Decoupling of Estuarine Hypoxia and Acidification as Revealed by Historical Water Quality Data

EPO Enhances Adaptation to Hypoxic Environment in the Freshwater Teleost (Micropterus salmoides) through the PI3K/AKT Pathway

Hypoxia has become one of the most common environmental stress events in the life history of aquatic organisms due to accelerated global warming. Exploring the adaptation mechanisms of aquatic organisms in hypoxic environments is important to deepen our understanding of environmental toxicology and to design breeding programs. In this study, the largemouth bass Micropterus salmoides exhibited greater hypoxic adaptability after 4 weeks of intermittent hypoxic exposure (IHE), with the O2 tension for loss of equilibrium decreased from 1.17 ± 0.20 to 0.66 ± 0.10 mg/L. Combined transcriptomics, biochemical detection, and immunostaining results revealed that the hypoxia-tolerant phenotype driven by IHE was strongly correlated with the activation of erythropoietin (EPO). EPO promoted phosphoinositide-3-kinase (PI3K)/protein kinase B (AKT) signaling to alleviate hepatic damage under acute hypoxic exposure (AHE) by selectively regulating the expression of genes related to antioxidant defense, antiapoptosis, and cell proliferation, which plays an important role in regulating hypoxic adaptation. The inhibition of EPO impaired cell survival in hypoxic environments, but intervention with the PI3K agonist 740 Y-P reversed this process. This novel finding provides insights into exploring how aquatic organisms cope with the challenges of hypoxia under increasing environmental risks.

Ocean acidification and its regulating factors in the East China Sea off the Yangtze River estuary

This study examines the seasonal variations in carbonate system parameters in the East China Sea (ECS) off the Yangtze River estuary (YRE) and analyzes the contributions of anthropogenic CO₂ and eutrophication to acidification. Carbonate parameters data were collected during summer 2019 and combined winter 2011. During winter, acidification is primarily driven by rising atmospheric CO₂, with minimal impact from biological processes. In contrast, summer presents a different pattern: enhanced photosynthesis due to eutrophication in surface waters helps mitigate the acidification effects of atmospheric CO₂ increases, while in bottom waters, the combined pressures of atmospheric CO₂ and intensified aerobic respiration leads to more severe acidification. Notably, biological processes now contribute more to acidification than increasing atmospheric CO₂ in the bottom waters. Our projections indicate that the summer bottom waters will experience the most pronounced acidification, with average pH levels expected to decline from 8.04 to 7.82 and aragonite saturation state (Ωar) values decreasing from 2.24 to 1.38 between 2000 and 2100. Additionally, our study indicates that winter acidification trends are also concerning, with pH only slightly higher than in summer bottom waters. The buffering capacity and the DIC:TA ratio play significant roles in determining the rate of future pH and Ωar declines. The strong buffering capacity in summer surface waters mitigates the pH decline, while the low DIC:TA ratio results in a rapid drop in Ωar.

Response of bottom dissolved oxygen reduction to net ecosystem production observed by a wave-driven profiler in the Changjiang River Plume

Coastal hypoxia, exacerbated by the combined influence of eutrophication and global warming, presents a significant environmental challenge. However, the lag correlation between organic matter (OM) export from the upper layers and bottom dissolved oxygen (DOBOT) reduction still lack clear elucidation. This study investigated the coupling between net ecosystem production (NEP, representing the maximum OM export) and DOBOT in the Changjiang River plume (CRP), using a wave-driven profiler system. The high-resolution profiles revealed rhythmic fluctuations in water column NEP, with sediment-water exchange (-74.6%) and NEP (-4.0%) dominating DOBOT reduction. Notably, surface NEP impacts DOBOT with a lag time of 25.65 h, indicating an OM sinking speed of 1.32 mm s-1. NEP at a depth of 3.4 m exerted the most significant influence on DOBOT, explaining a 12% reduction. These findings elucidate the response mechanism of DOBOT reduction to upper OM export and provide insights for hypoxia prediction in coastal and estuarine areas.

Community coalescence under variable hydrochemical conditions of the Chesapeake Bay shaped bacterial diversity and functional traits

Community coalescence related to bacterial mixing events regulates community characteristics and affects the health of estuary ecosystems. At present, bacterial coalescence and its driving factors are still unclear. The present study used a dataset from the Chesapeake Bay (2017) to address how bacterial community coalescence in response to variable hydrochemistry in estuarine ecosystems. We determined that variable hydrochemistry promoted the deterioration of water quality. Temperature, orthophosphate, dissolved oxygen, chlorophyll a, Secchi disk depth, and dissolved organic phosphorus were the key environmental factors driving community coalescence. Bacteria with high tolerance to environmental change were the primary taxa accumulated in community coalescence, and the significance of deterministic processes to communities was revealed. Community coalescence was significantly correlated with the pathways of metabolism and organismal systems, and promoted the co-occurrence of antibiotic resistance and virulence factor genes. Briefly, community coalescence under variable hydrochemical conditions shaped bacterial diversity and functional traits, to optimise strategies for energy acquisition and lay the foundation for alleviating environmental pressures. However, potential pathogenic bacteria in community coalescence may be harmful to human health and environmental safety. The present study provides a scientific reference for ecological management of estuaries.

