River nutrient water and sediment measurements inform on nutrient retention, with implications for eutrophication

Inorganic nitrogen and phytoplankton dynamics in a subtropical reservoir during low-flow winter periods: Implications for nitrogen management

The dynamics of nutrient cycling in inland waters, particularly during algal blooms, play a critical role in shaping aquatic ecosystems. However, the interaction between phytoplankton and inorganic nitrogen during low-flow periods remains poorly understood. This study presents high-frequency monitoring of ammonium (NH4-N), nitrate (NO3-N), nitrite (NO2-N) and phytoplankton communities in a subtropical river-type reservoir during the low-flow winter period, characterized by blooms dominated by cryptophytes and green algae. Our results revealed that NH4-N concentrations exhibited a diurnal pattern of decreasing during the day and increasing during the night, which was negatively correlated with total algal biomass at the intraday fluctuation scale (coefficient = -0.378, p = 0.028), indicating strong algal uptake of ammonium during daytime. NO3-N and NO2-N concentrations, however, did not show clear diurnal co-varied patterns with algae. On the day-to-day scale, the external nitrogen inputs resulting from rainfall contributed to the changes, particularly after extended dry periods. We observed low NH4-N concentrations and total algal biomass during the end of algal bloom. However, 3-4 days later ammonium thrived, followed by another algal bloom. Algal bloom occurrences caused large diurnal fluctuations in reservoir NH4-N concentrations (daily differences >5 μmol L-1), resulting the maximum nighttime NH4-N flux reaching up to five times the minimum daytime flux. Our study highlights the advantages of high-frequency synchronous monitoring of nutrient-algae dynamics to understand their interactions, and the importance of ammonium control on preventing algal blooms during low-flow winter periods.

Effects of nitrogen phosphorus ratio and light on phosphorus removal by microalgae in high-phosphorus wastewater

The removal of phosphorus from wastewater has consistently posed a major focus in the field of wastewater treatment. Microalgae-based phosphorus removal is widely acknowledged as an effective biological approach. However, ensuring the microalgae-mediated high phosphorus concentration removal remains a persistent challenge. In this study, a kind of multicellular microalgae, Klebsormidium sp., was used to explore its ability to remove phosphorus in high-phosphorus wastewater. The phosphorus removal rate by Klebsormidium sp. in highly concentrated (>20 mgP/L) wastewater can exceed 90%. To investigate the phosphorus absorption process, various nitrogen and phosphorus concentrations along with light conditions were employed. The results showed that 50% to 80% of the total phosphorus absorbed by microalgae entered the intracellular polymer. The phosphorus concentration and light intensity did not exert any significant effects on the absorption of phosphorus by microalgae. However, the nitrogen concentration and the light-to-dark ratio significantly influenced the storage of phosphorus by microalgae. At a nitrogen concentration over 300 mgN/L, phosphorus absorption by microalgae was inhibited. A higher light-to-dark ratio increased phosphorus transfer by microalgae, while the light duration exceeds 16 h inhibited it. Microalgae have emerged as promising materials for phosphorus removal in high-phosphorus sewage, the study offering potential solutions for a cleaner and more sustainable future.

Evaluation of the importance of sediment organic matter composition for CH4 production by microcosm tests with and without addition of natural sources (cyanobacterial biomass and riparian pasture) in two subtropical, eutrophic reservoirs

The biochemical composition of sediments, which depends on the origin of the organic matter (OM), is decisive in methane (CH4) production. This study aimed to determine the CH4 produced under anaerobic conditions from different substrates: native reservoir sediments and sediments with the addition of complex OM from Microcystis spp. blooms and terrestrial plants (pasture), alongside the biochemical characterization of the substrates used. The biochemical composition of the sediments explained the differences in CH4 production rate (µmol/g OM/day). Positive correlations were found between CH4 production rate and proteins (PRT) (r = 0.695), lipids (LIP) (r = 0.582), TN (r = 0.605), and biopolymeric carbon (BPC) (r = 0.784). Principal component analysis showed an association of CH4 production rate with PRT, LIP, TN, and BPC concentrations, sharing the same direction of the vectors. The addition of Microcystis bloom and pasture to the sediments significantly increased the rate and production of CH4 compared to native sediments, with higher values for bloom addition. In the studied reservoirs, cyanobacteria biomass is a very important source of organic matter to the sediments and a support for methanogenesis. Nonetheless, OM from surrounding vegetation, mainly pastures, could also play a significant role during events that increase reservoir levels, generating important CH4 emission "hot spots" at the periphery of the reservoirs.

Effects of water and sediment chemistry variables on aquatic macroinvertebrate community structuring in a subtropical Austral river system

Riverine physical and chemical characteristics are influencing ecosystem integrity while shaping and impacting species richness and diversity. Changes in these factors could potentially influence community structuring through competition, predation and localised species extinctions. In this study, eight sampling sites over multiple seasons were assessed along the streams draining the City of Nelspruit, South Africa, to examine river health based on water and sediment quality, while using macroinvertebrates as bioindicators for pollution. All water variables with the exception for salinity were found to be significantly different among seasons, with sites having significant differences among all water variables. All sediment chemistry variables were also found to be significantly different among sites and seasons, with the exception of K for sites and Zn and Ca for seasons. The PCA factor loadings and two-cluster analysis identified two groupings, i.e. group 1 that consisted of all metals apart from K and Na and group 2 with K and Na metals. A total of 4470 macroinvertebrate taxa were identified, with Crustacea Caridina nilotica and Diptera Chironominae being dominant across seasons, with macroinvertebrate communities being found to be significantly different among sites and seasons. The most common functional feeding groups across sites were the collector-gatherers (52.2%), followed by collector-filters (26.8%), predators (16.4%), scrapers (4.4%) and shredders (0.1%). Boosted regression trees indicated that high variation in species richness occurred with change in resistivity, P, water pH, ORP, conductivity and S concentrations. These results evidence a strong linkage among the sediment, water quality, substratum embeddedness and habitat structure and community structure. It is important to protect the integrity of aquatic ecosystems through effective monitoring due to the increasing water and sediment quality pressures that arise from various anthropogenic activities.

