Hypoxia is a major stressor in aquatic environments and it is frequently linked with excess nutrients resulting from sewage effluent discharges and agricultural runoff, which often also contain complex mixtures of chemicals. Despite this, interactions between hypoxia and chemical toxicity are poorly understood. We exposed male three-spined stickleback during the onset of sexual maturation to a model anti-androgen (flutamide; 250 μg/L) and a pesticide with anti-androgenic activity (linuron; 250 μg/L), under either 97% or 56% air saturation (AS). We assessed the effects of each chemical, alone and in combination with reduced oxygen concentration, by measuring the transcription of spiggin in the kidney, as a marker of androgen signalling, and 11 genes in the liver involved in some of the molecular pathways hypothesised to be affected by the exposures. Spiggin transcription was strongly inhibited by flutamide under both AS conditions. In contrast, for linuron, a strong inhibition of spiggin was observed under 97% AS, but this effect was supressed under reduced air saturation, likely due to interactions between the hypoxia inducible factor and the aryl hydrocarbon receptor (AhR) pathways. In the liver, hypoxia inducible factor 1α was induced following exposure to both flutamide and linuron, however this was independent of the level of air saturation. This work illustrates the potential for interactions between hypoxia and pollutants with endocrine or AhR agonist activity to occur, with implications for risk assessment and management.

Hypoxia off the Changjiang Estuary (CE) and its adjacent waters is purported to be the most severe in China, attracting considerable concern from both the scientific community and the general public. Currently, continuous observations of dissolved oxygen (DO) levels covering hypoxia from its appearance to disappearance are lacking. In this study, twelve consecutive monthly cruises (from February 2015 to January 2016) were conducted. The consecutive spatiotemporal variations in hypoxia throughout the annual cycle were elucidated in detail, and the responses of annual variations in hypoxia to the different influential factors were explored. Overall, hypoxia experienced a consecutive process of expanding from south to north, then disappearing from north to south. The annual variations in hypoxia were mainly contingent on stratification variations. Among different stages, there was significant heterogeneity in the dominant factors. Specifically, low-DO waters initially appeared from the intrusion of nearshore Kuroshio branch current (NKBC), as NKBC intrusion provided a low-DO background and triggered stratification. Thereafter, stratification was enhanced and gradually expanded northward, which promoted the extension of low-DO areas. The formation of hypoxia was regionally selective, and more intense organic matter decomposition at local regions facilitated the occurrence and discontinuous distribution of hypoxia. Hypoxic zones were observed at the Changjiang bank and Zhejiang coastal region from August (most extensively at 14,800 km²) to October. Thereafter, increased vertical mixing facilitated the dissipation of hypoxia from north to south.

We investigated the effect of hypoxic-anoxic range of dissolved oxygen (DO) on metal release/bioavailability at sediment-water interface (SWI) in the Ganga River. Here, we consider eight sites in the main river stem along 518 km; sixty sites downstream two point sources and two tributary confluences covering 630 km; and an incubation experiment to verify these results. We found higher concentrations of metals and bioavailable fractions at SWI at two locations of main stem and up to 700 m, 1000 m, 400 m and 500 m downstream Assi drain, Wazidpur drain, Ramganga confluence and Varuna confluence respectively where DO at SWI (DOₛw) was <2.0 mgL⁻¹. The incubation experiment did show higher levels of metal- and P-release and bioavailability under anoxic-hypoxic range of DO. The risk assessment code and eutrophication index indicated high to very high risks of contaminated river sediment and water to aquatic environment at sites with hypoxic-anoxic range of DOₛw. Further, the principal component analyses separated metals and bioavailable fractions opposite to FDAase indicating greater risk at these locations. The study, which forms the first report on benthic hypoxia/anoxia-driven metal release, potential bioavailability and risk to the Ganga River ecosystem will help understanding how human-driven perturbations influence geochemical cycling of metals and ecosystem responses in large rivers.

