This paper attempts to understand and explore the problem of eutrophication in the context of agriculture with the help of a nexus perspective. Eutrophication is significantly linked to water and energy resources with theoretically well-defined trade-offs and threshold levels. While looking at the linkages between water and land resources comprehensively, our paper questions the present approach to designing and implementing watershed management, and analyses the effects of agricultural intensification, especially in dry regions. Eutrophication is the process by which excessive nutrient loads in water bodies lead to undesirable water-quality problems and the degradation of the overall aquatic ecosystem. Due to limited information and knowledge on water and soil quality in most countries, farmers continue to use fertilizers at an increasing rate and agricultural run-off has been carrying ever more nitrogen and phosphorus into water bodies. This is likely to become a vicious cycle of eutrophication affecting food and water security. Of late, soil- and water-conservation interventions, like watershed development, are further reducing run-off. It is argued that there is a need to rethink the assumptions under which watershed interventions are designed and implemented.

We make the case that phosphorus (P) is inextricably linked to an increasingly fragile, interconnected, and interdependent nexus of water, energy, and food security and should be managed accordingly. Although there are many other drivers that influence water, energy, and food security, P plays a unique and under-recognized role within the nexus. The P paradox derives from fundamental challenges in meeting water, energy, and food security for a growing global population. We face simultaneous dilemmas of overcoming scarcity of P to sustain terrestrial food and biofuel production and addressing overabundance of P entering aquatic systems, which impairs water quality and aquatic ecosystems and threatens water security. Historical success in redistributing rock phosphate as fertilizer to enable modern feed and food production systems is a grand societal achievement in overcoming inequality. However, using the United States as the main example, we demonstrate how successes in redistribution of P and reorganization of farming systems have broken local P cycles and have inadvertently created instability that threatens resilience within the nexus. Furthermore, recent expansion of the biofuels sector is placing further pressure on P distribution and availability. Despite these challenges, opportunities exist to intensify and expand food and biofuel production through recycling and better management of land and water resources. Ultimately, a strategic approach to sustainable P management can help address the P paradox, minimize tradeoffs, and catalyze synergies to improve resilience among components of the water, energy, and food security nexus.

Closing nutrient loops in terrestrial and aquatic ecosystems is integral to achieve resource security in the food-energy-water (FEW) nexus. We performed multiyear (2005–2008), monthly sampling of instream dissolved inorganic nutrient concentrations (NH₄–N, NO₃–N, soluble reactive phosphorus-SRP) along a ∼ 300-km arid-land river (Rio Grande, NM) and generated nutrient budgets to investigate how the net source/sink behavior of wastewater and irrigated agriculture can be holistically managed to improve water quality and close nutrient loops. Treated wastewater on average contributed over 90% of the instream dissolved inorganic nutrients (101 kg/day NH₄–N, 1097 kg/day NO₃–N, 656 kg/day SRP). During growing seasons, the irrigation network downstream of wastewater outfalls retained on average 37% of NO₃–N and 45% of SRP inputs, with maximum retention exceeding 60% and 80% of NO₃–N and SRP inputs, respectively. Accurate quantification of NH₄–N retention was hindered by low loading and high variability. Nutrient retention in the irrigation network and instream processes together limited downstream export during growing seasons, with total retention of 33–99% of NO₃–N inputs and 45–99% of SRP inputs. From our synoptic analysis, we identify trade-offs associated with wastewater reuse for agriculture within the scope of the FEW nexus and propose strategies for closing nutrient loops in arid-land rivers.