The state of nitrogen in rivers and streams across sub-Saharan Africa

The nutrient status of rivers and streams is less researched in sub-Saharan Africa than in many other inhabited regions of the world. Given the expected population growth, intensification of agriculture, increased pressure on natural ecosystems and projected climate change in sub-Saharan Africa, it is crucial to quantify and understand drivers behind spatiotemporal patterns of nitrogen concentrations and loads in rivers and streams. Such knowledge can support sustainable management of water resources with the goal to provide clean water, create and maintain healthy ecosystems and prevent excessive pollution of water resources with nitrogen compounds, as is found in large parts of North America, Europe and Asia. This review provides a synthesis of the current available data from peer-reviewed literature (n = 243) on particulate and dissolved nitrogen in rivers and streams in sub-Saharan Africa, looking into seasonal and land cover-related differences. The review shows that data on nitrogen concentrations in rivers and streams is available for 32 out of the 48 countries (67 %) in sub-Saharan Africa, highlighting large data gaps given the size of the region. Differences in nitrogen concentrations between land cover types are reported, with highest median total nitrogen (3.9 mg N L-1) and nitrate (1.2 mg N L-1) concentrations observed at sites characterised by settlement and industry. In contrast, natural land cover types, like forest, have higher median (N:P) ratios (> 14.6) than cropland and urban areas (< 12.0). The analysis of paired samples from dry and wet seasons reveals varying effects of seasonality on the concentration of different nitrogen compounds between land cover types. However, the processes driving these spatiotemporal differences are still poorly understood. These findings highlight the need for a targeted research agenda for Africa to advance our understanding of the role of rivers and streams in nitrogen cycling in different ecosystems and their interaction with anthropogenic and natural drivers of change.

The role of microorganisms in phosphorus cycling at river-lake confluences: Insights from a study on microbial community dynamics

River-lake confluences are key zones in the river-lake network, essential for managing contaminant transport and transformation. However, the role of biogeochemical transformations, particularly in phosphorus (P) dynamics, has been underexplored. As a result, this study looks into the dynamics of microbial communities and how important microbes are to the cycling of P. It was revealed that microorganisms contribute differently to phosphorus cycling in different hydraulic regions. Regions with higher-velocity and finer sediment showed increased microbial diversity and enhanced capabilities for organic phosphorus (OP) mineralization and inorganic phosphorus (IP) solubilization due to lower bio-available P (bio-P) concentrations. In areas characterized by flow deflection (FD), flow stagnation (FST), and flow separation (FSE), distinct P fraction distributions were observed: Total phosphorus (TP) and bio-P were found to be more abundant in the FST and FD regions, but residual phosphorus (Res-P) and calcium phosphorus (Ca-P) were more prevalent in the FSE region. Sediment characteristics, including P species like aluminum-phosphorus (Al-P), OP, iron-associate phosphorus (BD-P), and sediment mid-diameter (D50), significantly influence microbial community composition. These results improve our comprehension of the distribution of microbial community distribution and its role in the phosphorus cycle at river-lake confluence, providing useful provide valuable information for managing river-lake confluences and protecting aquatic ecosystems.

Assessing chlorophyll-a and water quality dynamics in arid-zone temporary pan systems along a disturbance gradient

Temporary pans are susceptible to various anthropogenic effects such as pollution, resource extraction, and land use intensification. However, given their small endorheic nature, they are almost entirely influenced by activities close to their internally drained catchments. Human-mediated nutrient enrichment within the pans can lead to eutrophication, resulting in increased primary productivity and decreased associated alpha diversity. The Khakhea-Bray Transboundary Aquifer region and the pan systems that characterise the area are understudied area with no records available of the biodiversity therein. Additionally, the pans are a major water source for the people in these areas. This study assessed differences in nutrients (i.e., ammonium, phosphates) and their effect on chlorophyll-a (chl-a) concentrations in pans along a disturbance gradient in the Khakhea-Bray Transboundary Aquifer region, South Africa. Physicochemical variables, nutrients, and chl-a were measured from 33 pans representing variable anthropogenic exposure during the cool-dry season in May 2022. Five environmental variables (i.e., temperature, pH, dissolved oxygen, ammonium, and phosphates) showed significant differences between the undisturbed and disturbed pans. The disturbed pans generally had elevated pH, ammonium, phosphates and dissolved oxygen compared to the undisturbed pans. A strong positive relationship was observed between chl-a and temperature, pH, dissolved oxygen, phosphates and ammonium. Chlorophyll-a concentration increased as surface area, and the distance from kraals, buildings and latrines decreased. Anthropogenic activities were found to have an overall effect on the pan water quality within the Khakhea-Bray Transboundary Aquifer region. Therefore, continuous monitoring strategies should be established to better understand the nutrient dynamics through time and the effect that this may have on productivity and diversity in these small endorheic systems.

Hemato-biochemical alteration in the bronze featherback Notopterus notopterus (Pallas, 1769) as a biomonitoring tool to assess riverine pollution and ecology: a case study from the middle and lower stretch of river Ganga

Fishes are poikilothermic animals and are rapid responders to any sort of ecological alteration. The responses in the fish can be easily assessed from their hematological and biochemical responses. To study the variation in the hemato-biochemical parameters in retort to ecological alteration and ecological regime, a study was conducted at six different sampling stations of the middle and lower stretches of river Ganga. Various hematological and biochemical responses of fishes were also monitored in response to multiple ecological alterations. For the assessment of ecological alteration, various indices were calculated such as the water pollution index (WPI), National Sanitation Foundation-water quality index (NSF-WQI), and Nemerow's pollution index (NPI) has been calculated based on various water quality parameters such as dissolved oxygen (DO), pH, total dissolved solids (TDS), total alkalinity (TA), total hardness (TH), electrical conductivity (EC), biochemical oxygen demand (BOD), chlorinity (CL), total nitrogen (TN), and total phosphorus (TP). The hematological parameters such as WBC, RBC, platelet, hemoglobin, and hematocrit were monitored. The serum biochemical parameters such as SGPT, SGOT, ALP, amylase, bilirubin, glucose, triglyceride (TRIG), and cholesterol (CHOL) were investigated. The study revealed that NSF-WQI varied from 45.08 at Buxar to 110.63 at Rejinagar and showed a significantly positive correlation with SGPT, SGOT, ALP, TRIG, CHOL, and WBC, whereas a significantly negative correlation was observed between TRIG and RBC. WPI varied from 19 to 23 and showed a significant positive correlation with SGOT and a negative correlation was observed with total nitrogen. The PCA analysis illustrated the significance of both natural as well as anthropogenic factors on riverine ecology. Strong positive loading was observed with SGPT, SGOT, ALP, and platelet. The study signified the need for monitoring the hemato-biochemical responses of fishes in response to alterations in the ecological regime.

