Anthropogenically enhanced sediment oxygen demand creates mosaic of oxygen deficient zones in the Ganga River: Implications for river health

Assessing the pollution and ecotoxicological status of the Iguaçu River, southern Brazil: A review

The degradation of water resources available for human consumption is increasing with the continuous release of chemicals into aquatic environments and their inefficient removal in wastewater treatment. Several watersheds in Brazil, such as the Iguaçu River, are affected by multiple sources of pollution and lack information about their pollution status. The Iguaçu River basin (IRB) has great socioeconomic and environmental relevance to both the supply of water resources and its considerable hydroelectric potential, as well as for the high rate of endemism of its ichthyofauna. Also, the IRB is home to large conservation units, such as the Iguaçu National Park, recognized by UNESCO as a natural World Heritage Site. Thus, this article discusses the chemical pollution in the IRB approaching: (i) the main sources of pollution; (ii) the occurrence of inorganic and organic micropollutants; (iii) the available ecotoxicological data; and (iv) the socioeconomic impacts in three regions of the upper, middle, and lower IRB. Different studies have reported relevant levels of emerging contaminants, persistent organic pollutants, toxic metals, and polycyclic aromatic hydrocarbons detected in the water and sediment samples, especially in the upper IRB region, associated with domestic and industrial effluents. Additionally, significant concentrations of pesticides and toxic metals were also detected in the lower IRB, revealing that agricultural practices are also relevant sources of chemicals for this watershed. More recently, studies indicated an association between fish pathologies and the detection of micropollutants in the water and sediments in the IRB. The identification of the main sources of pollutants, associated with the distribution of hazardous chemicals in the IRB, and their potential effects on the biota, as described in this review, represent an important strategy to support water management by public authorities for reducing risks to the local endemic biodiversity and exposed human populations. Integr Environ Assess Manag 2024;20:1280-1305. © 2023 SETAC.

The impact of bottom water light exposure on electrical and sediment remediation performance of sediment microbial fuel cells

Sediment microbial fuel cells (SMFCs) generate bioelectricity from benthic sediments and thus providing both bioelectricity generation and sediment remediation. However, the high internal resistance of the cathode leads to a low power output, which requires research on cathode treatment. In this study, we explored the influence of light irradiation on bioelectricity production and nutrient removal in the SMFC system. The microcosm experiment of the SMFC system was designed with artificial illumination of 500 lux (light-SMFC) and compared with dark conditions of 15 lux (dark-SMFC), which showed that the current increases during photoperiods. The study reveals that light-illuminated SMFC consistently produced the highest voltage, with the highest voltage (553 mV) being 1.3 times higher than the dark-SMFC (440 mV). The polarization curves show a significant reduction in internal cathodic resistance under light condition, resulting in increased voltage generation. The light-SMFC exhibits the highest maximum power density of 35.93 mW/m2, surpassing the dark SMFC of 31.13 mW/m2. It was found that light illumination in the SMFC system increases oxygen availability in the cathodic region, which supports the oxygen reduction reaction (ORR) process. At the same time, the high bioelectricity output contributes to the highest sediment remediation by greatly reducing the chemical oxygen demand (COD) and phosphate (PO4-P) concentrations. The study highlights the potential of light illumination in mitigating cathodic limitation to improve SMFC performance and nutrient removal.

The potential linkage between sediment oxygen demand and microbes and its contribution to the dissolved oxygen depletion in the Gan River