Recently, we reported that hypoxia disrupts the endocrine system and causes metabolic abnormalities in prawns. Although transgenerational impairment effects of hypoxia have become a hot topic in vertebrate, it is unknown whether hypoxia could exert cross-generational effects on testicular function crustaceans. The present study aimed to investigate hypoxia’s toxic effects on the testicular function of oriental river prawns (Macrobrachium nipponense) and offspring development. Hypoxia disrupted testicular germ cells quality, caused sex hormone imbalance (testosterone and estradiol), and delayed testicular development. The F1 generation derived from male prawns exposed to hypoxia showed retarded embryonic development, and reduced hatching success and larval development, despite not being exposed to hypoxia. Analysis of the transcriptome the F0 generation (exposed to hypoxia) showed that the impaired testicular functions were associated with changes to mitochondrial oxidative phosphorylation, apoptosis, and steroid biosynthesis. Interestingly, quantitative real-time PCR confirmed that hypoxia could significantly suppress the expression of antioxidant and gonad development-related genes in the testis of the F1 generations, with and without continued hypoxia exposures. In addition, paternal exposure to hypoxia could result in a higher production of reactive oxygen species in offspring testis tissue compared with those without hypoxia exposure. The cross-generational effects of testicular function implied that the sustainability of natural freshwater prawn populations would be threatened by chronic hypoxia.

The ongoing transition to renewable fuel sources has led to increased use of wood and other biomass fuels. The physiochemical characteristics of biomass combustion derived aerosols depends on appliances, fuel and operation procedures, and particles generated during incomplete combustion are linked to toxicity. Frequent indoor wood burning is related to severe health problems such as negative effects on airways and inflammation, as well as chronic hypoxia and pathological changes in placentas, adverse pregnancy outcome, preterm delivery and increased risk of preeclampsia. The presence of combustion-derived black carbon particles at both the maternal and fetal side of placentas suggests that particles can reach the fetus. Air pollution particles have also been shown to inhibit trophoblast migration and invasion, which are vital functions for the development of the placenta during the first trimester. In this study we exposed a placental first trimester trophoblast cell line to wood smoke particles emitted under Nominal Burn rate (NB) or High Burn rate (HB). The particles were visible inside exposed cells and localized to the mitochondria, causing ultrastructural changes in mitochondria and endoplasmic reticulum. Exposed cells showed decreased secretion of the pregnancy marker human chorionic gonadotropin, increased secretion of IL-6, disrupted membrane integrity, disrupted proliferation and contained specific polycyclic aromatic hydrocarbons (PAHs) from the particles. Taken together, these results suggest that wood smoke particles can enter trophoblasts and have detrimental effects early in pregnancy by disrupting critical trophoblast functions needed for normal placenta development and function. This could contribute to the underlying mechanisms leading to pregnancy complications such as miscarriage, premature birth, preeclampsia and/or fetal growth restriction. This study support the general recommendation that more efficient combustion technologies and burning practices should be adopted to reduce some of the toxicity generated during wood burning.

The Bohai Sea is a shallow-water, semi-enclosed marginal sea of the Northwest Pacific. Since the late 1990s, it has suffered from nutrient over-enrichment. To better understand the eutrophication characteristics of this important coastal sea, we examined four survey datasets from summer (June 2011), late autumn (November 2011), winter (January 2016), and early spring (April 2018). Nutrient conditions in the Bohai Sea were subject to seasonal and regional variations. Survey-averaged N/P ratios in estuarine and nearshore areas were 20–133. In contrast, the central Bohai Sea had mean N/P ratios of 16.9 ± 3.4 in late autumn, 16.1 ± 3.0 in winter and 13.5 ± 5.8 in early spring, which are close to the traditional N:P Redfield ratio of 16. In summer, both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP) were used up in the surface waters of the central Bohai Sea, suggesting that the biological consumption of DIN and DIP may also follow the Redfield ratio. Wintertime nutrient budgets of the central Bohai Sea water were then established based on a mass balance study. Our results suggest that the adjacent North Yellow Sea supplied additional DIP to the central Bohai Sea via wintertime water intrusion, balancing terrigenous excess DIN that was introduced in summer. A water-mixing simulation combining these two nutrient sources with atmospheric nitrogen deposition suggests that eutrophication in the central Bohai Sea will likely be enhanced by the large-scale accumulation of anthropogenic nitrogen in adjacent open oceans. Such changes in nutrients may have fundamentally contributed to the recent development of algal blooms and seasonal hypoxia in the central Bohai Sea.

