Nutrient Export From Watersheds Research Articles

Excessive nitrogen and phosphorus in European rivers: 2000–2050

Rivers export nutrients to coastal waters. Excess nutrient export may result in harmful algal blooms and hypoxia, affecting biodiversity, fisheries, and recreation. The purpose of this study is to quantify for European rivers (1) the extent to which N and P loads exceed levels that minimize the risk of harmful algal blooms and (2) the relative shares of sources of N and P in rivers. This may help to identify effective management strategies to reduce coastal eutrophication. We focus on 48 rivers in 27 countries of the European Union (EU27). We used the Global Nutrient Export from Watersheds (NEWS) model to analyze nutrient export by rivers and the associated potentials for coastal eutrophication as reflected by Indicator for Coastal Eutrophication Potential (ICEP). In 2000, 38 of the 48 EU rivers indicated in our study had an ICEP>0, indicating a relatively high potential for harmful algal blooms. These 38 rivers cover 60% of EU27 land area. Between 2000 and 2050 nutrient export by European rivers is projected to decrease. However, by 2050 still 34EU rivers, covering 48% of the land area, have an ICEP>0. This indicates that in these scenarios little progress is made in terms of environmental improvement. About one-third of the rivers with ICEP>0 are N limited, and about two-thirds P limited. In N-limited rivers reducing N loads is a more effective way to reduce the risk for coastal eutrophication than reducing P, and vice versa. For N-limited rivers agriculture or sewage are the dominant sources of nutrients in river water. In P-limited rivers, sewage is found to be the dominant source of P, except for rivers draining into the Atlantic Ocean, where agriculture can also be dominant. A basin-specific approach is needed to effectively reduce N and P loads.

Coastal eutrophication in Europe caused by production of energy crops

In Europe, the use of biodiesel may increase rapidly in the coming decades as a result of policies aiming to increase the use of renewable fuels. Therefore, the production of biofuels from energy crops is expected to increase as well as the use of fertilisers to grow these crops. Since fertilisers are an important cause of eutrophication, the use of biodiesel may have an effect on the water quality in rivers and coastal seas. In this study we explored the possible effects of increased biodiesel use on coastal eutrophication in European seas in the year 2050. To this end, we defined a number of illustrative scenarios in which the biodiesel production increases to about 10–30% of the current diesel use. The scenarios differ with respect to the assumptions on where the energy crops are cultivated: either on land that is currently used for agriculture, or on land used for other purposes. We analysed these scenarios with the Global NEWS (Nutrient Export from WaterSheds) model. We used an existing Millennium Ecosystem Assessment Scenario for 2050, Global Orchestration (GO2050), as a baseline. In this baseline scenario the amount of nitrogen (N) and phosphorus (P) exported by European rivers to coastal seas decreases between 2000 and 2050 as a result of environmental and agricultural policies. In our scenarios with increased biodiesel production the river export of N and P increases between 2000 and 2050, indicating that energy crop production may more than counterbalance this decrease. Largest increases in nutrient export were calculated for the Mediterranean Sea and the Black Sea. Differences in nutrient export among river basins are large.

Possible future effects of large-scale algae cultivation for biofuels on coastal eutrophication in Europe

Biodiesel is increasingly considered as an alternative for fossil diesel. Biodiesel can be produced from rapeseed, palm, sunflower, soybean and algae. In this study, the consequences of large-scale production of biodiesel from micro-algae for eutrophication in four large European seas are analysed. To this end, scenarios for the year 2050 are analysed, assuming that in the 27 countries of the European Union fossil diesel will be replaced by biodiesel from algae. Estimates are made for the required fertiliser inputs to algae parks, and how this may increase concentrations of nitrogen and phosphorus in coastal waters, potentially leading to eutrophication. The Global NEWS (Nutrient Export from WaterSheds) model has been used to estimate the transport of nitrogen and phosphorus to the European coastal waters. The results indicate that the amount of nitrogen and phosphorus in the coastal waters may increase considerably in the future as a result of large-scale production of algae for the production of biodiesel, even in scenarios assuming effective waste water treatment and recycling of waste water in algae production. To ensure sustainable production of biodiesel from micro-algae, it is important to develop cultivation systems with low nutrient losses to the environment.

