The research, led by Md. Bokthier Rahman of Patuakhali Science and Technology University together with colleagues from several Bangladeshi and Australian institutions, represents the first comprehensive attempt to quantify heavy metal contamination across the entire Andharmanik ecosystem. Using inductively coupled plasma mass spectrometry, one of the most sensitive analytical techniques available, the team measured eleven metals: arsenic, cadmium, cobalt, chromium, copper, iron, manganese, nickel, lead, selenium and zinc. Samples of sediment, water and mud crab, Scylla serrata, were collected from five stations along the river during two contrasting seasons, the dry winter months of 2023 and the wet pre-monsoon summer of 2024.

The choice of seasons was far from arbitrary. During winter, when rainfall is minimal and river discharge drops, the Andharmanik’s flushing capacity weakens dramatically, allowing contaminants to linger and concentrate. Temperatures of 18 to 22 degrees Celsius prevail, and reduced freshwater inflow means less dilution of dissolved metals. Summer tells the opposite story: higher temperatures of 28 to 34 degrees Celsius and pre-monsoon rains swell the river, diluting dissolved metals and stirring up bottom sediments into resuspension. This seasonal hydrology proved decisive in shaping the study’s central finding: concentrations of nearly every metal were significantly higher in winter than in summer across all three sample types, a pattern the researchers attribute to the dilution effect of greater water volumes during the wet season.

In the sediments, the picture was stark. Arsenic, cobalt, chromium, copper, manganese, nickel and zinc all exceeded recommended guideline values established by the Food and Agriculture Organization, the World Health Organization and the United States Environmental Protection Agency. Iron dominated the metal profile, reaching concentrations as high as 3,028.65 milligrams per kilogram of dry weight in winter, while cadmium registered the lowest values. The overall abundance followed the sequence iron greater than manganese greater than nickel greater than chromium greater than zinc greater than copper greater than cobalt greater than lead greater than selenium greater than arsenic greater than cadmium. Statistical analysis revealed significant differences between stations and seasons for most metals, with station 3 consistently emerging as the most contaminated site in both seasons.

That station sits in a stretch of river surrounded by a dense cluster of pollution sources. The construction of Payra seaport and a power generation plant has transformed Kalapara into a rapidly industrializing area, and the study documents a litany of discharges reaching the water: ash from rice processing industries dumped directly at the riverbank, tar, dye and steel from ship breaking activities, sewage from lavatories installed along the coast, garbage from major fish landing centers at Mohipur, Alipur BFDC and Kuakata, and pesticide-laden agricultural runoff. The researchers also noted that poisonous substances were reportedly being used to catch fish in the Tegachia canal, which connects directly to the river near station 2. Spearman’s correlation analysis showed significant positive correlations among many pairs of metals in both seasons, suggesting they share common origins in these anthropogenic activities rather than in the river’s natural geology.

The water column itself offered a rare note of reassurance. Every metal measured in water samples remained below recommended guideline values, and the calculated Risk Index ranged from just 3.77 to 10.45, indicating that the river water does not currently pose a significant ecological threat. The sediment Risk Index, spanning 32.27 to 85.77, likewise fell within the low ecological risk category under the classification framework developed by Hakanson. In other words, the river’s open waters and its bed sediments, while showing concerning exceedances of some guidelines, have not yet crossed the threshold of serious ecological harm. The story changes dramatically, however, when the researchers turned their attention to the crabs.

Mud crabs are ideal sentinels of contamination because they live in close contact with sediments, the repository where metals concentrate, and they bioaccumulate a wide range of chemical contaminants in their tissues. They are also a dietary staple in coastal Bangladesh and a valuable export commodity: in the 2023-24 fiscal year alone, the country shipped roughly 10,782 metric tons of crab abroad. When the team analyzed crab muscle tissue, they found that arsenic, cadmium, chromium, copper, iron, lead and zinc all exceeded recommended limits set by Bangladeshi, European Union, United States and international food safety authorities. Iron again led the accumulation profile, followed by zinc and copper, with concentrations of iron reaching nearly 288 milligrams per kilogram of dry weight.

