Mangrove and salt-marsh wetlands are important coastal carbon sinks. In order to quantify carbon export via pore water exchange and to evaluate subsequent fate of the exported carbon, we carried out continuous observations in a mangrove-Spartina alterniflora ecozone in the Zhangjiang River Estuary, China. The carbon fluxes via pore water exchange were estimated using ²²²Rn and ²²⁸Ra as tracers to be (2.15 ± 0.63) mol/(m²∙d) for dissolved inorganic carbon (DIC) and (–0.008 ± 0.07) mol/(m²∙d) for dissolved organic carbon (DOC) in the wet season and (3.02 ± 0.65) mol/(m²∙d) for DIC and (–0.15 ± 0.007) mol/(m²∙d) for DOC in the dry season in the mangrove-dominated creek (M-creek), while (2.52 ± 0.82) mol/(m²∙d) for DIC and (0.02 ± 0.09) mol/(m²∙d) for DOC in the dry season in the S. alterniflora-dominated creek (SA-creek). The negative value means that pore water was a sink of DOC in the creek. The total carbon via pore water exchange in the tidal creeks in the mangroves accounted for 41%–55% of the net carbon fixed by mangrove vegetation and was 3–4 times as much as the soil carbon accretion in the mangroves. The exported carbon in the form of DIC contributed all of the carbon outwelling from the M-creek and 79% of the carbon outwelling from the SA-creek, implying effective fixation of carbon by the wetland ecosystem. Moreover, it resulted in 54% in the dry season, 75% in the wet season of the carbon dioxide released from the M-creek to the atmosphere, and 84% of the release from the SA-creek. Therefore, quantification of pore water exchange and related soil carbon loss is essential to trace the fate of carbon fixed in intertidal wetlands.

Subterranean estuaries (STE) are important seawater-groundwater mixing zones with complex biogeochemical processes, which play a vital role in the migration and transformation of dissolved materials. In this study, we first investigated the spatial distributions of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorous (DIP), dissolved inorganic silicon (DSi) and metal elements (As, Ba, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) in STE including upper intertidal, seepage face and subtidal zones. We then estimated submarine groundwater discharge (SGD) and associated nutrient and metal element fluxes. From the generalized Darcy’s law method, SGD was estimated to be 30.13 cm/d, which was about 7 times larger than the inflow (4.16 cm/d). The nutrient and metal fluxes from SGD were estimated to be (5.33 ± 4.99) mmol/(m²·d) for DIN, (0.22 ± 0.03) mmol/(m²·d) for DIP, (16.20 ± 2.05) mmol/(m²·d) for DSi, (1325.06 ± 99.10) μmol/(m²·d) for Fe, (143.41 ± 25.13) μmol/(m²·d) for Mn, (304.06 ± 81.07) μmol/(m²·d) for Zn, (140.21 ± 13.33) μmol/(m²·d) for Cu, (84.49 ± 2.94) μmol/(m²·d) for Pb, (37.38 ± 5.51) μmol/(m²·d) for Ba, (27.88 ± 3.89) μmol/(m²·d) for Cr, (10.10 ± 6.33) μmol/(m²·d) for Ni, and (6.25 ± 3.45) μmol/(m²·d) for As. The nutrient and metal fluxes from SGD were relatively higher than those from the inflow, suggesting that nearshore groundwater acted as the sources of nutrients and metal elements discharging into the sea. The environmental potential pollution of coastal seawater was evaluated by pollution factor index (P_i), comprehensive water quality index (CWQI), and ecological risk index (ERI). Pb mainly caused potential danger of nearshore environment with considerable contamination (P_i = 5.78 ± 0.19), heavy pollution (CWQI = 4.09) and high ecological risk (ERI = 18.00). This study contributed to better understanding the behavior of nutrients and metal elements and improving the sustainable management of STE under the pressure of anthropogenic activities and climate change.

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.

Phosphorus (P) is an essential nutrient for many organisms in the ocean, which plays a central role in the stability of ecosystems and the evolution of the environment. The distribution, occurrence and source-sink process of P in offshore waters are highly influenced by mariculture activities. P transformation in water-sediment system is the key process in P cycling, however, the mechanism is poorly documented in the coastal sea which is influenced by human activities. Based on the comprehensive surveys in the adjacent waters outside Rushan Bay in May, July and August 2014 and February 2015, the form and transformation of P in the suspended particulate matter (SPM) and surface sediment were analyzed. The results showed that contents of total P, authigenic P and organic P of SPM increased with the increase in distance from the shoreline off Rushan Bay, and the detrital-P decreased. The partition coefficient of P between water and SPM was related to the chemical activity of different forms of P, and a higher reactivity of inorganic P associated with SPM was observed. Hypoxia induced by mariculture changes the distribution and morphological composition of P in SPM and sediment in this typical aquaculture area, which can result in a conversion of sink to source of P in the sediment, thereby having a significant impact on the regional nutrient budget and associated with eutrophication.

