Studies of microplastics (MPs) in remote, trans-boundary and alpine rivers are currently lacking. To understand the sinks and transport mechanisms of MPs, this study investigated the distributions and sources of MPs in the surface waters and sediments of five tributaries of the Koshi River (KR), a typical alpine river in the Himalayas between China and Nepal. Mean abundances of MPs in water and sediment were 202 ± 100 items/m³ and 58 ± 27 items/kg, dry weight, respectively. The upstream tributary, Pum Qu in China, had the smallest abundance of MPs, while the middle tributary, Sun Koshi in Nepal, had the greatest abundance. Compared to international values in rivers, contamination of the KR with MPs was low to moderate. Fibers represented 98% of all MP particles observed, which consisted of polyethylene, polyethyleneterephthalate, polyamide, polypropylene, and polystyrene. Blue and black MPs were prevalent, and small MPs (<1 mm) accounted for approximately 60% of all MPs. Atmospheric transmission and deposition were considered to be the principal sources of MPs in the upstream tributary. The results imply that point sources associated with mostly untreated sewage effluents and solid wastes from households, major settlements, towns, and cities were most important sources of MPs in the KR. Non-point sources from agricultural runoff and atmospheric transport and deposition in the middle stream tributaries also contribute a part of microplastics, while the least amount was from fishing in the downstream tributary. Urbanization, agriculture, traffic, and tourism contributed to pollution in the KR by MPs. Equations to predict abundances of MPs based on river altitudes revealed that different trends were affected by both natural and human factors within the KR basin. This study presents new insights into the magnitude of MP pollution of a remote alpine river and provides valuable data for developing MP monitoring and mitigation strategies in similar environments worldwide.

Freshwater environments are susceptible to possible contamination by residual antibiotics that are released through different sources, such as agricultural runoffs, sewage discharges and leaching from nearby farms. Freshwater environment can thus become reservoirs where an antibiotic impact microorganisms, and is an important public health concern. Degradation and dilution processes are fundamental for predicting the actual risk of antibiotic resistance dissemination from freshwater reservoirs. This study reviews major approaches for detecting and quantifying antibiotic resistance bacteria (ARBs) and genes (ARGs) in freshwater and their prevalence in these environments. Finally, the role of dilution, degradation, transmission and the persistence and fate of ARB/ARG in these environments are also reviewed. Culture-based single strain approaches and molecular techniques that include polymerase chain reaction (PCR), quantitative polymerase chain reaction (qPCR) and metagenomics are techniques for quantifying ARB and ARGs in freshwater environments. The level of ARBs is extremely high in most of the river systems (up to 98% of the total detected bacteria), followed by lakes (up to 77% of the total detected bacteria), compared to dam, pond, and spring (<1%). Of most concern is the occurrence of extended-spectrum β-lactamase producing Enterobacteriaceae, methicillin resistant Staphylococcus aureus (MRSA) and vancomycin resistant Enterococcus (VRE), which cause highly epidemic infections. Dilution and natural degradation do not completely eradicate ARBs and ARGs in the freshwater environment. Even if the ARBs in freshwater are effectively inactivated by sunlight, their ARG-containing DNA can still be intact and capable of transferring resistance to non-resistant strains. Antibiotic resistance persists and is preserved in freshwater bodies polluted with high concentrations of antibiotics. Direct transmission of indigenous freshwater ARBs to humans as well as their transitory insertion in the microbiota can occur. These findings are disturbing especially for people that rely on freshwater resources for drinking, crop irrigation, and food in form of fish.

With increasing demand for recycled wastewater for irrigation purposes, there is a need to evaluate the potential for manufactured nanomaterials in waste water to impact crop production and agroecosystems. Copper oxide nanoparticles (CuO NPs) have previously been shown to negatively impact the growth of duckweed (Landoltia punctata) a model aquatic plant consumed by water fowl and widely found in agricultural runoff ditches in temperate climates. However, prior studies involving CuO NP toxicity to duckweed have focused on systems without the presence of dissolved organic matter (DOM). In the current study, duckweed growth inhibition was shown to be a function of aqueous Cu²⁺ concentration. Growth inhibition was greatest from aqueous CuCl₂ and, for particles, increased with decreasing CuO particle size. The dissolution of CuO NPs in ½ Hoagland's solution was measured to increase with decreasing particle size and in the presence of Suwannee river humic and fulvic acids (HA; FA). However, the current results suggest that HA, and to a lesser extent, FA, decrease the toxicity of both CuO NPs and free ionized Cu to duckweed, likely by inhibiting Cu availability through Cu-DOM complex formation. Such results are consistent with changes to Cu speciation as predicted by speciation modeling software and suggest that DOM changes Cu speciation and therefore toxicity in natural systems.

DNA-derived measurements of biological composition have the potential to produce data covering all of life, and provide a tantalizing proposition for researchers and managers. We used metabarcoding to compare benthic eukaryote composition from five estuaries of varying condition. In contrast to traditional studies, we found biotic richness was greatest in the most disturbed estuary, with this being due to the large volume of extraneous material (i.e. run-off from aquaculture, agriculture and other catchment activities) being deposited in the system. In addition, we found strong correlations between composition and a number of environmental variables, including nutrients, pH and turbidity. A wide range of taxa responded to these environmental gradients, providing new insights into their sensitivities to natural and anthropogenic stressors. Metabarcoding has the capacity to bolster current monitoring techniques, enabling the decisions regarding ecological condition to be based on a more holistic view of biodiversity.