Aquatic ecosystems are used for extensive rice-shrimp culture where the available water alternates seasonally between fresh and saline. Poor water quality has been implicated as a risk factor for shrimp survival; however, links between shrimp, water quality and their main food source, the natural aquatic biota inhabiting these ponds, are less well understood. We examined the aquatic biota and water quality of three ponds over an entire year in the Mekong Delta, Vietnam, where the growing season for the marine shrimp Penaeus monodon has been extended into the wet season, when waters freshen. The survival (30–41%) and total areal biomass (350–531 kg ha⁻¹) of shrimp was constrained by poor water quality, with water temperatures, salinity and dissolved oxygen concentrations falling outside known optimal ranges for several weeks. Declines in dissolved oxygen concentration were matched by declines in both shrimp growth rates and lipid content, the latter being indicative of nutritional condition. Furthermore, as the dry season transitioned into the wet, shifts in the taxonomic composition of phytoplankton and zooplankton were accompanied by declines in the biomass of benthic algae, an important basal food source in these systems. Densities of the benthic invertebrates directly consumed by shrimp also varied substantially throughout the year. Overall, our findings suggest that the survival, condition and growth of shrimp in extensive rice-shrimp ecosystems will be constrained when poor water quality and alternating high and low salinity negatively affect the physiology, growth and composition of the natural aquatic biota. Changes in management practices, such as restricting shrimp inhabiting ponds to the dry season, may help to address these issues and improve the sustainable productivity and overall condition of these important aquatic ecosystems.

As an illustration of the usefulness of the concept of water-agro-food systems, this paper describes the mechanisms behind the nitrogen cascade from agricultural soils to the coastal sea rivers through aquifers, riparian wetlands, rivers and streams and the estuary, for the case study of the Seine river watershed and its receiving coastal sea. It is stressed that the structure of the agro-food system, i.e. the way food production and trade, as well as consumption and waste management are organized, is the main determinant of the quality of ground- and surface water as well as of aquatic ecosystem functioning.

The Coastal California basin aquifer system (CCB), USA.Understanding the controls on nonpoint source (NPS) nitrate (NO3−) contamination in groundwater is motivated by the widespread detection of NO3−, implications for human health and aquatic ecosystems, groundwater sustainability, and a growing realization that such understanding across spatial scales promotes management and policy choices that optimize the Water–Energy–Food (WEF) Nexus. In the CCB, the conflicts in the WEF Nexus are apparent because of intensive agricultural practices that have contributed to chronic NO3− loading to groundwater. We evaluate the scale-dependent relations among source, transport, and attenuation (STA) factors that control NPS NO3− contamination in recently recharged CCB groundwater. Logistic regression models are presented for sub-regional and regional model domains.Dissolved oxygen (DO) (attenuation) in groundwater is often the most significant STA factor in all model domains, indicating that DO is an important, scale-invariant factor controlling NPS NO3− contamination. Farm fertilizer (source) is also a significant scale-invariant factor, while many of the transport factors are scale-dependent factors. We present vulnerability maps that illustrate the spatial patterns of predicted probability of detected elevated NO3−. Findings here improve knowledge about the scale dependence of STA factors, which help decision makers develop best management strategies and policies that advances groundwater sustainability and optimizes the WEF Nexus.

Global aquatic ecosystems are essential to human existence and have deteriorated seriously in recent years. Understanding the influence mechanism of habitat variation on the structure of the food-web allows the effective recovery of the health of degraded ecosystems. Whereas most previous studies focused on the selection of driving habitat factors, the impact of habitat variation on the food-web structure was rarely studied, resulting in the low success rate of ecosystem restoration projects globally. This paper presents a framework for exploring the effects of spatial variations in water quality and hydrological habitat factors on the food-web structure in city waters. Indices for the evaluation of the food-web structure are first determined by integrating model-parameter extraction via literature refinement. The key water quality and hydrological factors are then determined by coupling canonical correspondence analysis with partial least squares regression. Their spatial variation is investigated using spatial autocorrelation. Finally, fuzzy clustering is applied to analyze the influence of the spatial variations in water quality and hydrological factors on the food-web structure. The results obtained in Ji'nan, the pilot city of water ecological civilization in China, show that the Shannon diversity index, connectance index, omnivory index, and the ratio of total primary production to the total respiration are important indicators of food-web structural change. They show that the driving factors affecting the aquatic food-web structure in Ji'nan are hydrological factors (e.g., river width, water depth, and stream flow), physical aspects of water quality (e.g., air temperature, water temperature, electrical conductivity, and transparency), and chemical aspects (e.g., potassium, dissolved oxygen, calcium, and total hardness). They also show that the stability of the food-web is more prone to spatial variations in water quality than in hydrological factors. Higher electrical conductivity, potassium, total hardness, and air temperature lead to deteriorated food-web structures, whereas better transparency improves structure and stability. We found that water and air temperature are the most important factors in the spatial variation of the food-web structure in the study area, followed by total hardness. Transparency is the least important factor. Large disparities and varied spatial distributions exist in the driving effects of water quality and hydrological factors across regions attributable to differences in geographical environments, water salinity (fresh vs. sea water), and environmental factors (e.g., water pollution). The above methods and results serve as a theoretical and scientific basis for a high success rate of aquatic ecosystem restoration projects in the study area and other cities worldwide.