Submarine groundwater discharge (SGD) plays a major role as a conveyor of metals to coastal waters. However, the seasonal change of metal fluxes derived through SGD is unclear. Here, we evaluated the behaviours and fluxes of trace metals (Mn, Fe, Ba, Pb, U, Cr, Zn, Cu) in an estuary under different seasonal conditions. The behaviours of trace metals revealed that SGD was the source of Mn (3.51 mmol/(m²·d)), Fe (0.174 mmol/(m²·d)) and Ba (0.024 mmol/(m²·d)), but the Cu sink (−0.55 μmol/(m²·d)) and other metals exhibited a seasonal source‒sink conversion. The seasonal variation of dissolved organic matter and the fresh groundwater proportion in subterranean estuaries may have an important effect on metals fluxes especially for the Fe, Mn and Ba. Our result shows that the single seasonal metal fluxes estimation applied to the annual scale will cause a large deviation, up to 3.6 times for Fe, 5.5 times for Mn, and 15 times for Ba. Therefore, the influence of seasonal fluctuations on SGD-derived metal fluxes cannot be ignored, and our findings will be important for comprehending the metal budget and cycle in nearshore environment.

Hypoxia is a common phenomenon in the sea adjacent to the Changjiang River Estuary (CJE), one of the global major estuaries. Submarine groundwater discharge (SGD) is a widely recognized pathway for terrestrial materials entering the sea, and has been found to be significant off the CJE. We used a ²²²Rn mass balance model to estimate the SGD fluxes off the CJE and showed that it is linked to seasonal dissolved oxygen (DO) variations. Average SGD fluxes were estimated to be (0.012 ± 0.010) m³/(m²·d) in winter, (0.034 ± 0.015) m³/(m²·d) in summer, and (0.020 ± 0.010) m³/(m²·d) in autumn. We found a significant negative correlation between DO concentrations and SGD rates with groundwater discharge being highest in the summer flood season. In addition, distribution patterns of SGD and hypoxia zones in summer are spatially overlapped, indicating that SGD is an important contributor to summer hypoxia in this region.

In order to characterize the features of radium isotopes in estuaries of Tianjin, a continuous survey and sampling of typical estuaries were conducted from 2013 to 2017 in this study. The activities of natural radioactive radium isotopes (²²³Ra, ²²⁴Ra, and ²²⁸Ra) in groundwater and surface water were measured by the radium-delayed coincidence counting (RaDeCC) system. The non-conservative behavior of the radium isotopes was investigated under hydrogeochemical conditions and urbanization. The results indicated that in terms of horizontal distribution, the activities of radium in groundwater (Hangu, Tanggu, and Dagang) showed an upward trend from north to south and demonstrated a higher figure than surface water (Haihe River and Duliujian River). Concerning the vertical distribution, the activitives of radium at a 15 m burial depth was higher than that at a 30 m burial depth in all measurements. The activities of radium isotopes in the study area increased with the increase of total dissolved solids, and their desorption behavior on Fe-Mn oxides was constrained by the redox intensity. Different hydrogeological conditions resulted in variations in the vertical profile of radium activities. The activity of radium was regulated by seasonal variation and precipitation in groundwater and surface water. In addition, the rapid urbanization has caused a significant impact on the features of radium isotopes in typical estuaries of Tianjin. Meanwhile, radium isotopes can be applied to reflect the impact of urbanization on surface water-groundwater systems. Clarifying and cleverly utilizing the relationship between behavior of radium isotopes and urbanization will promote the development of the Tianjin Binhai New Area in a healthy way.