The health risk calculations are where the findings turn genuinely alarming. Using standard formulas that incorporate consumption rates of 49.5 grams per day and body weights of 70 kilograms for adults and 35 kilograms for children, the team computed target hazard quotients for each metal. For adults, all individual metals fell below the safety threshold of 1 except arsenic. For children, both arsenic and iron exceeded the limit. More critically, the total target hazard quotient, which sums the risks across all eleven metals, surpassed the acceptable value of 1 for both age groups, indicating medium to high potential non-carcinogenic health hazards from regular crab consumption. The carcinogenic risk assessment delivered an equally sobering verdict: while adults remained within the acceptable range of 10 to the power of minus 6 to 10 to the power of minus 4, the values for chromium and lead in children exceeded the safe threshold, suggesting that prolonged consumption of contaminated crab could pose a significant cancer risk for young consumers.

These results place the Andharmanik within a wider and worrying pattern across South Asian waterways. Comparable studies on the Rupsa, Dhaleshwari, Halda and upper Meghna rivers in Bangladesh, and on mud crab populations in India, Malaysia and Indonesia, have repeatedly found that benthic species accumulate higher metal loads than fish swimming in mid or upper water layers, and that rapidly industrializing coastlines are the dominant drivers. What distinguishes the new study is its completeness: by sampling water, sediment and crab simultaneously across seasons, it traces the full pathway by which industrial effluents enter a river, settle into its bed, and climb the food chain into human diets. The paradox is painful. The very features that made the Andharmanik a sanctuary, its sheltered depositional zones and productive mixing of fresh and salt water, are the same features that trap and concentrate its pollutants.

The researchers argue that the window for action remains open but is narrowing. Because the water and sediments have not yet reached high ecological risk levels, aggressive monitoring and enforcement of environmental laws could still prevent the river from sliding past the point of no return. They call on the relevant authorities to identify and control effluent sources, and they urge future research into bioaccumulation in fish and into stricter quality assurance protocols for contamination monitoring. For the communities who depend on the Andharmanik’s fisheries, and for the children who eat its crabs, the study’s message is unambiguous: a sanctuary on paper cannot protect an ecosystem that the law fails to shield in practice, and every season of unchecked discharge makes the eventual cleanup harder, costlier and less certain to succeed.

Subject of Research: Seasonal heavy metal contamination in the water, sediment and mud crabs of the Andharmanik River fish sanctuary in coastal Bangladesh and its associated human health risks

Article Title: Seasonal variations of heavy metals contamination in sediment, water and crab at a vital fish sanctuary connected to the Bay of Bengal and health risk assessment

Article References: Rahman, M. B., Smriti, S., Hussain, M., Yasmin, F., Begum, M., Ullah, H., & Rashid, H. (2026). Seasonal variations of heavy metals contamination in sediment, water and crab at a vital fish sanctuary connected to the Bay of Bengal and health risk assessment. Discover Oceans, 3(1), Article 34. https://doi.org/10.1007/s44289-026-00145-1

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00145-1

Keywords: heavy metals, Andharmanik River, Bay of Bengal, fish sanctuary, sediment contamination, mud crab, Scylla serrata, health risk assessment, carcinogenic risk, ICP-MS, seasonal variation, Bangladesh

The synthesis, led by Lei Liu and Xuejun Liu of China Agricultural University together with an international team spanning more than twenty institutions, compiles evidence from national monitoring networks and atmospheric chemistry models to reconstruct how reactive nitrogen deposition across China has shifted since 1980. The headline finding is striking: deposition climbed relentlessly for three decades, peaking at 16.4 teragrams of nitrogen per year during 2010 to 2012, before turning a corner. By 2020, total deposition had fallen 14 percent to 13.3 teragrams per year, a decline the authors attribute overwhelmingly to stringent industrial controls on nitrogen oxide emissions.

The mechanics of that turnaround deserve attention. Nitrogen oxides are produced when fossil fuels burn at high temperatures, in power plants, factories, cement kilns, and vehicle engines. Once emitted, they transform in the atmosphere into nitric acid and nitrate aerosols that are scavenged by precipitation or deposited dry onto canopies and soils. Beginning in the early 2010s, China rolled out aggressive emission reduction programs, including ultra-low emission standards for the power sector and tightening vehicle regulations. Satellite observations of nitrogen dioxide columns documented steep drops in pollution hotspots, and the deposition record followed. The review reports that oxidized nitrogen deposition declined by 34 percent, accounting for nearly all of the observed national reduction.