The role of sediment oxygen demand (SOD) in causing dissolved oxygen (DO) depletion is widely acknowledged, with previous studies mainly focusing on chemical and biological SOD separately. However, the relationship between the putative functions of sediment microbes and SOD, and their impact on DO depletion in overlying water, remains unclear. In this study, DO depletion was observed in the downstream of the Gan River during the summer. Sediments were sampled from three downstream sites (YZ, Down1, and Down2) and one upstream site (CK) as a control. Aquatic physicochemical parameters and SOD levels were measured, and microbial functions were inferred from taxonomic genes through analyses of the 16S rRNA gene. The results showed that DO depletion sites exhibited a higher SOD rate compared to CK. The microbial community structure was influenced by the spatial variation of Proteobacteria, Chloroflexi, and Bacteroidota, with total organic carbon (TOC) content acting as a significant environmental driver. A negative correlation was observed between microbial diversity and DO concentration (p < 0.05). Aerobic microbes were more abundant in DO depletion sites, particularly Proteobacteria. Microbes involved in various biogeochemical cycles, such as carbon (methane oxidation, methanotrophs, and methylotrophs), nitrogen (nitrification and denitrification), sulfur (sulfide and sulfur compound oxidation), and manganese cycles (manganese oxidation), exhibited higher abundance in DO depletion sites, except for the iron cycle (iron oxidation). These processes were negatively correlated with DO concentration and positively with SOD (p < 0.05). Overall, the results highlight that aerobic bacteria's metabolic processes consume oxygen, increasing the SOD rate and contributing to DO depletion in the overlying water. Additionally, the study underscores the importance of targeting the removal of in situ microbial molecular mechanisms associated with toxic H2S and CH4 to support reoxygenation efforts in rehabilitating DO depletion sites in the Gan River, aiding in identifying factors controlling DO consumption and offering practical value for the river's restoration and management.

Dynamics of sediment phosphorus in the middle and lower stretch of River Ganga, India: insight into concentration, fractionation, and environmental risk assessment of phosphorus

Despite continuous efforts, eutrophication is still occurring in freshwater and phosphorus (P) is the most important nutrients that drive the eutrophication in rivers and streams. However, little information is available about the distribution of P fractions in river sediment. Here, the sequential extraction approach was used to evaluate the sediment P fractionation and its content in the anthropogenically damaged river Ganga, India. Different sedimentary P fractions viz. exchangeable (Ex-P), aluminum bound (Al-P), iron bound (Fe-P), calcium bound (Ca-P), and organically bound phosphorus (Org-P), were quantified. Significantly higher level of total P was recorded in pre-monsoon season (438.5 ± 95.8 mg/kg), than other [winter (345.7 ± 110.6 mg/kg), post-monsoon (319.2 ± 136.3 mg/kg), and monsoon (288.6 ± 77.3 mg/kg)] seasons. Different P fractions such as Ex-P, Al-P, Fe-P, Ca-P and Org-P varied from 2.88-12.8 mg/kg, 7.64-98.8 mg/kg, 32.2-179.2 mg/kg, 51.97-286.1 mg/kg and 9.3-143.7 mg/kg, respectively, which correspondingly represented 0.5-10.54%, 3.41-20.18%, 17.27-37.82%, 37.35-60.2%, 4.15-25.88% of the Total P with a rank order of P-fractions was Ca-P > Fe-P > Org-P > Al-P > Ex-P. Bio-available P contributes a considerable portion (37.9-46.0%) of total P which may increase the eutrophication to overlying water. Results demonstrate that inorganic P species control the P bio-availability in both time and space. However, an estimated phosphorus pollution index based on sediment total P content showed no ecological risk of phosphorus to Ganga River sediment.

Impact of turbidity on the gill morphology and hypoxia tolerance of eastern sand darter (Ammocrypta pellucida)

Anthropogenic stressors such as agriculture and urbanization can increase river turbidity, which can negatively impact fish gill morphology and growth due to reduced oxygen in the benthic environment. We assessed the gill morphology, field metabolic rate (FMR), and two hypoxia tolerance metrics (oxygen partial pressure at loss of equilibrium, PO2 at LOE, and critical oxygen tension, Pcrit) of eastern sand darter (Ammocrypta pellucida), a small benthic fish listed as threatened under the Species at Risk Act in Canada, from rivers in southern Ontario. Field trials were conducted streamside in the Grand River (August 2019; mean NTU 8) and in the comparatively more turbid Thames River (August 2020; mean NTU 94) to test the effect of turbidity on each physiological endpoint. Gills were collected from incidental mortalities and museum specimens, and were assessed using hematoxylin and eosin and immunofluorescent staining. The between-river comparison indicated that turbidity significantly increased interlamellar space and filament width but had no significant influence on other gill morphometrics or FMR. Turbidity significantly increased PO2 at LOE (i.e., fish had a lower hypoxia tolerance) but did not significantly impact Pcrit. Therefore, although turbidity influences hypoxia tolerance through LOE, turbidity levels were not sufficiently high in the study rivers to contribute to measurable changes in gill morphology or metabolism in the wild. Determining whether changes in gill morphology or metabolism occur under higherturbidity levels would help resolve the ecological importance of turbidity on species physiology in urban and agricultural ecosystems.

