Dissolved organic carbon (DOC) is not only a critical component of global and regional carbon budgets, but also an important precursor for carcinogenic disinfection byproducts (DBP) generated during drinking water disinfection process. The lack of process based watershed scale model for carbon cycling has been a limiting factor impeding effective watershed management to control DOC fluxes to source waters. Here, we integrated terrestrial and aquatic carbon processes into the widely tested Soil and Water Assessment Tool (SWAT) watershed model to enable watershed-scale DOC modeling (referred to as SWAT-DOC hereafter). The modifications to SWAT mainly fall into two groups: (1) DOC production in soils and its transport to aquatic environment by different hydrologic processes, and (2) riverine transformation of DOC and their interactions with particular organic carbon (POC), inorganic carbon and algae (floating and bottom). We tested the new SWAT-DOC model in the Cannonsville watershed, which is part of the New York City (NYC) water supply system, using long-term DOC load data (from 1998 to 2012) derived from 1399 DOC samplings. The calibration and verification results indicate that SWAT-DOC achieved satisfactory performance for both streamflow and DOC at daily and monthly temporal scales. The parameter sensitivity analysis indicates that DOC loads in the Cannonsville watershed are controlled by the DOC production in soils and its transport in both terrestrial and aquatic environments. Further model uncertainty analysis indicates high uncertainties associated with peak DOC loads, which are attributed to underestimation of high streamflows. Therefore, future efforts to enhance SWAT-DOC to better represent runoff generation processes hold promise to further improve DOC load simulation. Overall, the wide use of SWAT and the satisfactory performance of SWAT-DOC make it a useful tool for DOC modeling and mitigation at the watershed scale.

Hypoxia (O₂ ≤ 2 mg L⁻¹) can severely threaten the survival of marine life and alter the biogeochemical cycles of coastal ecosystems. Its impacts are dependent on its duration. In the present study, hypoxia was observed in autumn at the end of October 2011. It may be one of the latest recorded annual hypoxic events in the East China Sea (ECS). In the hypoxic regions, a large amount of nutrients and dissolved inorganic carbon were observed to regenerate. Also, acidification (low pH) was observed. On the other hand, hypoxic dissipation may be due to the destratification caused by the upwelling of the hypoxic regions in the ECS. These results suggest that hypoxia may occur for longer periods of time than expected and, accordingly, the effects of hypoxia on the ECS ecosystems should be reconsidered and further evaluated.

Microbial communities in subterranean estuaries play important roles in the biogeochemical cycle. However, the microorganisms associated with biogeochemical behaviors in subterranean estuaries have received little attention. Here, the bacterial communities were compared between the fresh and saline groundwater in a subterranean estuary. Correlation analysis between bacterial groups and salinity indicated that different species represented different groundwater types. The key bacterial groups found along the subterranean estuaries have been shown to influence organic pollutant degradation and nitrate utilization. These species may be potential candidates for the in situ bioremediation of subterranean estuaries that are contaminated with pollutants. The utilization of nitrate and organic pollutants by bacteria in subterranean estuaries serves as a nitrate sink and inorganic carbon source. Our results show the role of bacteria in remediating pollutants through submarine groundwater discharge (SGD) to the coastal ocean, and specific species may be helpful in selecting reasonable groundwater end-members and reducing SGD uncertainties.

Copper can be beneficial or harmful to coral at environmentally relevant levels, making environmental monitoring a challenging. Membrane lipids make the cell a dynamic environment according to the circumstances; thus, the lipid profile should be indicative of an environmental/physiological state. To gain more insight into the copper effect on coral health and be a basis of biomonitoring, glycerophosphocholine profiling of coral exposed to microenriched copper levels was conducted in this study. The copper microenrichments resulted in a diacritical effect of decreasing carbonic anhydrase activity, following a supplementation effect, on coral lipid metabolism. Microdifferences in copper levels are critical to determine the coral metabolic state and were therefore included in this study. In addition, an excellent quantitative model correlating the coral lipid variation with the exposed copper levels or the induced physiological effect was obtained to demonstrate its performance for biomonitoring.