Toxicity determination, pollution source delineation, and microbial diversity evaluation of PAHs-contaminated sediments for an urban river

The Feng-Sang River is a metropolitan river in Kaohsiung City, Taiwan. In this study, Feng-Sang River sediments were analyzed to investigate the distributions and sources of polycyclic aromatic hydrocarbons (PAHs). The Sediment Quality Guidelines (SQGs), potentially carcinogenic PAHs (TEQcarc), and toxic equivalence quotient (TEQ) were applied to evaluate influences of PAHs on ecosystems and microbial diversities. Results indicate that PAHs concentrations varied between seasons and locations. The concentrations of ∑16PAHs ranged from 73.6 to 603.8 ng/kg in dry seasons and from 2.3 to 199.3 ng/kg in wet seasons. This could be because of the flushing effect during wet seasons, which caused the movement and dilution of the PAH-contaminated sediments. Diagnostic ratio analysis infers that high PAHs levels were generated by combustion processes and vehicle traffic, and results from multivariate descriptive statistical analysis also demonstrate that the vehicular traffic pollution could be the major emission source of PAHs contamination. Comparisons of PAHs with SQGs indicate that PAHs concentrations in sediment were below the effects range low (ERL) values, and thus, the immediate threat to organisms might not be significant. The diagnostic ratio analyses are effective methods for PAH source appointment. The metagenomic assay results imply that sediments contained essential microbial species with eminent diversity. The detected PAH-degrading bacteria (Desulfatiglans, Dechloromonas, Sphingomonas, Methylobacterium, Rhodobacter, Clostridium, and Exiguobacterium) played a key role in PAHs biotransformation, and Dechloromonas and Rhodobacter had a higher relative abundance. Results of microbial diversity analyses indicate that the contaminated environment induced the changes of governing microbial groups in sediments. PRACTITIONER POINTS: Diagnostic ratio analyses are effective methods for PAHs source appointment. Microbial composition in sediments are highly affected by anthropogenic pollution. Combustion and vehicle traffic contribute to urban river sediments pollution by PAHs. Dechloromonas and Rhodobacter are dominant PAHs-degrading bacteria in sediments.

Recovery, regeneration and sustainable management of spent adsorbents from wastewater treatment streams: A review

Adsorption is the most widely adopted, effective, and reliable treatment process for the removal of inorganic and organic contaminants from wastewater. One of the major issues with the adsorption-treatment process for the removal of contaminants from wastewater streams is the recovery and sustainable management of spent adsorbents. This review focuses on the effectiveness of emerging adsorbents and how the spent adsorbents could be recovered, regenerated, and further managed through reuse or safe disposal. The critical analysis of both conventional and emerging adsorbents on organic and inorganic contaminants in wastewater systems are evaluated. The various recovery and regeneration techniques of spent adsorbents including magnetic separation, filtration, thermal desorption and decomposition, chemical desorption, supercritical fluid desorption, advanced oxidation process and microbial assisted adsorbent regeneration are discussed in detail. The current challenges for the recovery and regeneration of adsorbents and the methodologies used for solving those problems are covered. The spent adsorbents are managed through regeneration for reuse (such as soil amendment, capacitor, catalyst/catalyst support) or safe disposal involving incineration and landfilling. Sustainable management of spent adsorbents, including processes involved in the recovery and regeneration of adsorbents for reuse, is examined in the context of resource recovery and circular economy. Finally, the review ends with the current drawbacks in the recovery and management of the spent adsorbents and the future directions for the economic and environmental feasibility of the system for industrial-scale application.

Modeling Chlorophyll a with Use of the SWAT Tool for the Nielba River (West-Central Poland) as an Example of an Unmonitored Watercourse

The majority of eutrophication studies focuses on lacustrine processes, thus riverine systems remain less recognized in this context. Moreover, since the availability of data related to parameters affecting this phenomenon is quite limited, modeling efforts should be considered. The current study verifies the SWAT model’s capability to simulate chlorophyll a loads for unmonitored watercourse. The analyses of the relationships between individual parameters, directly involved in the eutrophication process, help in the exploration of its dominant trends in SWAT modeling. The results obtained for the Nielba River pilot catchment (west-central Poland) showed a strong correlation of chlorophyll a with flow and surface runoff, but no relationship with temperature or solar radiation. Moreover, an impact of local conditions (hydrological features) on chlorophyll a load simulation could be traced in detail. The research specified the limitations and impact of generalization in the SWAT model on the results. Furthermore, intricacies related to the dataset statistical treatment (e.g., outliers) have been presented.

Hurricanes Substantially Reduce the Nutrients in Tropical Forested Watersheds in Puerto Rico

Because nutrients including nitrogen and phosphorus are generally limited in tropical forest ecosystems in Puerto Rico, a quantitative understanding of the nutrient budget at a watershed scale is required to assess vegetation growth and predict forest carbon dynamics. Hurricanes are the most frequent disturbance in Puerto Rico and play an important role in regulating lateral nitrogen and phosphorus exports from the forested watershed. In this study, we selected seven watersheds in Puerto Rico to examine the immediate and lagged effects of hurricanes on nitrogen and phosphorous exports. Our results suggest that immediate surges of heavy precipitation associated with hurricanes accelerate nitrogen and phosphorus exports as much as 297 ± 113 and 306 ± 70 times than the long-term average, respectively. In addition, we estimated that it requires approximately one year for post-hurricane riverine nitrogen and phosphorus concentrations to recover to pre-hurricane levels. During the recovery period, the riverine nitrogen and phosphorus concentrations are 30 ± 6% and 28 ± 5% higher than the pre-hurricane concentrations on average.