Response of hypoxia to future climate change is sensitive to methodological assumptions

Climate-induced changes in hypoxia are among the most serious threats facing estuaries, which are among the most productive ecosystems on Earth. Future projections of estuarine hypoxia typically involve long-term multi-decadal continuous simulations or more computationally efficient time slice and delta methods that are restricted to short historical and future periods. We make a first comparison of these three methods by applying a linked terrestrial-estuarine model to the Chesapeake Bay, a large coastal-plain estuary in the eastern United States. Results show that the time slice approach accurately captures the behavior of the continuous approach, indicating a minimal impact of model memory. However, increases in mean annual hypoxic volume by the mid-twenty-first century simulated by the delta approach (+ 19%) are approximately twice as large as the time slice and continuous experiments (+ 9% and + 11%, respectively), indicating an important impact of changes in climate variability. Our findings suggest that system memory and projected changes in climate variability, as well as simulation length and natural variability of system hypoxia, should be considered when deciding to apply the more computationally efficient delta and time slice methods.

Ecological risk assessment under the PSR framework and its application to shallow urban lakes

Shallow urban lakes are naturally vulnerable to ecosystem degradation. Rapid urbanization in recent decades has led to a variety of aquatic problems such as eutrophication, algal blooms, and biodiversity loss, increasing the risk to lake-wide ecological sustainability. Instead of a simple binary assessment of ecological risk, holistic evaluation frameworks that consider multiple stressors and receptors can provide a more comprehensive assessment of overall ecological risk. In this study, we analyzed a combined dataset of government statistics, remote sensing images, and 1 year of field measurements to develop an index system for urban lake ecological risk assessment based on the pressure-state-response (PSR) framework. We used the developed ecological safety index (ESI) system to evaluate the ecological risk for three urban lakes in Jiangsu Province, China: Lake Yangcheng-LYC, Lake Changdang-LCD, and Lake Tashan-LTS. LYC and LTS were classified as "mostly safe" and "generally recognized as safe," respectively, while LCD was assessed as having "potential ecological risk." Our data suggest that socioeconomic pressure and aquatic health are the two main factors affecting the ecological risk in both LYC and LCD. The ecological risk of LTS could be improved more effectively if regional management plans are well implemented. Our study highlights the pressure of external wastewater loading, low forest-grassland coverage, and lake shoreline damage on the three selected urban lakes. The findings of this study can inform watershed management for lake ecosystem restoration and environmental sustainability.

Geographical and seasonal patterns in the carbonate chemistry of Narragansett Bay, RI

This study examined geographical and seasonal patterns in carbonate chemistry and will facilitate assessment of acidification conditions and the current state of the seawater carbonate chemistry system in Narragansett Bay. Direct measurements of total alkalinity, dissolved inorganic carbon, dissolved oxygen percent saturation, water temperature, salinity and pressure were performed during monthly sampling cruises carried out over three years. These measurements were used to calculate the following biologically relevant carbonate system parameters: total pH ( p H T ) , the partial pressure of carbon dioxide in the gas phase p C O 2 , and the aragonite saturation state Ω A . The information provided by carbonate chemistry analysis allowed for the characterization of acidification events which have the potential to disrupt the species composition and ecological functioning of coastal biological communities and threaten commercially important aquatic life. We found very robust relationships between salinity and total alkalinity R adjusted 2 = 0.82 and between salinity and dissolved inorganic carbon R adjusted 2 = 0.81 that persisted through all regions, seasons, and depth-layers with mixing of coastal waters with freshwater entering in the upper bay being an important driver on alkalinity and dissolved inorganic carbon distributions. We compared the metabolically linked calculated carbonate system parameters with dissolved oxygen (DO) saturation and found high correlation, with DO percent saturation exhibiting robust correlation with the calculated carbonate system parameters total pH ( r = 0.70 ) and with partial pressure of carbon dioxide in the gas phase ( r = - 0.71 ) . Using a statistical model to correct for the confounded effects of time and space that are a common challenge in marine survey design, we found that acidification events occurred in the Northern Region of the bay, primarily during the Summer and Fall, and likely due to a combination of microbial respiration and stratification. These acidification events, especially in the Northern Region, have the potential to adversely impact aquatic life.

Distribution of nitrogen (N) and phosphorus (P) in seasonal low-oxygen marine ranching in northern Yellow Sea, China

Seasonal low-oxygen in marine ranching in the northern Yellow Sea has been one of the major environmental problems in coastal waters in recent years. Nitrogen (N) and phosphorus (P) are important nutrients, which are susceptible to the concentration of dissolved oxygen (DO). This article studied the effects of low-oxygen on nutrients represented by N and P fractions in marine ranching in the northern Yellow Sea. The results showed that there were significant layer differences in temperature and salinity during the low-oxygen period. In the seawater, the nutrient distribution in the death disaster zone of sea cucumbers and the non-disaster zone was similar, and DO had a strong positive correlation with dissolved inorganic nitrogen (DIN). In the sediment, significant regional differences existed in nutrient concentration, and the concentration of total phosphorus (TP) decreased significantly with the increase in DO content. The results showed that the sources and sinks of nitrogen and phosphorus nutrients were inconsistent in this zone, and migration and transformation of the existing form of nitrogen with the seasonal changes in the water environment was a main factor for N distribution. This study extended the understanding of the effects of seasonal low-oxygen on N and P.