Transition of carbon-nitrogen coupling under different anthropogenic disturbances in subtropical small mountainous rivers

The commonly observed inverse relationship between dissolved organic carbon (DOC) and nitrate (NO₃^-) concentrations in aquatic systems can be explained by stoichiometric and thermodynamic principles regulating microbial assimilation and dissimilation processes. However, the interactive effects of human activities and dissolved oxygen (DO) on the DOC and DIN (dissolved inorganic nitrogen, mainly composed of NO₃^--N and NH₄⁺-N) relations are not well identified, particularly in subtropical small mountainous rivers (SMRs). Here, we investigated the exports and relations of DOC-DIN in 42 Taiwan SMRs under different anthropogenic disturbances. Results showed that the island-wide mean concentrations of the three solutes in streams are generally low, yet the abundant rainfall and persistent supply contrarily lead to disproportional high DOC and DIN yields. The inverse DOC-NO₃^--N relation does not appear under well‑oxygenated conditions, regardless of low or high human disturbance. However, a significant inverse relationship between DOC-NO₃^--N would emerge in highly-disturbed watersheds under low-oxygenated conditions (mean annual DO <6.5 mg L^-1), where excess N accumulates as NH₄⁺-N rather than NO₃^--N. The controlling mechanism of DOC-DIN relations would shift from energetic constraints to redox constraints in low-oxygenated conditions. Although riverine concentrations of DOC, NO₃^--N, and NH₄⁺-N could be elevated by human activities, the transition of DOC-DIN relation pattern is directly linked to DO availability. Understanding the mechanism that drives CN coupling is critical for assessing the ecosystem function in the delivery and retention of DOC and DIN in aquatic ecosystems.

Trophic interactions regulate microbial responses to environmental conditions and partially counteract nitrogen transformation potential in urban river bends

River bends are distinguished by high biodiversity and elevated rates of biogeochemical activities due to complex hydromorphological processes that form diverse geomorphic units, making it challenging to elucidate the impact of trophic interactions on community assembly and biogeochemical processes. Here, we clarify the effect of trophic interactions in determining the assembly of multi-trophic microbial communities and the impact on nitrogen transformation potential by distinguishing the direct and cascading effects of environmental conditions based on 32 samples collected from a typical urban river bends. It was found that both bacterial and micro-eukaryotic communities were determined by homogeneous selection (indicated by β-nearest taxon index, accounted for 85% and 48.3%, respectively), whereas the dominant environmental factors were different, being sediment particle size (P < 0.05) and nitrogen (P < 0.05), respectively. Both the microbial co-occurrence network and the significant association (P < 0.05) between β-nearest taxon index and trophic transfer efficiency changes showed that the trophic interactions strongly shaped microbial communities in the urban river bends. The path modeling suggested that environmental conditions resulted in an increase in abundance of multi-trophic microbial communities via direct effects (mean standardized effects = 0.21), but reductions in abundance of bacteria via cascading effects, i.e., trophic interaction (mean standardized effects = -0.1). When considering direct and cascading effects together, environmental conditions in urban river bends were found to enhance the abundance of microbial communities, with decreasing magnitude at the higher trophic level. Analogously, the path modeling also indicated the nitrogen transformation potential enhanced by environmental conditions via direct effects, but partly counteracted by trophic interactions via cascading effects. The obtained results could provide a theoretical basis for the regulation and restoration of urban rivers.