In a context of future population increase and intensification of water cycle by climate change, water demand for irrigation is projected to double. However, freshwater resources have been degraded the last decades especially in rivers via fragmentation, dam contraction and pollution. Flow alteration and degradation lead to 80% of freshwater ecosystem species loss. In this thesis, a robust and reliable Environmental Flow (EF) method was developed for global scale: the Variable Monthly Flow (VMF) method. This method allowed estimating EF deficit at global scale including its origin, timing, frequency and magnitude. By setting EFRs as priority user in a global vegetation and hydrological model (LPJmL), irrigation loss due to EFRs implementation were assessed at 30% leading to 5% global calorie loss. To maintain water allocation to humans and ecosystems under global change, food imports would require to increase by 15% especially from Latin America to South of Asia.

In a context of future population increase and intensification of water cycle by climate change, water demand for irrigation is projected to double. However, freshwater resources have been degraded the last decades especially in rivers via fragmentation, dam contraction and pollution. Flow alteration and degradation lead to 80% of freshwater ecosystem species loss. In this thesis, a robust and reliable Environmental Flow (EF) method was developed for global scale: the Variable Monthly Flow (VMF) method. This method allowed estimating EF deficit at global scale including its origin, timing, frequency and magnitude. By setting EFRs as priority user in a global vegetation and hydrological model (LPJmL), irrigation loss due to EFRs implementation were assessed at 30% leading to 5% global calorie loss. To maintain water allocation to humans and ecosystems under global change, food imports would require to increase by 15% especially from Latin America to South of Asia.

The Three Gorges Reservoir (TGR) of China, the largest hydropower project over the world, has attracted much attention to the water impoundment and water-level manipulation. In this study, we evaluated potential effects of water impoundment and seasonal water-level manipulation on the bioaccumulation, trophic transfer and health risk of HMs (Cu, Fe, Zn, Hg, Cd and Pb) in food web components (seston, aquatic invertebrate and fish) in TGR. Our results show that, after the impoundment for eight years (2003–2010), all of the six metal concentrations in aquatic biota fell within the criteria of safety quality guidelines. The concentrations of Cu, Fe, Zn and Hg in fish and aquatic invertebrates were higher than those before impoundment, whereas Cd and Pb were lower than those before impoundment. Nonetheless, Hg, Cd and Pb in aquatic consumers underwent an increasing trend during the entire impoundment, implying potential reservoir effect in the future. Only the concentrations of Hg, Cd and Pb in aquatic consumers exhibited a declining trend towards the dam, showing consistent with the background level at the three reaches. Seasonal variations in HM concentrations of fish and aquatic invertebrates were ascribed to the water-level manipulation associated with reservoir management. Our findings show that Hg or Cd biomagnified through aquatic food web during different hydrological periods, whereas Pb, Cu, Fe and Zn exhibited weak biomagnification power. Overall, Hg, Cd and Pb showed a higher risk than that of Cu, Fe and Zn.