Nuisance phytoplankton transport is enhanced by high flow in the main river for drinking water in Uruguay

Eutrophication, climate change, and river flow fragmentation are the main cause of nuisance algal blooms worldwide. This study evaluated the conditions that trigger the growth and occurrence of nuisance phytoplankton in the Santa Lucía River, a subtropical floodplain lotic system that supplies drinking water to 60% of the population of Uruguay. The main variables that explained phytoplankton biovolume were extracted from generalized linear models (GLM). The potential impact of nuisance organism advection on water utility was estimated by the phytoplankton biovolume transport (BV_TR, m³ day^-1), an indicator of biomass load. Santa Lucía River had a wide flow range (0.7×10⁵-1438×10⁵ m³ day^-1) and eutrophic conditions (median, TP: 0.139 mg L^-1; TN: 0.589 mg L^-1). GLMs indicated that phytoplankton biomass increased with temperature and soluble reactive phosphorus. Contrary to expectations, the presence of cyanobacteria was positively associated with periods of high flow that result in high cyanobacterial biovolume transport, with a probability of 3.35 times higher when flow increased by one standard deviation. The cyanobacterial biovolume transported (max: 9.5 m³ day^-1) suggests that biomass was subsidized by allochthonous inocula. Biovolume from other nuisance groups (diatoms, cryptophytes, and euglenophytes) was positively associated with low-flow conditions and high nutrient concentrations in the main river channel, thereby indicating that these conditions boost eukaryote blooms. The evaluation of BV_TR allows a better understanding of the dynamics of fluvial phytoplankton and can help to anticipate scenarios of nuisance species transport.

Use of biomarkers in monitoring pollution status of urban rivers, Limpopo, South Africa

This study investigated the use of enzymes and histology as potential biomarkers in Sand and Blood Rivers in Limpopo Province, South Africa. Physico-chemical parameters were analyzed at sites S1, S2, S3, S4, and S5 of the Sand River and sites B1 and B2 of the Blood River. Sites S1 and B1 were selected as the reference sites and located upstream of the Polokwane and Seshego wastewater treatment plants (WWTPs) effluent discharge points, respectively. Water quality parameters assessed were water temperature, dissolved oxygen (DO), biological oxygen demand (BOD), total dissolved solids (TDS), turbidity, ammonia, nitrogen, phosphorus, and pH. Sites downstream the WWTPs recorded elevated levels of nitrogen, phosphorus, and ammonia. DO, ammonia, nitrogen, and phosphorus were above the targeted water quality range (TWQR) for aquatic ecosystems. The Canadian council of ministers of the environment water quality index (CCME WQI) showed that the reference sites had good water condition while sites downstream (S2, B2, S3, and S4) had poor water condition. Acetylcholinesterase (AChE) enzyme activity in the brain of Clarias gariepinus from sites downstream was lower than those from sites upstream of the effluent discharge points. The liver of Clarias gariepinus from sites downstream showed higher lactate dehydrogenase (LDH) enzyme activity compared to fish caught at the reference sites. Furthermore, Clarias gariepinus from sites downstream showed a number of histological alterations in gills and liver than fish from upstream. Histological alterations observed in gills included fusion of the secondary lamellae, hyperplasia of the interlamellar, epithelial lifting of secondary lamellae, and hyperplasia of the secondary lamellae. In the liver, histological alterations observed included melanomacrophage centers (MMCs), macrovesicular steatosis, sinusoid congested with kupffer cells, nuclei pleomorphism, and vacuolation. The gill and liver indices were significantly lower at the reference sites compared to sites downstream. Evidently, AChE, LDH, and gill and liver histology can be used as early warning signs of aquatic degradation in rivers that are recipients of poorly treated sewage effluent.

Interactions of microalgae-bacteria consortia for nutrient removal from wastewater: A review

Nitrogen and phosphorus pollution can cause eutrophication, resulting in ecosystem disruption. Wastewater treatment systems employing microalgae-bacteria consortia have the potential to enhance the nutrient removal efficiency from wastewater through mutual interaction and synergetic effects. The knowledge and control of the mechanisms involved in microalgae-bacteria interaction could improve the system's ability to transform and recover nutrients. In this review, a critical evaluation of recent literature was carried out to synthesize knowledge related to mechanisms of interaction between microalgae and bacteria consortia for nutrient removal from wastewater. It is now established that microalgae can produce oxygen through photosynthesis for bacteria and, in turn, bacteria supply the required metabolites and inorganic carbon source for algae growth. Here we highlight how the interaction between microalgae and bacteria is highly dependent on the nitrogen species in the wastewater. When the nitrogen source is ammonium, the generated oxygen by microalgae has a positive influence on nitrifying bacteria. When the nitrogen source is nitrate, the oxygen can have an inhibitory effect on denitrifying bacteria. However, some strains of microalgae have the capability to supply hydrogen gas for hydrogenotrophic denitrifiers as an energy source. Recent literature on biogranulation of microalgae and bacteria and its application for nutrient removal and biomass recovery is also discussed as a promising approach. Significant research challenges remain for the integration of microalgae-bacteria consortia into wastewater treatment processes including microbial community control and process stability over long time horizons.