But here is the twist that gives the review its urgency: while oxidized nitrogen fell, reduced nitrogen, the ammonia and ammonium compounds largely traced to agriculture, kept rising. Reduced nitrogen now constitutes 60 to 70 percent of total deposition across China, a share that has transformed the chemistry of the problem. Ammonia escapes from fertilized fields, manure heaps, and livestock operations, and unlike nitrogen oxides it has faced almost no targeted regulation. Adding to the complexity, declining sulfur dioxide emissions have reduced the formation of ammonium sulfate aerosols, which in turn leaves more free ammonia in the atmosphere and can even enhance ammonia and ammonium deposition, a feedback known as the ammonia compensating effect.

The ecological consequences of this agricultural dominance are documented in sobering detail. Widespread soil acidification is depleting base cations such as calcium and magnesium from croplands and forests, with long-term measurements across Chinese forest ecosystems showing significant pH declines. Freshwater systems are suffering too; atmospheric nitrogen input to lakes such as Taihu contributes measurably to eutrophication, fueling algal blooms that degrade drinking water supplies. Biodiversity is under pressure as nutrient enrichment favors fast-growing species over the specialized plants of grasslands and other nutrient-poor habitats, with experimental nitrogen addition studies in Chinese grasslands and tropical forests documenting species losses and shifts in community composition.

The review quantifies the scale of the policy challenge with a critical loads analysis, the standard framework for assessing how much nitrogen an ecosystem can absorb before harm occurs. As of 2020, roughly 15 percent of China’s land area still receives reactive nitrogen deposition exceeding the critical load for eutrophication, meaning ecosystems in those zones are being over-fertilized beyond their capacity to cope. The authors warn that climate change will make matters worse, because warming and intensified precipitation extremes are projected to reduce ecosystem resilience, thereby expanding the terrestrial area where deposition exceeds critical loads even if emissions remain flat.

To put China’s situation in global perspective, the team calculated what reductions would be needed to match the average nitrogen deposition levels currently experienced in the United States and Western Europe, regions that themselves wrestled with and partially tamed this problem over recent decades. The answer is dramatic: China would need to cut ammonia deposition by 56 to 76 percent and nitrogen oxide deposition by 53 to 60 percent. Those numbers underscore that despite genuine progress, Chinese ecosystems remain bathed in nitrogen at rates several times higher than their Western counterparts, with the gap driven primarily by the unrestrained agricultural ammonia component.

Why has ammonia escaped regulation for so long? Part of the answer is practical. Agricultural ammonia emissions come from millions of smallholder farms spread across vast territories, making them far harder to monitor and control than a few hundred power plants. Fertilizer overuse remains endemic in parts of Chinese agriculture, and manure management is often rudimentary. Yet the review notes that cost-effective mitigation options exist, from optimized fertilizer application and enhanced-efficiency products to improved livestock housing and manure storage. Research on smallholder ammonia mitigation campaigns has shown that air quality can improve while cereal yields are maintained, and economic analyses suggest the societal benefits of halving agricultural ammonia emissions in China far exceed the abatement costs.

The authors argue that the way forward requires integrating agricultural ammonia management into the broader architecture of climate and air quality policy, rather than treating it as a separate agricultural issue. This means connecting nitrogen policy to food system reform, including improvements in nitrogen use efficiency across the entire chain from fertilizer production to livestock feed to human diets. It also means anticipating the interactions between pollution control and climate, since a warmer, wetter future will alter both the emissions of ammonia from soils and livestock and the atmospheric processes that deposit nitrogen back to the surface. The review’s framework positions nitrogen deposition abatement as inseparable from the sustainability of China’s food systems.

For the world beyond China, the study offers both a template and a warning. The 34 percent drop in oxidized nitrogen deposition proves that determined industrial emission control can bend the curve on one of the most stubborn forms of air pollution, a lesson relevant to rapidly developing economies across Asia and Africa where nitrogen oxide emissions are still climbing. But the simultaneous rise in reduced nitrogen shows that solving the industrial half of the problem while ignoring agriculture simply shifts the burden. As global food demand grows and nitrogen fertilizer use expands, the Chinese experience makes clear that comprehensive nitrogen management, spanning smokestacks, tailpipes, fields, and barns alike, is the only route to protecting ecosystems while feeding a nation.

