Assessing seasonal nitrogen export to large tropical lakes

Rivers are exporting increasing amounts of nitrogen (N) to lakes, which is leading to eutrophication. However, the seasonality apparent in nutrient loading, especially in tropical areas, is thus far only partially understood. This study aims to better understand the seasonality and the sources of dissolved inorganic nitrogen (DIN) inputs from sub-basins to tropical lakes. We integrated existing approaches into a seasonal model that accounts for seasonality in human activities, meteorology and hydrology, and we applied the model to the sub-basins of a representative tropical lake: Lake Tana, Ethiopia. The model quantifies the river export of DIN by season, source and sub-basin and also accounts for open defecation to land as a diffuse source of N in rivers. Seasonality parameters were calibrated, and model outputs were validated against measured nitrogen loads in the main river outlets. The calibrated model showed good agreement with the measured nitrogen loads at the outflow of the main rivers. The model distinguishes four seasons: rainy (July–September), post-rainy (October–December), dry (January–March) and pre-rainy (April–June). The river export of DIN to Lake Tana was about 9 kton in 2017 and showed spatial and temporal variability: It was highest in the rainy and lowest in the dry seasons. Diffuse sources from agriculture were important contributors of DIN to rivers in 2017, and animal manure was the dominant source in all seasons. Our seasonal sub-basins and rivers model provides opportunities to identify the main nutrient sources to the lake and to formulate effective water quality management options. An example is nutrient application level that correspond to the crop needs in the sub-basins. Furthermore, our model can be used to analyse future trends and serves as an example for other large tropical lakes experiencing eutrophication.</p

Water pollution from food production: lessons for optimistic and optimal solutions

Food production is a source of various pollutants in aquatic systems. For example, nutrients are lost from fertilized fields, and pathogens from livestock production. Water pollution may impact society and nature. Large-scale water pollution assessments, however, often focus on single pollutants and not on multiple pollutants simultaneously. This study draws lessons from air pollution control for large-scale water quality assessments, where multi-pollutant approaches are more common. To this end, we present a framework for future water pollution assessments searching for optimistic and optimal solutions. We argue that future studies could shift their focus to better account for societal and economic targets. Participatory approaches can help to ensure the feasibility of future solutions to reduce water pollution from food production

Assessing eutrophication indicators in lake basins for water quality management

Eutrophication in freshwater lakes is a global water quality issue. The cumulative concentration of nutrients such as nitrogen (N), phosphorus (P), or aggregated water quality indices (e.g., trophic state index) are frequently used metrics to monitor lake water quality. Such monitoring fails to identify the key causes of eutrophication. As a result, management and policy-making are not sufficiently informed, while it is necessary to understand the effects of the changing climate and socioeconomic development. It is therefore critical to develop and promote indicators for water quality monitoring that include sources of nutrient emissions and their pathways into the lake basin. In this study, we present indicators of drivers and pressures that account for anthropogenic emissions, socio-economic variables, and land-use. To assess the indicators, we implement an integrated modeling framework that consists of GLOBIOM-CWatM-MARINA-Lakes, which combine land use, hydrology and anthropogenic nutrient emissions to lakes. The Lake Victoria basin in Africa will be used as a case study to assess the drivers and pressures and develop a better understanding of the impacts of socio-economy and sources of nutrient emissions on lake water quality. Such drivers and pressures of a lake basin can be used as proxies, particularly in data scarce regions, to fill the gap in water quality monitoring data, and to assist in design of nutrient management policies and plans

Cost-effective management of coastal eutrophication: A case study for the yangtze river basin