The golden tide, caused by the brown algae Sargassum horneri, exerts severe influences on the Pyropia aquaculture of Jiangsu coast, China. To study the outbreak of the golden tide in response to increasing greenhouse gas emissions, S. horneri was cultured under four conditions: ambient condition (10 °C, 400 μatm), elevated temperature condition (14 °C, 400 μatm), elevated CO₂ level (10 °C, 1000 μatm), and potential greenhouse condition (14 °C, 1000 μatm). The growth, photosynthetic performances, and inorganic carbon affinity of S. horneri were studied. The results showed that elevated temperature exerted a more pronounced positive influence on S. horneri growth, photosynthesis, and carbon assimilation than CO₂ enrichment. The growth of S. horneri was significantly improved by moderately elevated temperatures, especially under concurrently elevated CO₂ levels. This suggests that the greenhouse effect will benefit growth and carbon sequestration of S. horneri, which may enhance the frequency and scale of golden tides.

Various biogeochemical processes complicate carbon dioxide (CO₂) behaviour in coastal oceans. Through eight summer surveys, detailed variations in CO₂ mechanisms in the urbanized Jiaozhou Bay, China, were analysed. During the rainless period, respiration and dissolved inorganic carbon input from treated wastewater made the northeastern region a strong CO₂ source, while the western region with cleaner seawater was a weak source because calcium carbonate (CaCO₃) precipitation exceeded primary production. Rainfall events with different intensities and locations caused significantly different effects. When rainfall occurred over the sea, enhanced primary production caused a CO₂ sink; when rainfall induced little terrestrial pollutant input, CaCO₃ precipitation exceeded net primary production, leading to a CO₂ source. When heavy rain caused bulk runoff, the northeastern region was a strong CO₂ source because rivers flowing through downtown regions inputted considerable organic matter, while in the western region, runoff through suburbs and wetlands led to a strong sink.

Methane (CH₄) emissions from reservoirs have received widespread attention. The central urban area of Chongqing in the Three Gorges Reservoir area was selected as the study area in 2020. The temporal and spatial distribution of dissolved CH₄ concentration and flux, key generation pathways, and influencing factors have been studied. The dissolved CH₄ concentration in low-water-level period and impoundment period varied from 0.037~0.12 μmol·L⁻¹ and 0.11~0.23 μmol·L⁻¹, with the average values of (0.066 ± 0.0067) μmol·L⁻¹ and (0.13 ± 0.034) μmol·L⁻¹. The CH₄ flux was (0.941 ± 0.217) μmol·m⁻²·h⁻¹ in low-water-level period and (1.915 ± 0.204) μmol·m⁻²·h⁻¹ in impoundment period. CH₄ was produced by CO₂ reduction and acetic acid fermentation, accounting for 17.95% and 82.05% of the total CH₄ production, respectively. The dissolved CH₄ concentration was significantly positively correlated with DO and NO₃⁻-N, and it is opposite with dissolved inorganic carbon. The dissolved CH₄ concentration in this study area is affected by water environment (33.42%), inorganic nitrogen (29.60%), organic carbon (23.88%), and inorganic carbon (13.10%), and anthropogenic influences promoted dissolved CH₄ concentration.

Concentrations and isotopic compositions of mercury in surface water and surface sediment of the Jiulong River Estuary (JRE) were measured. Differences in total mercury (THg) concentrations in surface water between dry and wet seasons were insignificant, which was also the case with surface sediment THg levels. Large variations in Hg isotopic compositions were observed both in surface water (δ²⁰²Hg: − 1.53 to 0.61‰; Δ¹⁹⁹Hg: − 0.22 to 0.20‰) and surface sediment (δ²⁰²Hg: − 2.64 to − 0.96‰; Δ¹⁹⁹Hg: − 0.27 to 0.25‰). Concentration correlations between mercury and dissolved inorganic nitrogen (DIN) implied that mercury in surface sediment may undergo resuspension and be re-emitted to the water surface. The negative Δ¹⁹⁹Hg in most of the surface sediments and the correlation between mercury and DIN in the wet season indicate that soil erosion is the major source of mercury. The main sources of mercury in surface water are precipitation and natural soil. The correlation between mercury and dissolved inorganic carbon, perfluoroalkyl substances, polycyclic aromatic hydrocarbons, and dissolved Cd indicates that the contribution of underground water and industrial and urban waste cannot be ignored, which is supported by the positive signature of Δ¹⁹⁹Hg. The results of the isotopic analysis also indicate that atmospheric dry deposition is another source of mercury in surface water. The study suggests that the mercury distribution in the estuary is related to some other pollutants. Both using mercury isotope signatures and the distribution links between mercury and other pollutants can be used to better understand the processes and sources of mercury in the estuary.