Response of the microbial community to salt stress and its stratified effect in constructed wetlands

Nitrogen removal in constructed wetlands (CWs) may be inhibited by salinity. The clarification of the response of microbial community to salt stress is a premise for developing strategies to improve nitrogen removal efficiency in CWs under saline conditions. Results showed that the ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), and total nitrogen (TN) removal percentages significantly (p < 0.05) decreased in CWs with increasing salinity. The structure and abundance of the microbial community varied with different salinity levels and sampling depths in CWs. Compared with a non-saline condition, the abundances of some bacteria with a denitrification function (e.g., Arthrobacter) significantly (p < 0.05) decreased in CWs under saline conditions (i.e., EC of 15 and 30 mS/cm). Aerobic bacteria (e.g., Sphingomonas) exhibited more abundance in soil and upper gravel samples in CWs than those in bottom gravel samples, while the abundance of some denitrifying bacteria (e.g., Thauera and Azoarcus) was significantly (p < 0.05) higher in bottom gravel samples compared with soil and upper gravel samples, respectively. This study provides both microbiological evidence for explaining the impact of salt stress on nitrogen removal in CWs and scientific reference for developing enhanced strategies to improve the nitrogen removal capacity of CWs.

From Monitoring and Modeling to Management: How to Improve Water Quality in Brazilian Rivers? A Case Study: Piabanha River Watershed

Water quality has been a global concern, as evidenced by UN Sustainable Development Goals. The current paper has focused on the Piabanha River rehabilitation as a case study which can be generalized to other similar watersheds. A monitoring program during a hydrological year was carried out, and different databases were used to calibrate and validate the QUAL-UFMG water quality model. Sanitation is the major problem in the watershed, notably in its headwater catchments, which concentrate the most urbanized regions where water quality is worse in the dry season due to low river flows. Thus, simulations of the river water quality have been performed through computational modeling suggesting organic load reductions in some sub-basins. In conclusion, some strategies to improve water quality have been discussed: (i) The water quality rehabilitation must consider progressive goals of pollution reduction starting with an initial implementation in a reduced area. The monitoring should be based on a few parameters relevant and simple to monitor. (ii) Pollution reduction ought to be carried out strategically with deadlines and intermediate goals that must be agreed upon between the stakeholders in the watershed. (iii) Watershed committees should supervise projects to improve water quality in partnership with the State Prosecutor’s Office.

Benthic diatom-based indices and isotopic biomonitoring of nitrogen pollution in a warm temperate Austral river system

Rivers are impacted by pollutants from anthropogenic activities such as urbanisation and agricultural practices. Whilst point source pollution has been widely studied and in some cases remediated, non-point pollutant sources remain pervasive, particularly in developing countries that lack economic and human specialist capacity. Monitoring of pollution levels in many regions is additionally challenged by a lack of robust indicators for nitrogen inputs, however, diatom community indices and analysis of variation in microphytobenthos (MBP) stable isotope analysis variations have potential. The present study investigates variations and utilities in benthic diatom indices and MPB δ¹⁵N along different river sections (n = 31) of an austral river between two seasons (wet and dry), testing for relationships with key environmental variables (physical, water and sediment), in the context of N monitoring. One hundred and eighteen diatom taxa belonging to 36 genera were identified, with physical (water flow), water (nitrate, P and total dissolved solids) and sediment (B, Ca, Cr, Na, N, P, SOM, Pb and Zn) variables correlating to one or more of the 12 diatom indices presented. In particular, Biological Diatom Index, Biological Index of Water Quality, Central Economic Community, Index of Artois-Picardie Diatom (IDAP) and Sládeček's Index were strongly explained by sediment variables, whilst Descy's Pollution Index and Schiefele and Schreiner's Index were explained by water and physical variables. Whilst MPB δ¹⁵N were within the "no impact" level in the wet and dry seasons at reference (i.e. unpolluted) sites, all sites located in agricultural or urban areas, and downstream of sewage discharges, had a wider range that encompassed increasing organic impacts ("inorganic impacts" to high "organic impacts"). Temperature and turbidity (negative), as well as dissolved oxygen, waterway width and depth (positive), significantly affected MPB δ¹⁵N, whilst effects of chemistry variables were less apparent. Overall, we found that MPB δ¹⁵N signatures were significantly correlated with Trophic Diatom Index, the Specific Pollution Sensitivity Index and the Artois-Picardie Diatom Index, suggesting the utility of diatoms and MPB δ¹⁵N in assessments of aquatic pollution. In turn, MPB δ¹⁵N values are strong indicators of N pollution across spatial and seasonal gradients. Thus, the results showed the effects of sediment variables on diatoms to be strong, indicating that sediment rather than water characteristics more strongly structure diatom communities. Thus, sediment variables should be sampled when conducting bioassessment studies.

Identification of nitrate sources and transformations in basin using dual isotopes and hydrochemistry combined with a Bayesian mixing model: Application in a typical mining city

The external nitrogen load input caused by human activities exacerbates the eutrophication process of aquatic ecosystems in mining areas, causing water quality problems. However, knowledge of the sources and environmental behavior of nitrate in the surface water of mining areas is still very limited. This study investigated the nitrate content and spatiotemporal variation characteristics of surface water in the Linhuan mining area, identified the sources and transformation processes of nitrate using isotopes and hydrochemistry, and evaluated the contribution rates of different potential nitrate sources based on a Bayesian mixing model. The nitrogen pollution in the surface water in the mining area seriously exceeded class Ⅴ of the Environmental Quality Standard of Surface Water of China (GB3838-2002). The NO₃^- content ranged from 0.87 to 3.41 mg/L, showing obvious seasonal and spatial differences. Isotope and NO₃^-/Cl^- analysis indicated that nitrate in the subsidence area water (SAW) was mainly derived from chemical fertilizer (NF) and soil organic nitrogen (NS), while nitrate in the mainstream of the Huihe River water (HRW) was mainly derived from manure/sewage (MS). The nitrate in the tributary of the Baohe River water (BRW) was mainly derived from soil NS, and nitrification was a nitrogen conversion pathway in the soil. The results of the Bayesian mixing model showed that the main sources of nitrate in the BRW, HRW and SAW were NF (34.5%), MS (68.8%) and NF (40.8%) in the wet season, and NS (33.4%), MS (70.9%) and NF (58.1%) in the dry season, respectively. The results of this study provide a new integrated method for the identification of nitrate pollution sources in mining areas, and this method can be used to improve the biogeochemical information of nitrogen in the aquatic ecosystems of mining areas and help formulate relevant measures to reduce water nitrogen pollution.