The Soil and Water Assessment Tool (SWAT) was used to assess the impact of climate change on sediment, nitrate, phosphorus and pesticide (diazinon and chlorpyrifos) runoff in the San Joaquin watershed in California. This study used modeling techniques that include variations of CO2, temperature, and precipitation to quantify these responses. Precipitation had a greater impact on agricultural runoff compared to changes in either CO2 concentration or temperature. Increase of precipitation by ±10% and ±20% generally changed agricultural runoff proportionally. Solely increasing CO2 concentration resulted in an increase in nitrate, phosphorus, and chlorpyrifos yield by 4.2, 7.8, and 6.4%, respectively, and a decrease in sediment and diazinon yield by 6.3 and 5.3%, respectively, in comparison to the present-day reference scenario. Only increasing temperature reduced yields of all agricultural runoff components. The results suggest that agricultural runoff in the San Joaquin watershed is sensitive to precipitation, temperature, and CO2 concentration changes. Agricultural runoff is significantly affected by changes in precipitation, temperature, and atmospheric CO2 concentration.

Evidence of ecological impacts from pesticide runoff has prompted installation of vegetated treatment systems (VTS) along the central coast of California, USA. During five surveys of two on-farm VTS ponds, 88% of inlet and outlet water samples were toxic to Ceriodaphnia dubia. Toxicity identification evaluations (TIEs) indicated water toxicity was caused by diazinon at VTS-1, and chlorpyrifos at VTS-2. Diazinon levels in VTS-1 were variable, but high pulse inflow concentrations were reduced through dilution. At VTS-2, chlorpyrifos concentrations averaged 52% lower at the VTS outlet than at the inlet. Water concentrations of most other pesticides averaged 20–90% lower at VTS outlets. All VTS sediment samples were toxic to amphipods (Hyalella azteca). Sediment TIEs indicated toxicity was caused by cypermethrin and lambda-cyhalothrin at VTS-1, and chlorpyrifos and permethrin at VTS-2. As with water, sediment concentrations were lower at VTS outlets, indicating substantial reductions in farm runoff pesticide concentrations. Toxicity identification evaluations identified key pesticides in agricultural runoff, and their concentrations were reduced by farmer-installed vegetated treatment systems.

The beneficial uptake of nutrients by wetland plants is countered to some extent by nutrient release back into the aquatic environment due to vegetative die-back. This current study examined whether Leersia oryzoides, a common wetland plant, exhibits luxury uptake of nutrients from simulated farm runoff. The study also tested whether with subsequent decomposition, these nutrients are released back into the water column. When exposed to elevated (>2 mg/L N and P) runoff, L. oryzoides assimilated significantly higher concentrations of nitrogen (p < 0.001) and phosphorus (p < 0.001) in above-ground biomass as compared to non-enriched treatments (<0.05 mg/L N and P). Subsequently, senescence of enriched above-ground biomass yielded significantly higher concentrations of phosphorus (2.19 ± 0.84 mg P/L). Using L. oryzoides as our model, this study demonstrates nitrogen and phosphorus sequestration during the growing season and release of phosphorus in the winter. Release of sequestered nutrients during plant senescence.

A pesticide runoff event was simulated on two 10 m x 50 m constructed wetlands (one non-vegetated, one vegetated) to evaluate the fate of methyl parathion (MeP) (Penncap-M). Water, sediment, and plant samples were collected at five sites downstream of the inflow for 120 d. Semi-permeable membrane devices (SPMDs) were deployed at each wetland outflow to determine exiting pesticide load. MeP was detected in water at all locations of the non-vegetated wetland (50 m), 30 min post-exposure. MeP was detected 20 m from the vegetated wetland inflow 30 min post-exposure, while after 10d it was detected only at 10 m. MeP was measured only in SPMDs deployed in non-vegetated wetland cells, suggesting detectable levels were not present near the vegetated wetland outflow. Furthermore, mass balance calculations indicated vegetated wetlands were more effective in reducing aqueous loadings of MeP introduced into the wetland systems. This demonstrates the importance of vegetation as sorption sites for pesticides in constructed wetlands.

Coral reefs are amongst the most biodiverse ecosystems on earth, but are significantly impacted by agricultural runoff. Despite herbicides being commonly detected in coastal waters, the possibility of herbicide accumulation in coral reef species has largely been overlooked. We investigate the accumulation of several herbicides in five species of coral reef invertebrates collected from ten sites along the Maputaland coast, South Africa. Multiple herbicide residues were detected in 95% of the samples, with total average concentrations across sites ranging between 25.2 ng g⁻¹ to 51.3 ng g⁻¹ dw. Acetochlor, alachlor and hexazinone were the predominant herbicides detected at all sites, with atrazine and simazine detected less frequently. Significant interactive effects were detected between sites nested in reef complex crossed with species, based on multiple and total herbicide concentrations. In general, multivariate herbicide concentrations varied significantly between species within and across most sites. Contrastingly, the concentrations of the different herbicides and that of total herbicide did not differ between conspecifics at most sites nested in their respective reef complexes. On average, highest total herbicide concentrations were measured in soft coral (Sarcophyton glaucum; 90.4 ± 60 ng g⁻¹ and Sinularia gravis; 42.7 ± 25 ng g⁻¹) and sponge (Theonela swinhoei; 39.0 ± 40 ng g⁻¹) species, while significantly lower concentrations were detected in hard corals (Echinopora hirsutissima; 10.5 ± 5.9 ng g⁻¹ and Acropora austera; 5.20 ± 4.5 ng g⁻¹) at most sites. Agricultural runoff entering the ocean via the uMfolozi-St Lucia Estuary and Maputo Bay are likely sources of herbicide contamination to coral reefs in the region. There is an urgent need to assess the long-term effects of herbicide exposure on coral reef communities.

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