Springs drive downstream nitrate export from artificially-drained agricultural headwater catchments

Deteriorating waterways: The effect of nitrate pollution on the development and physiology of the endangered southern bell frog (Litoria raniformis)

Nitrogen-based fertilizers can increase agricultural yields and crop quality, but this comes at the risk of contaminating nearby waterways. Nitrate is the most stable and abundant form of inorganic nitrogen in the environment and chronic exposure can impair performance and fitness in aquatically respiring species. But it remains unknown if these impairments are linked to disruptions in energy homeostasis. Here, we investigated the energetic cost of living in nitrate contaminated waters during early, energy-limited, larval life stages in the endangered southern bell frog (Litoria raniformis). We hypothesised that chronic nitrate exposure during development would increase energetic costs, evidenced by reductions in growth rates and body sizes, and elevations in routine heart rate (RHR) and routine metabolic rate (RMR). Following hatching, larvae were exposed to one of three nitrate treatments (0, 25 and 50 mg NO3-L-1) for 12 weeks, and survival, growth, RHR, and RMR were measured. Survival rates were similar across all treatments. Nitrate exposure caused a reduction in growth rates, resulting in larvae with significantly smaller body sizes. Compared to controls, nitrate-exposed larvae were 12% and 18% smaller in total length in the 25 and 50 mgNO3-L-1 treatments, respectively. However, RHR and RMR were independent of nitrate exposure, indicating that the 'cost of living' was similar across treatments. Observed growth reductions were therefore independent of RHR and RMR, suggesting other mechanisms were involved. Taken together, these results highlight the vulnerability of L. raniformis to nitrate during early life and suggest that the application of nitrogen-based fertilizers near critical aquatic habitats will be harmful.

Differential sensitivity of offspring from four species of goodeine freshwater fish to acute exposure to nitrates

Nitrate-nitrogen (NO3-N) pollution related to anthropogenic activities is increasing in freshwater ecosystems. Knowledge about NO3-N sensitivity in freshwater wild fish is needed to understand the differential tolerance between species. Goodeinae is a subfamily of 41 endemic fishes that inhabit central Mexico, with 33 species in the IUCN red list and three extinct. Distributional patterns suggest tolerant and sensitive goodeines related to the conservation gradient of freshwater ecosystems. Four species with a differential distribution and tolerance were selected to evaluate their physiological responses to NO3-N. Fish were exposed to different NO3-N concentrations for 96 h and the median lethal concentration (LC50) was determined. Swimming disorders plus gill and liver histopathological indexes were estimated and incorporated into an Integrated Biomarker Response (IBR) for each species. Skiffia lermae (LC50 = 474.332 mg/L) and Xenotoca variata (LC50 = 520.273 mg/L) were more sensitive than Goodea atripinnis (LC50 = 953.049 mg/L) and Alloophorus robustus (LC50 = 1537.13 mg/L). The typical histological damage produced by NaNO3-N exposure was fusion of secondary lamellae in gills. This was present in all species and cellular degeneration was observed at the highest concentrations. Secondary lamellae aneurysms were only observed in G. atripinnis. Liver alterations included vascular dilation in hepatic sinusoids, hyperemia and nuclear hypertrophy; higher concentrations produced hepatocyte cytoplasmic vacuolation and reduced frequency of cell nuclei. Behavioral and histopathological alterations could explain the differential species sensitivity. The results suggest that species which preserve gill function and transfer the task of detoxification to the liver might have the best chance of surviving in polluted environments. Moreover, species previously considered as tolerant may be highly susceptible to NaNO3-N exposure. Therefore, it is necessary to closely monitor NaNO3-N concentrations in freshwater ecosystems and, if possible, reduce their levels to avoid the loss of wild populations.