Spatial and seasonal variation of water parameters, sediment properties, and submerged macrophytes after ecological restoration in a long-term (6 year) study in Hangzhou west lake in China: Submerged macrophyte distribution influenced by environmental variables

Submerged macrophyte restoration is the key stage in the reestablishment of an aquatic ecosystem. Previous studies have paid considerable attention to the effect of multiple environmental factors on submerged macrophytes. Meanwhile, few studies have been conducted regarding the spatial and seasonal characteristics of water and sediment properties and their long-term relationship with submerged macrophytes after the implementation of the submerged macrophytes restoration project. On a monthly basis, we monitored the spatial and seasonal variation in water parameters, sediment properties, and the submerged macrophyte characteristics of West Lake in Hangzhou from August 2013 to July 2019. From these measurements, we characterized the relationship between environmental factors and submerged macrophytes. Water nutrient concentrations continuously decreased with time, and the accumulation of sediment nutrients was accelerated as the submerged macrophyte communities developed on a long-term scale. The results indicated that the difference in water parameters was due to seasonal changes and land-use types in the watershed. The differences in the sediment properties were mainly attributed to seasonal changes and changes in the flow field. Redundancy analysis showed that the influence of water nutrients on the submerged macrophyte distribution was greater than that of sediment nutrients. The result also suggested that the developed root system, high stoichiometric homeostasis coefficients of P, and compensation ability of substantial leaf tissue may lead to a large distribution of Vallisneria natans in West Lake in Hangzhou. The correlation of water parameters and sediment properties with submerged macrophytes for a long time was very important as the restoration was achieved. To ensure the stability of the aquatic ecosystem after performing the submerged macrophyte restoration, a greater emphasis must be placed on reestablishing the entire ecosystem, including the restoration of aquatic animals and fish stocks. We expect these findings to serve as a reference for researchers and government agencies in the field of aquatic ecosystem restoration.

Metal Distribution and Sediment Quality Variation across Sediment Depths of a Subtropical Ramsar Declared Wetland

The study of wetlands is particularly important as these systems act as natural water purifiers and thus can act as sinks for contaminated particles. Wetland sediments are important as they provide an indication of potential contamination across temporal and spatial scales. The current study aimed to investigate the distributions of selected metals and nutrients in different sites in relation to sediment depth, and identify relationships among sediment metals. Significant differences in nutrient (i.e., N, P) and metal (i.e., K, Mg, Na, Fe, Cu, B) concentrations were found across study sites, whereas nutrients (i.e., N, P) and metals (i.e., Ca, Mg, Fe, Cu, Zn) were significantly different with sediment depths. When compared against Canadian sediment standards, most of the assessed metals were within the “no effect” level across the different sites and depths. The K, Ca, and Mg concentration showed extreme contamination across all sites and depths. The enrichment factor values for K, Ca, and Mg showed extremely high enrichment levels for all sites and sediment depths. The Na, Mn, Fe, Cu, Zn, and B concentration showed mostly background enrichment levels. All sediments across the different sites and sediment depths indicated deterioration of sediment quality. Pearson correlations suggest that most metals might have originated in a similar source as that of Mn and B, owing to a lack of significant differences. These results provide baseline information for the general management of the Nylsvley Wetland in relation to sediment metal pollution. The specific sources of metal contaminants also require further elucidation to further inform management efforts.

Influence of anaerobic digestion on the labile phosphorus in pig, chicken, and dairy manure

Phosphorus (P) loss from livestock and poultry industry causes serious threat to agro-ecological environments. Anaerobic digestion (AD), through recycling of P-containing resources and biogas production, prevails as a promising solution to the resource, energy, and environment trilemma. In this study, the dynamic transformation of P in batch AD processes fed with chicken, pig and dairy manures was investigated. Results showed that the Labile-P of total phosphorus (TP) in pig, chicken and dairy manure digestates decreased from 37.35% to 23.79%, 36.79% to 17.29%, and 60.47% to 20.39%, respectively, and was associated with an increase of NaOH-P during the AD process. However, the Labile-P in raw manures ranging from 64.67% to 81.10%, indicated that AD could reduce the pollution risk caused by the overuse of high Labile-P animal manure as fertilizer. Metal ions had a significant influence on P transformation because of their ability to combine with PO₄^3-/HPO₄^2-. During AD, the species of phosphates increased: AlPO₄, FePO₄, Mg₃(PO₄)₂, CaHPO₄, Mg(NH₄)PO₄·6H₂O and Ca₁₀(PO₄)₆(OH)₂ were the main phosphates qualified by X-ray diffraction (XRD). AD produced a satisfactory fertilizer for plants that were able to activate the precipitated P, which could provide readily available N and slow-release P. This study provides a meaningful theoretical guide for recycling P from animal manure resources.

Responses of phytoremediation in urban wastewater with water hyacinths to extreme precipitation

Climate change not only intensifies eutrophication and enhances the rainfall, but also elevates the contents of greenhouse gases, which can further increase the intensity and frequency of extreme precipitation events. The effectivity of phytoremediation of urban wastewaters by water hyacinths under an extreme rainfall event (up to 380 mm d^-1) was investigated using self-designed fabrications with six flow rates (2-15 m³ d^-1) in situ on pilot scale for 30 days. The results suggest that water hyacinths had high N and P removal capacities even under adverse conditions such as low dissolved oxygen concentrations (DO, <1 mg L^-1) and high ammonium concentrations (NH₄⁺-N, >7 mg L^-1). Specifically, the highest removal yields of N and P were 13.14 ± 0.47 g N·m^-2·d^-1 and 2.12 ± 0.04 g P·m^-2·d^-1, respectively. The results indicate that water hyacinths can be used for water treatment to reduce the amounts of NH₄⁺-N, dissolved organic nitrogen (DON) and phosphate (PO₄^3-) even during extreme precipitation events. Moreover, DO increased due to wet deposition, runoff and surface flows during the extreme rainfall event, resulting in shifts between nitrification and denitrification processes which significantly altered nitrogen forms in urban wastewater. Results of this study suggest that water hyacinths could be recommended as a cost-effective and eco-friendly technology for urban wastewater phytoremediation in areas suffering from frequent extreme precipitation events.