Subterranean estuaries, i.e., the mixing zone between terrestrial groundwater and recirculated seawater, host a wide range of microbiota. Here, field campaigns were conducted at the mouth of the subterranean estuary at the Sanggou Bay (Shandong Province, China) over four consecutive seasons at a seepage face (0−20 cm depth). The diversity of benthic microbiome was characterized via 16S rRNA gene sequencing and metagenomics, combined with physic-chemical parameters, e.g., organic carbon, total nitrogen and sulfate contents in sediments. During spring, the dominant species were assigned to the phylum Proteobacteria. Important opportunistic species was assigned to Acidobacteria, Actinobacteria and Bacteroidetes. The key components were identified to be species of the genera Pseudoalteromonas, Colwellia and Sphingobium, indicating the involvement of sediment microbiota in the degradation of sedimentary organic carbon, particularly that of pelagic origin, e.g., phytoplankton detritus and bivalve pseudo-feces. During spring, the microbial community was statistically similar along the depth profiles and among the three sampled stations. Similar spatial distributions were obtained in the remaining seasons. By contrast, the dominant species assemblages varied significantly among seasons, with key genera being Thioprofundum and Nitrosopumilus during summer and autumn and Thioprofundum and Ilumatobacter during winter. Network analysis revealed a seasonal shift in benthic nitrogen and sulfur metabolism associated with these variations in microbial community composition. Overall, our findings suggested that macro elements derived from pelagic inputs, particularly detrital phytoplankton, shaped the microbial community compositions at the seepage face, resulting in significant seasonal variations, while the influence of terrestrial materials transported by groundwater on the sediment microbiota at the seepage face found to be minor.

Subterranean estuaries (STEs) are characterized by the mixing of terrestrial fresh groundwater and seawater in coastal aquifers. Although microorganisms are important components of coastal groundwater ecosystems and play critical roles in biogeochemical transformations in STEs, limited information is available about how their community dynamics interact with hydrological, geochemical and environmental characteristics in STEs. Here, we studied bacterial and archaeal diversities and distributions with 16S rRNA-based Illumina MiSeq sequencing technology between surface water and groundwater in a karstic STE. Principal-coordinate analysis found that the bacterial and archaeal communities in the areas where algal blooms occurred were significantly separated from those in other stations without algal bloom occurrence. Canonical correspondence analysis showed that nutrients and salinity can explain the patterns of bacterial and archaeal community dynamics. The results suggest that hydrological, geochemical and environmental characteristics between surface water and groundwater likely control the bacterial and archaeal diversities and distributions in STEs. Furthermore, we found that some key species can utilize terrestrial pollutants such as nitrate and ammonia in STEs, indicating that these species (e.g., Nitrosopumilus maritimus, Limnohabitans parvus and Simplicispira limi) may be excellent candidates for in situ degradation/remediation of coastal groundwater contaminations concerned with the nitrate and ammonia. Overall, this study reveals the coupling relationship between the microbial communities and hydrochemical environments in STEs, and provides a perspective of in situ degradation/remediation for coastal groundwater quality management.

In the open ocean, radium isotopes are useful tracers of residence time and water-mass mixing. However, limited by the measurement resolution of commonly used gamma counters, the low activity of radium in the open ocean makes it necessary to enrich radium from large volumes of seawater and pretreat radium-enriched carriers prior to measurements. The commonly applied method of radium enrichment and pretreatment, however, has limitations of uneven coating of MnO₂ on cartridges, relatively expensive cartridges, time-consuming issues during cartridge-ashing, ash loss during transfer, and changes of gamma counters efficiency caused by different ash weights. To address these issues, in this study we optimized the enrichment and pretreatment of low-activity radium prior to measurements. Firstly, we replaced commonly used acrylic cartridges with cheaper polypropylene cartridges, which took 6 h to be ashed, 42 h shorter than for acrylic cartridges. Secondly, MnO₂-coated cartridges were prepared with a circulating hot acidic KMnO₄ solution to ensure homogeneous coating. The radium extraction efficiency of this MnO₂-coated cartridge was 20%–61% higher than that prepared by directly immersing cartridges in the solution. The radium delayed coincidence counter efficiency for MnO₂-coated cartridge was stable with a moisture content of 0.05–1. Lastly, after ashing cartridges, instead of directly transferring the ash to a measurement vial, a mixture of hydroxylamine hydrochloride and hydrochloric acid was used to completely leach the ash for long-lived radium, followed by coprecipitation by BaSO₄, to avoid potential loss of ash during transfer and variations in measurement geometry due to different ash weights. And the recovery of long-lived radium pretreatment was 94%–102%, which improved by 11% compared with the common method. In addition, the radium extraction efficiency of the MnO₂-coated cartridge varied from 3% to 4% within the in situ pump working flow rate of 4–7 L/min, which fell within the measurement errors.