Many water resources are threatened with nutrient pollution worldwide. This holds for rivers exporting increasing amounts of nutrients from the intensification of food production systems and further urbanization. This riverine nutrient transport causes coastal eutrophication. This study aims to identify cost-effective management options to simultaneously reach environmental targets for river export of nitrogen and phosphorus by the Yangtze River (China) in 2050. A newly developed modelling approach is used that integrates the Model to Assess River Inputs of Nutrients to seAs (MARINA) with a cost-optimization procedure, and accounts for socio-economic developments, land use and climate changes in a spatially explicit way. The environmental targets for river export of nutrients aim to reduce the gap between baseline and desirable nutrient export. Our baseline is based on MARINA projections for future river export of nutrients, while the desirable nutrient export reflects a low eutrophication potential. Results show the possibilities to close the gap in river export of both nutrients by 80–90% at a cost of 1–3 billion $ per year in 2050. Recycling of animal waste on cropland is an important cost-effective option; reducing synthetic fertilizer inputs provides an opportunity to compensate for the additional costs of the recycling and treatment of manure and wastewater. Our study provides new insights on the combination of cost-effective management options for sub-basins of the Yangtze. This can support the design of cost-effective and sub-basin specific management options for reducing future water pollution.Publishe

Sources and export of nutrients in the Zambezi River basin: status and future trend

In the past decades, nutrient enrichment in African water bodies has been frequently reported and associated to long-term ecological and socio-economic consequences, such as species extinction, unsafe drinking water and compromised local livelihood. Meanwhile, rapid population growth and land-use change towards intensified food production are projected in Africa. As a result, substantial increases are expected in human-induced nutrient inputs (e.g. human waste and fertilizer) to the terrestrial and aquatic environments. This may potentially further deteriorate African water bodies. As part of the Integrated Solution for Water, Land and Energy (IS-WEL) project, this study aims to assess the status and projected changes of nutrient sources, inputs to rivers and export to seas, shading light on possible solutions to minimize further nutrient-induced deterioration of the water bodies and maximize the availability of water of suitable quality for different sectors. This study focuses on the Zambezi river basin, the fourth largest transboundary basin draining through eight southern African countries. Nutrient sources, inputs to rivers and export to sea are estimated using the MARINA model (Model to Assess River Inputs of Nutrients to seAs) under current conditions and future climate, land use and socio-economic projections up to 2050. Results show that for the current period (2005-2010), inputs of nutrients (nitrogen and phosphorus) to rivers and their export to sea are mainly attributed to natural sources. These sources include nitrogen fixation by the natural ecosystems, phosphorus weathering, and leaching of organic nitrogen and phosphorus from non-agricultural areas. By 2050, nutrient sources will be at least doubled due to anthropogenic inputs in the basin. Consequently, the fraction of human-induced nutrient export are projected to increase considerably, especially for dissolved inorganic phosphorus from domestic wastewater. Additionally, nutrient export to sea is strongly influenced by the intra- and inter-annual precipitation and discharge variabilities in the region. The study highlights the need to simultaneously consider source control, infrastructure development and climate adaptation to minimize further nutrient-induced deterioration of water bodies

Comprehensive indicators for eutrophication in lakes

Anthropogenic influences such as urbanization, agricultural intensification, use of chemical fertilizers,and the release of sewage emit nutrients (nitrogen and phosphorus) into freshwater lakes. Excessiveamounts of nitrogen and phosphorus lead to eutrophication, which is a global water quality concern,with typical impacts such as the growth of harmful algal blooms, hypoxia and fish kills. The currentwater quality indicators used to monitor lake eutrophication focus only on water quality constituents(e.g., total phosphorus and total nitrogen). The response of lakesto nutrient inputs, however, dependson its interaction with climate, basin characteristics that include land use, hydrology andanthropogenic activities. In this study, we present a set of comprehensive indicators that account fornutrient sources and biogeochemical pathways in the lake basins and demonstrate their interactionsusing a causal network. The causal network provides a holistic perspective of nutrient dynamics andthe interactive effects of the indicators on water quality in lake basins, which is key to improving waterquality management. The indicators from this study can be used as proxies to monitor water qualitystatus. The comprehensive indicators improve mechanistic understanding of the lake nutrientdynamics, by systematically considering mechanisms of sources and factors of nutrients emission inthe basin. They can consequently fill the gap in water quality monitoring data, especially in data scarceregions such as Asia and Africa. These indicators can be used to set realistic water quality targets andare therefore beneficial in long-term policy making and sustainable water quality management

Increasing future human-induced nitrogen exports to rivers and sea in the Zambezi river basin