Land use/land cover change effect on soil erosion and sediment delivery in the Winike watershed, Omo Gibe Basin, Ethiopia

Information on soil loss and sediment export is essential to identify hotspots of soil erosion to inform conservation interventions in a given watershed. This study investigates the dynamics of soil loss and sediment export associated with land-use/land cover changes and identifying soil loss hotspot areas in the Winike watershed of the Omo-Gibe Basin of Ethiopia. Spatial data collected from satellite images, topographic maps, meteorological and soil data were analyzed. The land-use types in the study area were categorized into six: cultivated land, woodland, forest, grazing, shrubland, and bare land. The Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) of the sediment delivery ratio (SDR) model was used based on the analysis of land use/land cover and RUSLE factors. The results show that total soil loss increased from 774.86 thousand tons in 1988 to 951.21 thousand tons in 2018 while the corresponding sediment export increased by 3.85 thousand tons for the same period. These were subsequently investigated in each land-use type. Cultivated fields generated the highest soil erosion rate, increasing from 10.02 t/ha/year in 1988 to 43.48 t/ha/year in 2018 when compared with the grazing, shrub, forest, wood land and bare land-use types. This corresponds with the expansion of the cultivated area. This is logical as the correlation between soil loss and sediment delivery and expansion of cultivated area is highly significant (p < 0.001). Sub-watershed six (SW-6) showed the highest soil loss (23.17 t/ha/year) while sub-watershed two (SW- 2) has the lowest soil loss (5.54 t/ha/year). This is because SW-2 is situated in the lower reaches of the watershed under dense vegetation cover experiencing less erosion. The findings on the erosion hotspots presented in this study allow prioritizing the segments of the watershed that need immediate application of improved management interventions and informed decision-making processes.

Quantifying Metal Contamination and Potential Uptake by Phragmites australis Adans. (Poaceae) Along a Subtropical River System

Metal pollution is pervasive across terrestrial and aquatic ecosystems owing to anthropogenic activities. Sediments can accrue high concentrations of metals and act as secondary sources, and thus may be valuable indicators of metal contamination across spatiotemporal scales. In aquatic systems, the extent of metal pollution may be further mediated by transference among sediments and living organisms, with plant metal contaminants potentially predictive of underlying sediment concentrations. The present study thus quantifies the extent of metal pollutants (Na, K, Ca, Mg, Cu, Zn, Mn, B, Fe) across multiple study sites and seasons (cool-dry, hot-wet, hot-dry) in a subtropical river system. Furthermore, uptake by a key macrophyte species, Phragmites australis, was examined and correlated with sediment pollution levels among different plant parts. Overall, sediment pollution load indices differed seasonally, being significantly highest during the cool-dry season irrespective of sampling location, suggesting that periods with reduced water flows can exacerbate metal pollution levels in riverine sediments. Also, metal concentrations were highest in upstream wetland sites, indicating a capacity for metal sink effects in these areas. Overall, macrophytes contained high concentrations of select metals, however composition and concentrations differed across plant parts, with roots containing particularly high concentrations of Fe and B. Correlations between sediment and macrophyte concentrations were mostly non-significant, whilst stem Mn and Fe concentrations correlated significantly negatively and positively to sediment concentrations, respectively. The present study identifies key spatiotemporal differences in multiple metal contaminants in an understudied subtropical aquatic system that align with hydrological regime differences. Whilst macrophytes were not found to be major accumulators, or predictors, of metal contaminants in this study, they may collectively play a central role in concentration regulation in aquatic systems.

Assessment of nutrient contamination in the waters of the El Fuerte River, southern Gulf of California, Mexico

This study assessed the spatial and temporal nutrient variability in the El Fuerte River basin in northwestern Mexico, considering its effects on the water trophic status as well as the nutrient loading to the Gulf of California. Physicochemical parameters, inorganic species of nitrogen, phosphate, total nitrogen, total phosphorus, and chlorophyll-a (chl-a) were quantified at 16 selected sites along the river in April (dry season) and October (rainy season) 2017. Mean concentrations of nutrients during dry and rainy seasons were 36.3 ± 24.1 and 55.1 ± 74.6 μg L^-1 of total ammonia nitrogen, 3.4 ± 3.6 and 4.5 ± 3.5 μg L^-1 of NO₂^--N, 190.8 ± 256.0 and 163.6 ± 261.0 μg L^-1 of NO₃^--N, 42.4 ± 44.2 and 104.9 ± 76.2 μg L^-1 of PO₄^3--P, 1.0 ± 1.3 and 691 ± 2242 mg L^-1 of TN, 0.06 ± 0.06 and 0.08 ± 0.09 mg L^-1 of TP, and 0.9 ± 0.6 and 2.0 ± 0.9 μg L^-1 of chl-a with significant differences (p < 0.05) between sites and seasons. When waters are transported downstream, nutrient levels are enriched by 4 to 35 times compared to those upstream due to increased population and agriculture downstream, confirming the hypothesis of the study. The calculated TN and TP fluxes were 1.23 × 10⁴ and 3.57 × 10¹ ton year^-1, respectively. Factor analysis indicated that inorganic nitrogen species and phosphorus are the main factors affecting the river water quality. Despite N excess during the rainy season, the river reached mesotrophic waters due to phosphorus limitation. This suggests the need to establish a water quality monitoring program to understand the vulnerability of the river course to changes in its trophic state.