A hypoxia event occurred in the nearshore of the northern Yellow Sea in summer 2016, and a bloom of small jellyfish occurred in June of this year. As a key copepod species in the Yellow Sea, Calanus sinicus exhibits early recruitment mainly in this nearshore area. Therefore, this study assessed the effects of these two phenomena on C. sinicus especially on their early recruitment. Sampling in nearshore waters was performed every month from May to December, except for October 2016. The numerical abundance of eggs and nauplii (E + N), copepodites and adults (C + A) of C. sinicus was determined in relation to environmental factors. Results showed that dissolved oxygen (DO) concentrations decreased from June on, attaining a minimum in August, and showed a significant positive relationship with C. sinicus E + N abundance. This suggested that decreasing DO had potential negative effects on early recruitment of C. sinicus. The abundance of small jellyfish, dominated by Sugiura chengshanense (Hydromedusa), also showed a significant positive relationship with C. sinicus E + N abundance, indicating a potential predator-prey relationship between them. Spatial relationships were also examined between E + N abundance and DO concentration in July and with small jellyfish abundance in June. Both bottom DO (B-DO) and E + N abundance in inshore waters were significantly lower than those in offshore water in July, indicating a potential negative effect of low B-DO (<6 mg L⁻¹) in nearshore water. Spatial variation of small jellyfish abundance showed no correlation with C. sinicus abundance in June, but the non-overlapping patchy distribution also suggested their potential predator-prey relationship. Together, both the seasonal and spatial results indicated that low DO and a small jellyfish bloom in nearshore waters had detrimental effects on early recruitment of C. sinicus. These negative effects thus have the potential to indirectly impact the trophodynamics of fishery resources in the northern Yellow Sea.

Anthropogenic nutrient enrichment results in hypoxia, ocean acidification and elevated nutrients (HOAN) in coastal environments throughout the world. Here, we examined the composition of biofilm bacterial communities from a nutrient-excessive fish farm with low dissolved oxygen (DO) and pH levels using 16S rRNA gene sequencing. HOAN was accompanied by higher bacterial diversity and richness, and resulted in an altered community composition than the control site. HOAN resulted in more Flavobacteriales, Rhizobiales, Epsilonproteobacteria and Vibrionales, but less Oceanospirillales and Alteromonadales. Photobacterium sp. and Vibrio sp. were mostly found to be exclusive to HOAN conditions, suggesting that HOAN could possibly proliferate the presence of these potential pathogens. Our study suggests the complexity of bacterial communities to hypoxia and acidification in response to increased nutrient loads, along with identities of nutrient, oxygen and pH-susceptible bacterial groups that are most likely affected under this ocean trend.

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.

Coastal eutrophication and consequent oxygen depletion (hypoxia) occurs worldwide due to increased human activity. The paucity of genomic information of microbes in hypoxia prone coastal waters have hindered our understanding of microorganism related causation and adaption to the environment. Here, using metagenomic approach, we investigated microbial metabolic capability in heavily polluted Pearl River estuary. Our results highlighted the possible roles of microbial metabolic activity in the formation of bottom water hypoxia by revealing enriched organic degradation related microbial genes in the bottom layer beneath surface phytoplankton bloom. Microbial nitrate reduction in hypoxia layer was low, possibly due to the low pH and fluctuating oxygen level. On contrary, high abundance of sulfate-reducing, and antibiotic and metal resistance related genes were detected in bottom and surface layers, respectively, indicating microbial adaptation to oxygen depletion and pollution. Our study provides gene level information on the interactive relations between microbial functions and environmental stress.