Effects of COVID-19 lockdown on water quality, microbial extracellular enzyme activity, and sediment-P release in the Ganga River, India

This study investigates possible improvement in water quality and ecosystem functions in the Ganga River as influenced by COVID-19 lockdown in India. A total of 132 samples were collected during summer-2020 low flow (coinciding COVID-19 lockdown) for water (sub-surface and sediment-water interface) and 132 samples separately for sediment (river bottom and land-water interface) considering 518-km main river stem including three-point sources (one releases urban sewage and the other two add metal-rich industrial effluents) and a pollution-impacted tributary. Parameters such as dissolved oxygen deficit and the concentrations of carbon, nutrients (N and P), and heavy metals were measured in water. Sediment P-release was measured in bottom sediment whereas extracellular enzymes (EE; alkaline phosphatase, FDAase, protease, and β-D-glucosidase) and CO2 emission were measured at land-water interface to evaluate changes in water quality and ecosystem functions. The data comparisons were made with preceding year (2019) measurements. Sediment-P release and the concentrations of carbon, nutrients, and heavy metals declined significantly (p<0.05) in 2020 compared to those recorded in 2019. Unlike the preceding year, we did not observe benthic hypoxia (DO <2.0 mg L-1) in 2020 even at the most polluted site. The EE activities, which declined sharply in the year 2019, showed improvement during the 2020. The stability coefficient and correlative evidences also showed a large improvement in the water quality and functional variables. Positive changes in functional attributes indicated a transient recovery when human perturbations withdrawn. The study suggests that timing the ecosystem recovery windows, as observed here, may help taking management decision to design mitigation actions for rivers to recover from anthropogenic perturbations.

The molecular characteristics of dissolved organic matter in urbanized river sediments and their environmental impact under the action of microorganisms

The complex relationships between the molecular composition of dissolved organic matter (DOM) and microbial communities are essential for maintaining the stability of aquatic ecosystems. This study comprehensively analyzed the characteristics and potential effects of DOM molecular composition as well as the relationship between microbial communities and DOM molecular composition in sediments from the Beiyun River, Beijing, China. The results showed that the content of DOM in Beiyun River sediments was 9.93-41.57 g/kg, mainly composed of lignin-like (36.75%) and protein-like (17.79%) substances. Compared with other rivers affected by anthropogenic activities, the higher content of labile substances in the Beiyun River increased the risk of nutrient release. At the same time, 1402 molecules remained stable in each sample, most of which were refractory lignin-like substances and protein-like substances carrying ester groups. The agricultural section contained more common DOM molecules than the urban section, mainly tannin-like and lignin-like substances with unsaturated or cyclic structures. And, the intensity of anthropogenic activities was the main reason affecting the diversity of unique DOM molecular in each sample. Moreover, Dechloromonas as the dominant genus of Proteobacteria was closely related to the biological modification of low unsaturated (DBE < 15) condensed aromatic compounds (P < 0.05). Anaerolineaceae and Anaerolineae belonging to the Chloroflexi phylum have the potential to degrade medium and high molecular weight (M/Z > 400) liable substances (P < 0.05) and release lignin-like substances. In addition, the proportion of protein-like substances can indirectly reflect the risk of nutrient release in sediments affected by urbanization. Thus, the results of this study further reveal the impact of urbanization on rivers, and provide theoretical basis and guidance for pollution control of the Beiyun River and other urbanized rivers worldwide.

Integrating resilience with functional ecosystem measures: A novel paradigm for management decisions under multiple-stressor interplay in freshwater ecosystems

Moving beyond monitoring the state of water quality to understanding how the sensitive ecosystems "respond" to complex interplay of climatic and anthropogenic perturbations, and eventually the mechanisms that underpin alterations leading to transitional shifts is crucial for managing freshwater resources. The multiple disturbance dynamics-a single disturbance as opposed to multiple disturbances for recovery and other atrocities-alter aquatic ecosystem in multiple ways, yet the global models lack representation of key processes and feedbacks, impeding potential management decisions. Here, the procedure we have embarked for what is known about the biogeochemical and ecological functions in freshwaters in context of ecosystem resilience, feedbacks, stressors synergies, and compensatory dynamics, is highly relevant for process-based ecosystem models and for developing a novel paradigm toward potential management decisions. This review advocates the need for a more aggressive approach with improved understanding of changes in key ecosystem processes and mechanistic links thereof, regulating resilience and compensatory dynamics concordant with climate and anthropogenic perturbations across a wide range of spatio-temporal scales. This has relevance contexting climate change and anthropogenic pressures for developing proactive and adaptive management strategies for safeguarding freshwater resources and services they provide.

