Lipophilic shellfish toxins (LSTs) are widely distributed in marine environments worldwide, potentially threatening marine ecosystem health and aquaculture safety. In this study, two large-scale cruises were conducted in the Bohai Sea and the Yellow Sea, China, in spring and summer 2023 to clarify the composition, concentration, and spatial distribution of LSTs in the water columns and sediments. Results showed that okadaic acid (OA), dinophysistoxin-1 (DTX1) and/or pectenotoxin-2 (PTX2) were detected in 249 seawater samples collected in spring and summer. The concentrations of ∑LSTs in seawater were ranging of ND (not detected) -13.86, 1.60-17.03, 2.73-17.39, and 1.26-30.21 pmol L-1 in the spring surface, intermediate, bottom water columns and summer surface water layers, respectively. The detection rates of LSTs in spring and summer seawater samples were 97% and 100%, respectively. The high concentrations of ∑LSTs were mainly distributed in the north Yellow Sea and the northeast Bohai Sea in spring, and in the northeast Yellow Sea, the waters around Laizhou Bay and Rongcheng Bay in summer. Similarly, only OA, DTX1 and PTX2 were detected in the surface sediments. Overall, the concentration of ∑LSTs in the surface sediments of the northern Yellow Sea was higher than that in other regions. In sediment cores, PTX2 was mainly detected in the upper sediment samples, whereas OA and DTX1 were detected in deeper sediments, and LSTs can persist in the sediments for a long time. Overall, OA, DTX1 and PTX2 were widely distributed in the water column and surface sediments in the Bohai Sea and the Yellow Sea, China. The results of this study contribute to the understanding of spatial distribution of LSTs in seawater and sediment environmental media and provide basic information for health risk assessment of phycotoxins.

A two-year (2020-2021) survey dataset of six water quality parameters (pH, dissolved oxygen (DO), chemical oxygen demand (COD), dissolved inorganic nitrogen (DIN), soluble reactive phosphate (SRP), and petroleum pollutants) was used to investigate their spatial distribution in the Bohai Sea and quantify their potential sources. There were significant differences in spatial distribution of the parameters. High concentrations of COD, DIN and SRP were found in three bays, with terrestrial input being the main pollution source. Phosphorus-limiting conditions were present in the Bohai Sea. High petroleum pollutant concentrations were identified in port areas, offshore oilfields, and adjacent areas. The pH was above the global oceanic average and there were no signs of acidification. The contribution of the mixed terrestrial inputs, maritime transportation, and offshore oil exploitation sources, oceanic and associated biotic sources, and seawater-atmosphere exchange and atmospheric deposition sources to water quality were 63.4 %, 8.0 %, and 28.6 %, respectively.

Marginal seas play a crucial role in the cycling of dissolved organic nitrogen (DON) between the terrestrial and marine environments. However, very few studies have considered the molecular transformation of DON in marginal seas, leaving the DON molecular modifications in its cycling largely unknown. Therefore, this study examined DON cycling in the Bohai Sea and Yellow Sea, two semi-closed marginal seas in northern China, using stable isotopes (δ15N and δ13C), optical characteristics, and molecular compositions. Compared to the Yellow Sea, the Bohai Sea had a weaker exchange with the open ocean, resulting in higher concentrations, lower δ15N, and more recalcitrant properties in DON. The DON cycling showed significant differences inside and outside the Yellow Sea Cold Water (YSCW). Degradation was the major sink of DON in the YSCW, during which more highly unsaturated compounds and carboxyl-rich alicyclic molecules were produced. Nitrogen atoms were found to be removed from the molecules with more N atoms to those with fewer ones during the DON degradation. This study discovered the molecular modifications in DON cycling and highlighted the intrinsic mechanisms in the cycling of DON in marginal seas.