Applications of soft computing techniques for prediction of pollutant removal by environmentally friendly adsorbents (case study: the nitrate adsorption on modified hydrochar)

Artificial intelligence has emerged as a powerful tool for solving real-world problems in various fields. This study investigates the simulation and prediction of nitrate adsorption from an aqueous solution using modified hydrochar prepared from sugarcane bagasse using an artificial neural network (ANN), support vector machine (SVR), and gene expression programming (GEP). Different parameters, such as the solution pH, adsorbent dosage, contact time, and initial nitrate concentration, were introduced to the models as input variables, and adsorption capacity was the predicted variable. The comparison of artificial intelligence models demonstrated that an ANN with a lower root mean square error (0.001) and higher R² (0.99) value can predict nitrate adsorption onto modified hydrochar of sugarcane bagasse better than other models. In addition, the contact time and initial nitrate concentration revealed a higher correlation between input variables with the adsorption capacity.

Better together: Cross-tolerance induced by warm acclimation and nitrate exposure improved the aerobic capacity and stress tolerance of common carp Cyprinus carpio

Climate warming is a threat of imminent concern that may exacerbate the impact of nitrate pollution on fish fitness. These stressors can individually affect the aerobic capacity and stress tolerance of fish. In combination, they may interact in unexpected ways where exposure to one stressor may heighten or reduce the resilience to another stressor and their interactive effects may not be uniform across species. Here, we examined how nitrate pollution under a warming scenario affects the aerobic scope (AS), and the hypoxia and heat stress susceptibility of a generally tolerant fish species, common carp Cyprinus carpio. We used a 3 × 2 factorial design, where fish were exposed to one of three ecologically relevant levels of nitrate (0, 50, or 200 mg NO₃^- L^-1) and one of two temperatures (18 °C or 26 °C) for 5 weeks. Warm acclimation increased the AS by 11% due to the maintained standard metabolic rate and increased maximum metabolic rate at higher temperature, and the AS improvement seemed greater at higher nitrate concentration. Warm-acclimated fish exposed to 200 mg NO₃^- L^-1 were less susceptible to acute hypoxia, and fish acclimated at higher temperature exhibited improved heat tolerance (critical thermal maxima, CTMax) by 5 °C. This cross-tolerance can be attributed to the hematological results including maintained haemoglobin and increased haematocrit levels that may have compensated for the initial surge in methaemoglobin at higher nitrate exposure.

Numerical Representation of Groundwater-Surface Water Exchange and the Effect on Streamflow Contribution Estimates

The effects of streams and drainage representation in 3D numerical catchment scale models on estimated streamflow contribution were investigated. MODFLOW-USG was used to represent complex geology and a stream network with two different conceptualizations—one with equal cell discretization in the entire model domain and another with refined cell discretization along stream reaches. Both models were calibrated against a large data set including hydraulic heads and streamflow measurements. Though the optimized hydraulic parameters and statistical performance of both model conceptualizations were comparable, their estimated streamflow contribution differed substantially. In the conceptualization with equal cell discretization, the drainage contribution to the streamflow was 13% compared to 41% in the conceptualization with refined cell discretization. The increase in drainage contribution to streamflow was attributed to the increase in drainage area in proximity to the stream reaches arising from the refined discretization. e.g., the cell refinement along stream reaches reduced the area occupied by stream cells allowing for increased drain area adjacent to the stream reaches. As such, an increase in drainage area equivalent to 7% yielded a 146% increase in drainage contribution to streamflow. In-stream field measurements of groundwater-surface water exchange fluxes that were qualitatively compared to calculated fluxes from the models indicated that estimates from the refined model discretization were more representative. Hence, the results of this study accentuate the importance of being able to represent stream and drain flow contribution correctly, that is, to achieve representative exchange fluxes that are crucial in simulating groundwater–surface water exchange of both flow and solute transport in catchment scale modeling. To that end, the in-stream measurements of exchange fluxes showed the potential to serve as a proxy to numerically estimate drainage contribution that is not readily available at the catchment scale.