In the past decades, nutrient enrichment in African water bodies has been frequently reported and lead to water security challenges, such as unsafe drinking water and compromised local livelihood. Meanwhile, rapid population growth and land-use change towards intensified food production are projected in Africa. Substantial increases in anthropogenic nutrient inputs (e.g. human waste and fertilizer) to the terrestrial and aquatic environments are therefore expected. This may further deteriorate African water bodies and threaten water security. As part of the Integrated Solution for Water, Land and Energy (IS-WEL) project funded by Global Environmental Facility and IIASA, this study aims to assess the status and projected changes of nitrogen (N) sources, associated inputs to rivers and export to sea. The study focuses on the Zambezi river basin, the 4th largest transboundary basin in Africa draining through 8 countries. N inputs to rivers and export to sea are estimated using the MARINA model (Model to Assess River Inputs of Nutrients to seAs) under current conditions and future climate, land use and socio-economic scenarios towards 2050. In addition to global climate (Representative Concentration Pathways) and socioeconomic (Shared Socioeconomic Pathways) scenarios, the study co-developed regional scenarios through stakeholder engagement in cooperation with the Zambezi Watercourse Commission. The regional scenarios, especially for sanitation and agricultural development, will later be incorporated in the model. Preliminary results show that for the current period (2010), N input to rivers and export to sea are mainly from natural sources, namely N fixation by the natural ecosystems and organic N leaching from non-agricultural areas. By 2050, N sources in the basin will be more than doubled due to anthropogenic inputs based on the global scenarios. Consequently, the fraction of human-induced N export is projected to increase considerably. Additionally, N export to sea is strongly influenced by the intra- and inter-annual climate variabilities in the region. The study highlights the need to simultaneously consider source control and climate adaptation in the regional contexts to minimize further N-induced deterioration of water bodies and ensure regional water security

Recent advancement in water quality indicators for eutrophication in global freshwater lakes

Excessive nutrient (nitrogen and phosphorus) loadings to freshwater lakes cause eutrophication, which is a global water quality issue. Anthropogenic activities in lake basins emit nutrients, either as point- (e.g., sewage) or diffuse sources (e.g., agricultural runoff). Their typical impacts on lake water quality include the occurrence of harmful algal blooms, hypoxia and fish kills. These impacts are likely to worsen due to climate change, population growth and economic development. The response of lakes to a change in nutrient inputs depends on their interactions with the climate, land-use, hydrology and socio-economic conditions of a lake basin. These feedback mechanisms, however, are not often included in the eutrophication assessments for lakes. In this study, we present a new causal network of the drivers-pressure-state-impact-response (DPSIR) framework using a total of 58 sub-indicators to characterize all the DPSIR elements and systematically conceptualize the complex interactions of nutrients in freshwater lake basins. The network provides a holistic perspective on nutrient dynamics of multiple indicators and their interactive effects on water quality in lake basins, which is key to improving water quality management. Furthermore, we disentangle the complex eutrophication mechanisms using drivers and pressures, that represent different sources and nutrient pathways. The study highlights coupling of lake systems in water quality modeling frameworks and assessments which is required to understand its impact on water quality from human activities in the basin. The drivers and pressures can be used as proxies to provide meaningful information on nutrient emissions and biogeochemical pathways, that can fill the gap in water quality monitoring data, especially in data scarce regions such as Asia and Africa. These indicators can be used to set realistic water quality targets, and are, therefore, beneficial in long-term policy making and sustainable water quality management

Bridging global, basin and local-scale water quality modeling towards enhancing water quality management worldwide

Global water quality (WQ) modeling is an emerging field. In this article, we identify the missing linkages between global and basin/local-scale WQ models, and discuss the possibilities to fill these gaps. We argue that WQ models need stronger linkages across spatial scales. This would help to identify effective scale-specific WQ management options and contribute to future development of global WQ models. Two directions are proposed to improve the linkages: nested multiscale WQ modeling towards enhanced water management, and development of next-generation global WQ models based-on basin/local-scale mechanistic understanding. We highlight the need for better collaboration among WQ modelers and policy-makers in order to deliver responsive water policies and management strategies across scales