Future trends in urbanization and coastal water pollution in the Bay of Bengal: the lived experience

The Bay of Bengal includes coastal seas of several countries, including Bangladesh, India, and Myanmar. We present scenarios for future river export of eutrophying nutrients into the Bay of Bengal, and the role of urbanization therein. We used NEWS (Nutrient Export from WaterSheds) model to analyze trends over the period 1970–2050. The scenarios are based on the Millennium Ecosystem Assessment and indicate the number of people living in urban areas may increase from 22 % in 1970 to about 50 % in 2050. We show that this may considerably increase nutrient levels in rivers from sewage and other sources. For 2050, we calculate that harmful algal blooms may be a potential problem in coastal waters of about 95 % of the total drainage basin of the Bay of Bengal. In addition, we analyze Bangladeshi citizens’ expectations of future trends and how citizens with different worldviews would experience environmental changes (i.e., their lived experience). The citizens indicate that trends as envisaged in our scenarios may be a negative experience. However, some people may experience the trends as positive, because they expected worse.

The increasing impact of food production on nutrient export by rivers to the Bay of Bengal 1970–2050

The objective of this study is to assess the impact of food production on river export of nutrients to the coastal waters of the Bay of Bengal in the past (1970 and 2000) and the future (2030 and 2050), and the associated potential for coastal eutrophication. We model nutrient export from land to sea, using the Global NEWS (Nutrient Export from WaterSheds) approach. We calculate increases in river export of N and P over time. Agricultural sources account for about 70–80% of the N and P in rivers. The coastal eutrophication potential is high in the Bay. In 2000, nutrient discharge from about 85% of the basin area of the Bay drains into coastal seas contributes to the risk of coastal eutrophication. By 2050, this may be 96%. We also present an alternative scenario in which N and P inputs to the Bay are 20–35% lower than in the baseline.

Reducing future nutrient inputs to the Black Sea

Rivers export increasing amounts of dissolved inorganic (DIN, DIP) and organic (DON, DOP) nitrogen and phosphorus to the Black Sea causing coastal eutrophication. The aim of this study is to explore future trends in river export of these nutrients to the sea through a sensitivity analysis. We used the Global NEWS (Nutrient Export from WaterSheds) model to this end. We calculated that between 2000 and 2050 nutrient inputs to the Black Sea may increase or decrease, depending on the assumed environmental management. We analyzed the effects of agricultural and sewage management on nutrient inputs to the sea in 2050 relative to two Millennium Ecosystem Assessment (MEA) scenarios, Global Orchestration (GO) and Adaptive Mosaic (AM). In these baselines, total N and P inputs to the Black Sea decrease between 2000 and 2050, but not for all rivers and nutrient forms. Our results indicate that it is possible to reduce nutrient inputs to the sea further between 2000 and 2050 in particular for dissolved inorganic N and P and for many river basins, but not for all. For scenarios assuming combined agricultural and sewage management dissolved inorganic N and P inputs to the Black Sea are reduced by up to two-thirds between 2000 and 2050 and dissolved organic N and P inputs by one-third. River export of DIN is mainly affected by agricultural management and that of DIP by sewage management. On the other hand, in scenarios assuming increased fertilizer use for, for instance bioenergy crops, nutrient inputs to the sea increase. An increase in DIP inputs by southern rivers seems difficult to avoid because of the increasing number of people connected to sewage systems.

A comparison of NEWS and SPARROW models to understand sources of nitrogen delivered to US coastal areas

The relative contributions of different anthropogenic and natural sources of in-stream nitrogen (N) cannot be directly measured at whole-watershed scales. Hence, source attribution estimates beyond the scale of small catchments must rely on models. Although such estimates have been accomplished using individual models, there has not yet been a comparison of N source attribution predictions at large spatial scales. We compared results from two models applied to the continental US: Nutrient Export from WaterSheds (NEWS) and SPAtially Referenced Regressions On Watersheds (SPARROW). NEWS and SPARROW predictions for total N delivery to the US coastal zone were 373 and 429 kg N km−2 year−1, respectively, for the contemporary period. Despite differences in how inputs were represented and defined by the models, NEWS and SPARROW both identified the same single-largest N sources for 67 % of the surface area that drains to the US coastal zone. When only anthropogenic sources were considered, agreement increased to 78 % of surface area. Fertilizer and crop N-fixation were dominant in the Mississippi River Basin, where the bulk of agricultural lands are located. Sewage and population-related sources were most important in urban areas and natural N (primarily N-fixation on non-agricultural land) was most important in the Pacific Northwest. Attribution to fertilizer plus crop N-fixation, atmospheric deposition, and sewage and population-related sources was generally greater by SPARROW than NEWS, and the reverse was true for manure and natural sources. Nonetheless, both models agreed in attributing 62–81 % of N delivered to the coastal zone in the continental US to human activities.

