicate that the study of eddy current diffusion in offshore marine water facilitates a deeper understanding of the water mixing process and nutrient transport and migration. [ABSTRACT FROM AUTHOR]

210Po and 210Pb activity concentrations were determined in tissues of four fish species collected from the southern coast of China. Activity concentrations were found to vary from 0.16 to 59.0 Bq/kg (210Po) and from 0.10 to 2.26 Bq/kg (210Pb). The mean 210Po activity concentration in tissues decreased in the following order: liver > kidney > digestive system > gonad > gill > skin > muscle. The 210Po/210Pb activity ratio was higher than unity in all samples. The calculated annual committed effective dose from ingestion of 210Po in fish species for adults was in a range of 3.88 to 56.9 µSv/y. [ABSTRACT FROM AUTHOR]

210Po, 210Pb, and 234Th were determined in water columns of the East China Sea (ECS) to investigate their biogeochemical behaviors during a severe red tide event. Dissolved 210Po, 210Pb, and 234Th accounted for large fractions of the total phases. The abnormally high concentrations of dissolved 210Pb were observed. Partition behaviors of these radionuclides were influenced by particle content effect and particle composition based on distribution coefficient (Kd) vs. total suspended matter (TSM) content and Kd vs. ratios of particulate organic carbon and total suspended matter contents (POC/TSM content ratios). The peaks of mass specific activities of 210Po, 210Pb, and 234Th indicated that degraded particles could have an intensified enrichment ability for radionuclides compared with the surficial suspended matters. Fractionation factor of 210Po and 210Pb (FPo/Pb) (>1) and fractionation factor of 210Po and 234Th (FPo/Th) (>1) were much higher at algal blooming regions than that at non-blooming stations, indicating that algal blooms promoted the fractionation of 210Po against 210Pb and 234Th, and proving that 210Po exhibited a stronger affinity for biogenic particles than 210Pb and 234Th when POC content increased in the sea. POC/210Po, POC/210Pb and POC/234Th ratios (content/activity ratios) sharply decreased with depth in both algal bloom and non-bloom stations. The outbreak of algal bloom promoted the complexity of suspended particles and increased the variability of POC/tracer ratios (content/activity ratios) in the different depth of the shallow seas. More considerations should be taken to the difficulty of the selection of export interface and the suitable tracers when algal blooming occurs. [ABSTRACT FROM AUTHOR]