Subject of Research: Drivers, trends and ecological impacts of atmospheric reactive nitrogen deposition in China

Article Title: Drivers, trends and impacts of nitrogen deposition in China

Article References: Liu, L., Liu, X., Wang, X., Xu, W., Tang, A., Du, E., Duan, L., Pan, Y., Zhang, L., Shen, J., Song, L., Li, K., Zhou, X., Lu, X., Zhao, Y., Yu, Q., Li, M., Zhang, X., Wen, Z., … Zhang, F. (2026). Drivers, trends and impacts of nitrogen deposition in China. Nature Reviews Earth & Environment. https://doi.org/10.1038/s43017-026-00830-x

Image Credits: AI Generated

DOI: 10.1038/s43017-026-00830-x

Keywords: nitrogen deposition, reactive nitrogen, ammonia emissions, nitrogen oxides, soil acidification, eutrophication, critical loads, China, air quality, agriculture, ecosystem health, emission controls

The research addresses a difficult question: how much nitrogen is needed to support human development, and when does nitrogen use begin to undermine the very goals it is meant to advance? Although nitrogen makes up roughly 78 percent of Earth’s atmosphere, most plants and animals cannot use atmospheric nitrogen directly. It must first be converted into biologically available forms, such as ammonium and nitrate. Industrial fertilizer production, especially through the Haber–Bosch process, has made it possible to grow far more food than would otherwise be possible. But this extraordinary expansion has also created a global nitrogen imbalance, with large amounts escaping farms, factories and cities into the atmosphere and waterways.

The study’s central contribution is to quantify nitrogen’s role across multiple Sustainable Development Goals rather than treating fertilizer solely as an agricultural input or pollutant. Nitrogen can help advance food security by increasing crop yields, support poverty reduction by strengthening rural production and contribute to economic development through industrial and agricultural activity. At the same time, excess nitrogen can intensify harmful algal blooms, contaminate drinking water, generate fine particulate matter and contribute to greenhouse-gas emissions. The same chemical element can therefore function as a nutrient, an economic resource and a pollutant, depending on where it is used and how effectively it is retained.

That tension is especially visible in agriculture. Crops absorb only part of the nitrogen applied to fields. The remainder may be lost as ammonia, nitrous oxide, nitrate or dissolved organic nitrogen. Ammonia can react in the atmosphere to form particulate pollution, while nitrous oxide is a powerful greenhouse gas with a long atmospheric lifetime. Nitrate can move through soil into groundwater and rivers, eventually reaching coastal zones where nutrient over-enrichment can trigger oxygen depletion. These pathways are connected: a kilogram of nitrogen lost from a farm does not simply disappear; it may move through air, soil and water, affecting climate, human health and biodiversity in different locations.

By placing these pathways within the Sustainable Development Goals framework, the authors highlight why nitrogen policy cannot be designed around a single outcome. Increasing fertilizer access may improve harvests in regions where nutrients are scarce, but applying more fertilizer in already intensive systems can produce diminishing agricultural returns while increasing environmental damage. Conversely, reducing nitrogen losses does not necessarily mean reducing food production. Better timing, improved placement, precision application, crop rotations, biological nitrogen fixation and the recovery of nutrients from manure and wastewater can all increase what scientists call nitrogen-use efficiency—the proportion of applied nitrogen that ultimately supports desired production.

The study is part of a wider scientific shift toward viewing nitrogen as a global systems issue. Nitrogen circulates through farms, cities, oceans and the atmosphere, crossing national borders and connecting decisions made by consumers, producers and governments. Meat and dairy production, for example, influences nitrogen demand because animal feed must be grown and because livestock manure can release reactive nitrogen. Urban wastewater is another major pathway: sewage contains valuable nutrients, but conventional treatment often removes nitrogen at an energy cost rather than recovering it for reuse. Technologies that capture nitrogen from wastewater, recycle organic wastes and reduce losses across supply chains could turn pollution into a resource.