Applications of a UV optical nitrate sensor in a surface water/groundwater quality field study

Examples of the utility of UV optical nitrate sensors are provided for two field applications, investigating nitrate pollution in a lowland, peri-urban catchment. In one application, rapid, in-stream longitudinal nitrate surveys were made in summer and winter, by fixing an optical nitrate sensor operating in continuous measurement mode to a kayak that was paddled along 10 km of the mainstem of the low-order stream in under 4 h. Nitrate concentrations ranged between 3.45 and 6.39 mg NO₃-N/L. Nitrate hot-spots and cool-spots were mapped and found to relate to point discharges from spring-fed tributaries and land drains. Effective nitrate removal (dN/dx = - 0.08 mg N/L/km), inferred to be from assimilation reactions, was evident in the summer dataset, but not the winter nitrate dataset. In a second application, the optical sensor was configured with appropriate technology to establish an autonomous and fully automated nitrate monitoring station. The station makes daily nitrate measurements of surface water, and groundwater, sampled from a cluster of four multi-level wells. Quarterly maintenance of the nitrate sensor has proven sufficient to keep measurement errors under 5%. Most nitrate variation has been recorded at or near the water table where concentrations have ranged between 3.47 and 5.88 mg NO₃-N/L, and annual maxima have occurred in late winter/spring, which coincides with when most nitrate leaching occurs from agricultural land. Seasonal nitrate patterns are not evident in groundwater sampled from 8-m depth, or deeper. High-frequency monitoring has revealed that some infra-season, short-term variability also occurs in shallow groundwater nitrate, driven by storm events, and which on occasion results in a temporary inversion of the groundwater nitrate-depth profile.

Groundwater-dependent ecosystems as transfer vectors of nitrogen from the aquifer to surface waters in agricultural basins: The fontanili of the Po Plain (Italy)

The most spread groundwater-dependent ecosystems in the River Po valley are semi-natural lowland springs called "fontanili". They provide specific habitats and support high biodiversity, but are often strongly impaired by agricultural pollution. In the present study we seasonally monitored the discharge and nitrogen concentration of 48 fontanili from the Adda and the Ticino river basins. We observed a wide spatial variability of both NO₃-N concentrations and flows. The annual NO₃-N loads ranged from <1 to 75 t y^-1 and < 1 to 29 t y^-1 in the Adda and Ticino basins respectively. In the springs characterized by variable discharge the N loads were exported mostly during the summer season when water table level was elevated mainly due to irrigation. Upscaling the mean NO₃-N load to each river catchment based on the total number of springs, we obtained an aerial export of 33.2 ± 6.0 and 12.5 ± 3.2 kg y^-1 ha^-1. Such loads accounted for the 30.4 and 21.5% of the N surplus estimated for the Adda and Ticino basins respectively. Random Forest analysis was performed to identify the most important environmental variables influencing the nitrate contamination in the spring waters. A total of 22 explanatory variables related to N sources, land uses, intrinsic hydrogeologic and soil proprieties, in "situ" and remotely sensed variables were considered. The percent of soil cultivated with maize in a 500 m radius buffer area surrounding the sampling site, the N from manure and the distance of each spring from the main river were the most effective factors in controlling the NO₃-N concentration in the fontanili water. The outcomes of this work open up to achievable management prospects for the protection and recovery of fontanili waters, and can be particularly useful for water managers in identifying areas and sites where restoration plans should be a priority.

Thermal acclimation offsets the negative effects of nitrate on aerobic scope and performance

Rising temperatures are set to imperil freshwater fishes as climate change ensues unless compensatory strategies are employed. However, the presence of additional stressors, such as elevated nitrate concentrations, may affect the efficacy of compensatory responses. Here, juvenile silver perch (Bidyanus bidyanus) were exposed to current-day summer temperatures (28°C) or a future climate-warming scenario (32°C) and simultaneously exposed to one of three ecologically relevant nitrate concentrations (0, 50 or 100 mg l-1). We measured indicators of fish performance (growth, swimming), aerobic scope (AS) and upper thermal tolerance (CTmax) to test the hypothesis that nitrate exposure would increase susceptibility to elevated temperatures and limit thermal compensatory responses. After 8 weeks of acclimation, the thermal sensitivity and plasticity of AS and swimming performance were tested at three test temperatures (28, 32, 36°C). The AS of 28°C-acclimated fish declined with increasing temperature, and the effect was more pronounced in nitrate-exposed individuals. In these fish, declines in AS corresponded with poorer swimming performance and a 0.8°C decrease in CTmax compared with unexposed fish. In contrast, acclimation to 32°C masked the effects of nitrate; fish acclimated to 32°C displayed a thermally insensitive phenotype whereby locomotor performance remained unchanged, AS was maintained and CTmax was increased by ∼1°C irrespective of nitrate treatment compared with fish acclimated to 28°C. However, growth was markedly reduced in 32°C-acclimated compared with 28°C-acclimated fish. Our results indicate that nitrate exposure increases the susceptibility of fish to acute high temperatures, but thermal compensation can override some of these potentially detrimental effects.