Insight into the nitrogen accumulation in urban center river from functional genes and bacterial community

Along with urbanization, the intensified nitrogen pollution in urban rivers and the form of black-odor rivers has become one of the biggest concerns. Better understanding of the nitrogen transformations and microbial mechanisms occurring within urban rivers could help to manage their water quality. In this study, pollution characteristics, potential nitrogen removal rate, composition and function of bacterial community, and abundance of functional genes associated with nitrogen transformation were comparatively investigated in a typical urban river (FC) and a suburban river (LH). Compared with LH, FC was characterized by higher content of nutrients, lower potential nitrogen removal rate and lower abundance of functional genes associated with nitrogen transformation in both overlying water and sediment, especially in summer. Sediment dissolved organic matter characterized by excitation−emission matrix (EEM) showed that FC was more severely polluted by high nitrogen organic matter. Our results revealed that anammox was the main nitrogen removal pathway in both rivers and potential nitrogen removal rates decreased significantly in summer. Bacterial community analysis showed that the benthic communities were more severely influenced by the pollutant than aquatic ones in both rivers. Furthermore, the FC benthic community was dominated by anaerobic respiring, fermentative, sulfate reduction bacteria. Quantitatively, the denitrification rate showed a significant positive correlation with the abundance of denitrification genes, whilst the anammox rate was significantly negatively correlated with bacterial diversity. Meanwhile, NH₄⁺-N had a significant negative correlation to both denitrification and anammox in sediment. Taken together, the results indicated that the increased nitrogen pollutants in an urban river altered nitrogen removal pathways and bacterial communities, which could in turn exacerbate the nitrogen pollution to this river.

Benthic hypoxia in anthropogenically-impacted rivers provides positive feedback enhancing the level of bioavailable metals at sediment-water interface

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 (DO_sw) 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 DO_sw. 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.

Hypoxia and associated feedbacks at sediment-water interface as an early warning signal of resilience shift in an anthropogenically impacted river

Multiple human perturbations in the large rivers often cause habitat fragmentation creating patches of unpredictable structural and functional attributes. The resilience has been largely neglected in riverine studies, despite its pivotal importance in ecosystem recovery. We expect that a shift in sub-habitat conditions along a river transect subjected to frequent oxygen fluctuation and release of carbon, nutrients and other substances generate feedbacks to overstep the resilience and constrain ecosystem recovery. Because dissolved oxygen (DO) plays a regulatory role in ecosystem structure and functioning and feedbacks the denitrification and sediment-P release, we consider the mechanistic links among DO_sw, denitrification and sediment-P release to identify resilience level and to construct a dynamic fit model to uncover the level of resilience and critical transitions in the river. We investigated 180 sites downstream two point sources and two tributaries, each with a 1.4 km river segment, covering 630 km length of the Ganga River. The dynamic fit model intersecting the DO_sw at <1.5 mg L^-1, sediment-P release >7.03 mg m^-2 d^-1 and denitrification rate >1.0 mg N m^-2 hr^-1 at 25 m reach downstream point sources indicated a threat to natural/self-recovery of the Ganga River. The non-metric multidimensional scaling (NMDS) and neighbor-joining analysis indicated that locations up to 700 m downstream Wazidpur drain have overstepped the ecosystem resilience. We found almost similar results downstream Assi drain and study confluences. Our explicit incorporation of DO_sw, sediment-P release, and denitrification in an organized framework provides key insights to detect resilience and critical transitions in an anthropogenically impacted river ecosystem. Given the importance of the Ganga River for national water security and supply across several major states in India, research on the factors and status of resilience underpinning its recovery should be high on our national agenda.