The implications extend beyond climate and food. Nitrogen pollution is associated with respiratory health risks through the formation of fine particles, while nitrate contamination can threaten drinking-water safety. In lakes, rivers and coastal waters, excessive nutrient loading can alter species composition, reduce oxygen levels and create conditions hostile to fish and other aquatic organisms. Nitrogen deposition from the atmosphere can also change forests, grasslands and other ecosystems adapted to low-nutrient conditions. By connecting these effects to development targets, the research presents nitrogen management as a potential lever for achieving several goals simultaneously—provided that interventions are tailored to local conditions rather than imposed as a universal solution.

The challenge is political as much as technical. Regions facing undernutrition and low farm productivity may need greater access to nitrogen fertilizers, while heavily fertilized regions may need strict controls on losses and stronger incentives for efficiency. A global nitrogen strategy would therefore have to distinguish between nitrogen scarcity and nitrogen excess, while accounting for trade, consumption and unequal responsibility for pollution. The authors’ analysis reinforces the idea that progress should be measured not only by how much nitrogen enters an economy, but also by how much food, income and human well-being is produced per unit of nitrogen, and how much environmental harm is generated along the way.

For the public, the message is both alarming and hopeful. Nitrogen pollution is widespread, but it is not inevitable. Farmers can use digital tools, soil testing and improved management to match applications more closely to crop demand. Industries can reduce emissions and recover nitrogen from waste streams. Governments can coordinate fertilizer policy, water-quality standards, food systems and climate plans instead of managing them in isolation. Consumers also influence the nitrogen cycle through dietary choices and food waste. The study by Zhou and colleagues makes clear that meeting global development ambitions will require more than producing additional nitrogen or restricting it outright. The decisive goal is to use nitrogen intelligently: enough to nourish people and economies, but not so much that the excess destabilizes the planet’s climate, waters and living systems.

Subject of Research: Nitrogen’s role in achieving the global Sustainable Development Goals

Article Title: Quantifying the role of nitrogen in achieving global Sustainable Development Goals

Article References: Zhou, Y., Zhang, X., Zou, Y. et al. Quantifying the role of nitrogen in achieving global Sustainable Development Goals. Nature Communications (2026). https://doi.org/10.1038/s41467-026-76777-w

Image Credits: AI Generated

DOI: 10.1038/s41467-026-76777-w

Keywords: Nitrogen cycle, Sustainable Development Goals, nitrogen use efficiency, agriculture, food security, climate change, water pollution, biodiversity, public health, nutrient management

The Lancang River Basin, renowned for its extensive series of hydropower reservoirs, is experiencing significant environmental pressures due to rapid development and climatic shifts. These cascade reservoirs form a chain of impoundments controlling water flow for electricity generation, flood control, and irrigation. However, these engineered systems also alter natural water movement and mixing patterns, leading to water column stratification—an environmental condition where distinct thermal layers form in the reservoir. This stratification profoundly influences nutrient cycling and oxygen distribution, creating conditions that can favor algal blooms.

At the heart of the study is the investigation into how the reservoir’s stratification interacts with separated hydrologic regimes — essentially varied patterns of water inflow and outflow affected by seasonal variations and human operations. The researchers meticulously analyzed physical, chemical, and biological data from these reservoirs, revealing that stratification coupled with hydrologic separation can create nutrient hotspots, fostering favorable settings for cyanobacteria and other harmful algae to thrive.

The phenomenon of stratification in reservoirs typically results in warmer, nutrient-rich upper layers (epilimnion) and cooler, more oxygen-poor deeper layers (hypolimnion). This separation inhibits vertical mixing, often trapping nutrients in the lower strata during certain periods. However, in cascade reservoirs of the Lancang River Basin, the study found that periodic hydrologic separation due to reservoir operations disrupts this natural balance. Water retention times increase, and nutrient recycling intensifies, triggering algal biomass surges at critical junctures, particularly during warm seasons.

This complex interplay was shown to exacerbate bloom risks especially in large cascading reservoirs where water release and storage follow non-natural, technology-driven schedules rather than purely ecological rhythms. As inflows become more segmented and retention times extend, stratification strengthens, and the potential for HAB occurrences rises. The study underscores how anthropogenic changes to hydrologic regimes alter reservoir ecology, suggesting that current operational models may inadvertently contribute to environmental degradation.