Impact of application mode on the control of phosphorus release from sediments using zirconium-modified bentonite as geo-engineering material

In this study, the influence of zirconium-modified bentonite (ZMBT) addition, capping, and addition/capping on the transport and transformation of phosphorus (P) in sediments were comparatively investigated using incubation experiments to determine the effect of ZMBT application mode on the controlling efficiency. Results showed that the release of soluble reactive P (SRP) from sediment to the overlying water was effectively intercepted by all the ZMBT treatments. The inactivation of pore-water SRP, diffusive gradients in thin films-labile P (DGT-LP) and mobile P (Mob-IP) in sediment played a pivotal role in the regulation of SRP liberation from the sediment to the overlying water by ZMBT. An application mode change from capping and addition/capping to addition resulted in a decline of the reduction efficiency of overlying water SRP by the ZMBT treatment to some extent. The variation in the reduction efficiency of pore-water SRP and DGT-LP in the uppermost sediment were responsible for the change of the reduction efficiency of overlying water SRP by the ZMBT treatment. A change in application mode from capping to addition/capping and addition caused an obvious increase in the immobilization efficiency of pore-water SRP, DGT-LP and Mob-IP in the lower sediment by the ZMBT treatment. Results of this work indicate that, when the ZMBT capping layer on the top of sediment was completely mixed with the sediment, although the stability of P in the lower sediment obviously increases, the controlling efficiency of SRP liberating from the sediment to the overlying water decreases to some extent. Thus, the repeated addition of ZMBT to form a covering layer on the ZMBT-amended sediment is very necessary for the effective control of sediment-P release to the overlying water.

Fluorescence characteristics and biodegradability of dissolved organic matter (DOM) leached from non-point sources in southeastern China

Under the increasingly intensive measures for surface water restoration in China, point source discharge has been strictly regulated; however, for non-point sources, which constitute a large part of surface water pollutants, effective control has been difficult to reach. A comprehensive understanding of the characteristics of non-point source pollutants is essential for surface water improvement programs of cities such as Ningbo, on the southeast coast of China. Ningbo has made tremendous efforts in the past few years to control point source pollutants, but available data and management strategies on the non-point source pollutants are still limited. To this end, leachates of representative non-point source samples from the territory of Ningbo, including cropland and wetland soil, urban channel sediment, and poultry manure, were examined and compared focusing on the fluorescence characteristics and biodegradability of the dissolved organic matter (DOM). Results indicated that biodegradable dissolved organic carbon (BDOC) accounting for the total DOC was 46.7 ± 0.7% for cropland, wetland (56.3 ± 6.8%), non-sewage channel (60.1 ± 0.4%), sewage channel (74.5 ± 1.1%), and poultry manure (62.7 ± 4.5%). The leachates of the studied samples showed significant differences in both the amount and composition of DOM. However, a fluorescence component representing tryptophan-like substances identified by the excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis effectively predicted the BDOC variations among the studied samples. Moreover, under the studied nutrient concentrations, which were equivalent to Grade III water quality in China, nutrient limitation of microbial degradation was not observed. Threats to water quality, especially excessive consumption of dissolved oxygen, could be posed by the non-point source leachates due to their high bioavailability, large distribution, and weak nutrient restraint. Further investigations, including a quantitative evaluation of the non-point source pollution contribution, and pollutant blocking techniques are required.

Isotopic biomonitoring of N pollution in rivers embedded in complex human landscapes

The dynamic and hierarchical structure of rivers, together with disruption of the natural river continuum by human activities, makes it difficult to identify and locate sources of nutrient pollution affecting receiving waters and observe its dispersion, thus impairing monitoring efforts. The identification of reliable indicators of anthropogenic nitrogen inputs in catchments is therefore key to achieving effective management of polluted rivers. We tested the capacity of N isotopic signatures (δ¹⁵N) of epilithon and snails to provide useful indications of organic and inorganic anthropogenic N inputs in three Mediterranean rivers differing in terms of surrounding land use and physicochemical conditions. We used a combined approach based on (i) analysis of nutrient concentrations in water, (ii) CORINE land cover classification and drainage patterns in catchments and (iii) isotopic analysis of river biota to verify whether isotopic variations were indicative of anthropic activities in the watershed, the associated alteration of water quality, and the consequent impact on snail abundance and diversity. Variation in the δ¹⁵N of epilithon within and between rivers reflected localised and diffuse N inputs from inorganic and organic sources. Negative epilithon δ¹⁵N values (<0‰) indicated inorganic pollution from agriculture. Values between 4‰ and 8‰ and those above 8‰ respectively indicated moderate organic pollution from urban areas, and high organic pollution, mostly from waste waters. The diversity and abundance of snails decreased with increasing water pollution. While their isotopic variations reflected between-river differences, they failed to indicate within-river variations in anthropogenic N inputs, since the proportion of epilithon in their diet varied along the rivers. Concluding, epilithon was a reliable indicator of anthropogenic N sources across a wide range of nutrient concentrations and anthropogenic inputs, and the proposed approach allowed us to determine the nature of nitrogen pollutants, their sources, location and dispersion along rivers embedded in complex human landscapes.

Macroinvertebrate functional feeding group alterations in response to habitat degradation of headwater Austral streams

Protecting the structural and functional integrity of lotic ecosystems is becoming increasingly important as many ecological systems face escalating pressures from human population growth and environmental impacts. Knowledge on the functional composition of macroinvertebrates in austral temperate streams is generally lacking hindering the design and implementation of water management and restoration goals. Therefore, this study examined the effects of urban land-use activities on the benthic macroinvertebrate functional feeding guild structure among different stream orders in an austral river system (Bloukrans River) in the Eastern Cape Province of South Africa. Water quality and macroinvertebrate community data were collected across two seasons from 18 sites in two different stream order categories (i.e. 1, 2 + 3), following standard methods. We separated macroinvertebrates into functional feeding groups (FFGs), which we then used to assess the effects of riparian condition on FFG organization. Linear mixed effects model (LMM) results demonstrated that total dissolved solids (TDS), canopy cover, phosphate and channel width were the key variables that described the major sources of variation in macroinvertebrate FFGs. Based on FFG proportions, collector-gatherers were the most abundant in the Bloukrans River and represented 71.3% of the macroinvertebrate assemblages. The FFG ratios indicated that all the eighteen sites were strongly heterotrophic (i.e. streams received additional sources of energy from leaf litter and other organic matter), showed below expected linkage with riparian input and stable substrates were limited. The FFG ratios offered some insights into the overall functioning of Bloukrans River system. Our results highlight the importance of including macroinvertebrate functional diversity as a complementary approach to assess the ecological integrity in management and restoration plans of river systems.