Bromophenols (BrPs) are artificial precursors of some flame retardants and important substances with natural marine- or ocean-like flavors. This study studied the temporal variations and spatial distributions of BrPs in 150 mollusk samples (12 species) collected from 9 cities around the Bohai Sea from 2009 to 2019. Only 3 B.P. among the tested 19 congeners, 4-monobromophenol (4-mBrP), 2,4-dibromophenol (2,4-diBrP), and 2,4,6-tribromophenol (2,4,6-triBrP), were extensively detected with the detection frequency of 98.7 %, 86.7 %, and 98.0 %. The median concentration of 2,4,6-triBrP was 4.27 ng/g dw, followed by 4-mBrP (1.89 ng/g dw) and 2,4-diBrP (0.625 ng/g dw). The concentration of 3 detectable congeners, ∑3BrPs\' ranged from 0.152 to 703 ng/g dw with a median value of 8.08 ng/g dw. The highest concentrations of ∑3BrPs and 2,4,6-triBrP all occurred in Rapana venosa (49.2 and 45.1 ng/g dw) which belongs to Muricidae (2009 to 2019) that was at a relatively higher trophic level among the tested mollusks. The concentration of BrPs in Gastropoda is much higher than that in Bivalvia. The median concentrations of 2,4-diBrP, 2,4,6-triBrP, and ∑3BrPs in Gastropoda and Bivalvia from Shandong Province were higher than those in other administrative divisions at the provincial level because of the large volume of BrP production and brominated flame retardants. Temporal variations in Gastropoda and Bivalvia from Weihai showed that concentrations of ∑3BrPs, 2,4,6-triBrP, 4-mBrP, and 2,4-diBrP declined slowly from 2009 to 2019. Our results provide a systematic insight into the environmental occurrence and fate of BrPs in the Bohai Sea.

Free-living marine nematodes are valuable biological indicators for different environmental disturbances. Their taxonomic composition and functional traits often respond to environmental changes. In this study, marine nematodes, collected from the Bohai sea on the northeastern coast of China in 2014, were investigated in terms of their taxonomic composition and functional traits. Furthermore, the environmental quality of the investigated area was assessed based on nematode metrics. The studied nematode community showed spatial variation in taxonomic and functional composition, in response to changes in environmental variables such as sediment chlorophyll-a, phaeophytin-a, organic matter content, silt-clay content etc. Overall, high percentage of tolerant marine nematodes species or colonizers predominated, suggesting a disturbed environmental condition of the study area. Further results from the environmental quality assessment based on nematode metrics indicated a moderate quality status at the most investigated stations.

The microalgae located near the estuary of the Liaohe River along the coast of Panjin have long been in an area with large fluctuations in salinity, temperature, and nutrients, and have high-quality alternatives for high-value metabolites. Three strains of microalgae were screened and the biomass of microalgae could be optimized 0.313-0.790 g L-1 in 10 L bioreactor. The determination results of bioactive substances in these three microalgae showed that, the amount of fucoxanthin in the growth phase II (14 days) was maximum, at 5.354, 6.284 and 14.837 mg g-1 respectively. The diatoxanthin of Dut-wj-J1 in growth phase III (21 days) could reach 5.158 mg g-1. Dut-wj-J4 had the highest lipid production efficiency (9.45 mg L-1 d-1) followed by Dut-wj-J2 (8.49 mg L-1 d-1) and Dut-wj-J1 (8.18 mg L-1 d-1) respectively. These bioactive substances have inhibition zones of 7-13 mm against all four strains of bacteria ie., Acetobacter, Rhodococcus erythropolis, Escherichia coli and Bacillus subtilis Cohn respectively. In addition, these microalgae can play a potential role in nutrient enrichment in eutrophic seawater. The NO3- degradation rates of these three algae in the first 14 days were 75.0 %, 45.8 % and 100 % respectively, as well as the PO4- degradation rates in the first 7 days were 94.8 %, 100 % and 80.9 % respectively. This work manifests the plasticity of algae isolated from the Bohai Sea and provides useful insights for further joint production of bioactive substances.

The spread of pathogenic bacteria in coastal waters endangers the health of the local people and jeopardizes the safety of the marine environment. However, their dynamics during seasonal hypoxia in the Bohai Sea (BHS) have not been studied. Here, pathogenic bacteria were detected from the 16S rRNA gene sequencing database and were used to explore their dynamics and driving factors with the progressively deoxygenating in the BHS. Our results showed that pathogenic bacteria were detected in all samples, accounting for 0.13 to 24.65% of the total number of prokaryotic sequences in each sample. Pathogenic Proteobacteria was dominated in all samples, followed by Firmicutes, Actinobacteria, Tenericutes, and Bacteroidetes, etc. β-diversity analysis showed that pathogenic bacteria are highly temporally heterogeneous and regulated by environmental factors. According to RDA analysis, these variations may be influenced by salinity, ammonia, DO, phosphate, silicate, and Chl a. Additionally, pathogenic bacteria in surface water and hypoxia zone were found to be significantly separated in August. The vertical distribution of pathogenic bacterial communities is influenced by several variables, including DO and nutrition. It is noteworthy that the hypoxia zones increase the abundance of certain pathogenic genera, especially Vibrio and Arcobacter, and the stability of the pathogenic bacterial community increased from May to August. These phenomena indicate that the central Bohai Sea is threatened by an increasingly serious pathogenic community from May to August. And the developing hypoxia zone in the future may intensify this phenomenon and pose a more serious threat to human health. This study provides new insight into the changes of pathogenic bacteria in aquatic ecosystems and may help to make effective policies to control the spread of pathogenic bacteria.