The researchers employed advanced modeling techniques alongside in situ monitoring, allowing them to simulate various hydrologic scenarios and predict their impacts on algal bloom risk. Their models incorporated temperature profiles, nutrient fluxes, and reservoir water exchange dynamics, delivering a detailed picture of how physical and chemical factors converge to encourage or restrain harmful algae growth. These insights are pivotal for forecasting blooms and mitigating their effects.

One striking revelation of the study is the critical role of flow regime management. By altering the timing and magnitude of water releases, reservoir operators can potentially influence stratification patterns and nutrient availability, thus controlling or limiting bloom formation. The findings advocate for integrated water resource management approaches that consider ecological parameters, not solely hydroelectric output or irrigation needs.

Algal blooms bring about severe consequences: they consume oxygen, produce toxins, and contaminate water supplies, impacting aquatic life, human health, and local economies. The Lancang River Basin, a vital lifeline for millions, faces escalating risks due to these blooms, which threaten biodiversity and disrupt freshwater ecosystems. This study’s comprehensive approach fills a knowledge gap crucial for safeguarding this crucial water system’s future.

The significance of this research extends beyond the Lancang Basin. Similar cascade reservoir systems worldwide face comparable ecological challenges due to stratification and altered hydrologic regimes. The principles and models developed here can be adapted to other geographies, offering global relevance for managing reservoir ecology in an era increasingly defined by climate change and intensified human activity.

Furthermore, the study highlights environmental monitoring’s vital role in adapting reservoir management strategies. Continuous observation of water temperature profiles, nutrient levels, and algal populations provides early warning signals that can guide operational adjustments, preventing bloom outbreaks before they escalate.

This research also delves into the biogeochemical cycles within these reservoirs, particularly nitrogen and phosphorus dynamics, which are central to algal growth. The stratification regulates nutrient availability by impeding or promoting vertical nutrient transfer, while hydrologic disruptions influence external nutrient loading from upstream sources, creating a nexus of interacting factors that determine bloom severity.

The authors call for incorporating ecological considerations into hydropower and reservoir management policies. By balancing energy production demands with ecosystem health requirements, more sustainable operational regimes can be devised. These would reduce the frequency and intensity of algal blooms, preserving water quality and aquatic biodiversity.

In the context of climate change, the study foresees possible increases in stratification duration and intensity, as warming temperatures exacerbate thermal layering. The cascade reservoirs’ susceptibility to these changes makes it urgent to refine and implement management strategies based on dynamic ecological understanding.

Aside from operational interventions, ecological restoration strategies such as aeration, artificial mixing, and selective withdrawal could complement hydrologic management to disrupt stratification and reduce bloom risk. The study encourages multifaceted approaches combining engineering and ecological knowledge.

Guo and colleagues’ research opens avenues for future studies focused on real-time adaptive management technologies integrating sensor networks, predictive modeling, and automated control systems. Such innovations promise more responsive and effective prevention of HABs in cascade reservoirs globally.

Ultimately, this comprehensive investigation draws attention to the intricate balance between human infrastructure and natural systems within large reservoir networks. It underscores that managing water resources must go hand-in-hand with preserving ecological integrity to safeguard human well-being and environmental sustainability.

The insights emerging from this study not only contribute foundational scientific knowledge but also provide actionable guidance for policymakers, engineers, and environmentalists engaged in the critical challenge of managing reservoir ecosystems effectively. The Lancang River Basin’s experience serves as a vital case study illuminating these broader environmental dynamics at an important convergence of nature and technology.

Subject of Research: Risk of algal blooms in large cascade reservoirs due to water stratification and hydrologic regime separation in the Lancang River Basin, China.

Article Title: Risk of algal blooms by stratification and separated hydrologic regime: large cascade reservoirs in Lancang River Basin, China

Article References:
Guo, M., Wang, S., Yeager, K.M. et al. Risk of algal blooms by stratification and separated hydrologic regime: large cascade reservoirs in Lancang River Basin, China. Environ Earth Sci 84, 694 (2025). https://doi.org/10.1007/s12665-025-12692-5

Image Credits: AI Generated

DOI: https://doi.org/10.1007/s12665-025-12692-5