Combining Tools from Edge-of-Field to In-Stream to Attenuate Reactive Nitrogen along Small Agricultural Waterways

Reducing excessive reactive nitrogen (N) in agricultural waterways is a major challenge for freshwater managers and landowners. Effective solutions require the use of multiple and combined N attenuation tools, targeted along small ditches and streams. We present a visual framework to guide novel applications of ‘tool stacking’ that include edge-of-field and waterway-based options targeting N delivery pathways, timing, and impacts in the receiving environment (i.e., changes in concentration or load). Implementing tools at multiple locations and scales using a ‘toolbox’ approach will better leverage key hydrological and biogeochemical processes for N attenuation (e.g., water retention, infiltration and filtering, contact with organic soils and microbes, and denitrification), in addition to enhancing ecological benefits to waterways. Our framework applies primarily to temperate or warmer climates, since cold temperatures and freeze–thaw-related processes limit biologically mediated N attenuation in cold climates. Moreover, we encourage scientists and managers to codevelop N attenuation toolboxes with farmers, since implementation will require tailored fits to local hydrological, social, and productive landscapes. Generating further knowledge around N attenuation tool stacking in different climates and landscape contexts will advance management actions to attenuate agricultural catchment N. Understanding how different tools can be best combined to target key contaminant transport pathways and create activated zones of attenuation along and within small agricultural waterways will be essential.

Water-quality issues facing dairy farming: potential natural and built attenuation of nitrate losses in sensitive agricultural catchments

Context Dairy farming will be increasingly scrutinised for its environmental impacts, in particular for its impacts on freshwater quality in New Zealand and elsewhere. Management and mitigation of high nitrate losses is one of the greatest water-quality challenges facing dairy farming in New Zealand and other countries. Management of critical flow pathways and nitrate-attenuation capacity could offer potential solutions to this problem and help maintain dairy-farming productivity, while reducing its water-quality impacts. Aims The present paper reviewed the key water-quality issues faced by dairy farming and assessed potential of emerging edge-of-paddock technologies, and catchment-scale nutrient-attenuation practices, to reduce nitrate losses from dairy farming to receiving water bodies. Methods We developed a conceptual catchment-scale modelling analysis assessing potential natural and built attenuation of nitrate losses from dairy farming in the Tararua and Rangitikei catchments (located in the lower part of the North Island, New Zealand). Key results This exploratory analysis suggests that a reduction of greater than 25% in the river nitrate loads from dairy-farming areas could potentially be achieved by spatially aligning dairy land with areas of high subsurface nitrate-attenuation capacity, and by managing critical flow pathways using innovative edge-of-field technologies such as controlled drainage, drainage-water harvesting for supplemental irrigation, woodchip bioreactors, and constructed wetlands in the study catchments. Conclusions The research findings highlighted the potential to better understand, map and effectively utilise existing natural and new built-in nitrate-attenuation capacity to significantly reduce water-quality impacts from dairy farming across environmentally sensitive agricultural catchments. This knowledge and tools could help farmers close the gap between what can be achieved with current, in-field mitigation practises and the nitrogen-loss allocation imposed by regulatory authorities. Implications However, the research findings presented here are based on a coarse-scale, conceptual modelling analysis, and therefore further research is recommended to develop tools and practices to better understand, map and effectively utilise existing natural and new built-in nitrogen attenuation capacity at farm-scale to achieve productive and environmentally friendly pastoral dairy farming across agricultural landscapes.