Marine bacterial community plays a vital role in the formation of the hypoxia zone in coastal oceans. Yet, their dynamics in the seasonal hypoxia zone of the Bohai Sea (BHS) are barely studied. Here, the 16S rRNA gene-based high-throughput sequencing was used to explore the dynamics of their diversity, structure, and function as well as driving factors during the gradual deoxygenation process in the BHS. Our results evinced that the bacterial community was dominated by Proteobacteria, followed by Bacteroidetes, Firmicutes, Actinobacteria, and Cyanobacteria, etc. The abundant subcommunity dominated in the number of sequences (49%) while the rare subcommunity dominated in the number of species (99.61%). Although abundant subcommunity accounted for most sequences, rare subcommunity possessed higher diversity, richness and their population dramatically changed (higher turnover) during the hypoxia transition. Further, co-occurrence network analysis proved the vital role of rare subcommunity in the process of community assembly. Additionally, beta diversity partition revealed that both subcommunities possessed a higher turnover component than nestedness and/or richness component, implying species replacement could explain a considerable percentage of community variation. This variation might be governed by both environmental selection and stochastic processes, and further, it influenced the nitrogen cycle (PICRUSt-based prediction) of the hypoxia zone. Overall, this study provides insight into the spatial-temporal heterogeneity of bacterial and their vital role in biogeochemical cycles in the hypoxia zone of the BHS. These findings will extend our horizons about the stabilization mechanism, feedback regulation, and interactive model inside the bacterial community under oxygen-depleted ecosystems.

This study investigated the occurrence and profile of 14 organophosphate esters (OPEs) in surface water and sediment of the whole Bohai Sea. A total of 53 pairs of surface water and sediment samples were collected and the contained OPEs were quantified using a gas chromatography-mass spectrometry (GC-MS). The average concentrations of OPEs in surface water and sediment were in the range of 0-92.9 ng/L and 0.001-8.58 ng/g dry weight (dw), respectively, with tri (2-chloroethyl) phosphate (TCEP) as the predominant congener in both compartments. The total concentrations of 14 OPEs (∑14OPEs) in surface water and sediment were in the range of 10.9-516.4 ng/L and 1.42-52.9 ng/g dw, respectively. The inventories of ∑14OPEs were calculated to be 179.3 tons in the water and 101.5 tons in the sediment. Based on the risk quotients (RQs), the ecological risks of OPEs to the aquatic organisms in the Bohai Sea were considered to be negligible.

Microplastic (MP) ingestion has been recorded in 39 zooplankton species from 28 taxonomic orders, and marine zooplankton are the organisms most susceptible to MPs. However, few field studies have examined the characteristics and major influencing factors of MPs in marine zooplankton. The present study investigated the shape, color, size, chemical composition and quantity of MPs in zooplankton in the rainy and dry seasons in the Bohai Sea. Furthermore, the relationship between the MPs in zooplankton and the MPs in seawater was studied. The results showed that the MPs in zooplankton of the Bohai Sea were dominated by blue fibers. In the rainy and dry seasons, fibers accounted for 92% and 93%, respectively, of all ingested MPs, while 50% and 55%, respectively, of ingested MPs were blue. The average size of MPs in zooplankton was 1300 ± 1520 μm in the rainy season and 1040 ± 1060 μm in the dry season. Regarding the MP chemical composition, in the rainy and dry seasons, the ingested MPs were dominated by cellophane, which accounted for 53% and 68%, respectively, of MPs, followed by polyester (PET), which accounted for 18% and 20%, respectively, of MPs. The composition of MPs in zooplankton was mainly affected by the composition of MPs in seawater. No significant difference in the MP composition was observed between the two seasons or among the different zooplankton groups. The MP number was significantly higher in individual medusa than in individuals of other zooplankton groups. The mean quantity of MPs in the Bohai Sea zooplankton community in the rainy season was 2.03 ± 2.87 piece/m3, which was significantly higher than that in the dry season, 0.41 ± 0.38 piece/m3. The above results will provide a reference for marine ecological risk assessments based on the characteristics of MPs in natural seas.