Preface to special section on Past and Future Trends in Nutrient Export From Global Watersheds and Impacts on Water Quality and Eutrophication

[1] Coastal eutrophication is a worldwide problem [Diaz and Rosenberg, 2008; Selman et al., 2008]. It is largely caused by increased river transport of nutrients such as nitrogen (N), phosphorus (P), and carbon (C). Further distortion of coastal marine ecosystems is related to changing element ratios as a result of increasing N and P loading and decreasing river export of silica (Si). The main causes of increased river nutrient loading are the increasing livestock and crop production and fertilizer use, and increasing wastewater flows. There have been considerable gains in knowledge of current amounts and sources of N and P entering the terrestrial biosphere, and biogeophysical factors that control the amount of N and P ultimately exported by rivers to coastal ecosystems. Considerably less work has addressed how various socioeconomic factors and approaches to nutrient management affect N and P inputs to the terrestrial biosphere and subsequent impacts on river nutrient export. [2] This special section of Global Biogeochemical Cycles focuses on Past and Future Trends in Nutrient Export from Global Watersheds and Impacts on Water Quality and Eutrophication. It summarizes the recent work of the Global Nutrient Export fromWatersheds (NEWS) work group, using an integrated modeling approach connecting socioeconomic, nutrient management and biogeophysical factors to river export. [3] We present the second generation of the Global NEWS model [Mayorga et al., 2010; Seitzinger et al., 2010]. The first generation of the NEWS model was published in a 2005 Special Section of Global Biogeochemical Cycles [Beusen et al., 2005; Dumont et al., 2005; Harrison et al., 2005a; Harrison et al., 2005b; Seitzinger et al., 2005]. The NEWS model calculates river export of different forms of N, P, C and Si as a function of hydrology, morphology, land use and human activities in the river basin. More than 6000 river basins are included in the model. [4] Since 2005, three important developments took place on which we report in this special section: (1) several submodels for individual nutrient forms were updated [Beusen et al., 2009; Harrison et al., 2010; Mayorga et al., 2010; Seitzinger et al., 2010], such as including detergent sources of P in the DIP submodel [Harrison et al. 2010], and development of a submodel for river export of dissolved silica [Beusen et al., 2009]; (2) the models for different nutrient species were integrated into one modeling interface [Mayorga et al., 2010]; and (3) the model was used to analyze past trends (1970–2000) and four future scenarios (up to 2050). The future scenarios were based on the Millennium Ecosystem Assessment (MEA) [Alcamo et al., 2006]. We interpreted these scenarios to develop spatially explicit model inputs for diffuse sources of nutrients from agriculture and natural ecosystems [Bouwman et al., 2009] and sewage [Van Drecht et al., 2009], and hydrology [Fekete et al., 2010] consistent with the range of social, economic, policy, and ecological considerations in the four MEA scenarios. [5] Past and future trends in river nutrient export to coastal waters calculated from the NEWS model were used to analyze global trends [Seitzinger et al., 2010] as well as changes at the continental scale for South America andAfrica [van der Struijk and Kroeze, 2010; Yasin et al., 2010] or at the scale of individual river basins in Europe [Thieu et al., 2010; Yan et al., 2010], and a time series comparison of measured and modeled DIN export by the Changjiang River (1970–2003) [Yan et al., 2010]. We also developed a global perspective on indicators for nutrient inputs to land, and for changing nutrient ratios which could favor increases in nondiatom algal blooms [Billen et al., 2010; Garnier et al., 2010]. [6] Our results indicate that since 1970, river export of nutrients has been increasing considerably in many world regions. During the coming decades, DIN andDIP loads were projected to continue to increase in many world regions; however, particulate loads were projected to decrease. Perhaps the most surprising result of our analyses was that for large world areas, at least in one scenario, we calculate lower nutrient export rates in the future than in 2000, despite inInstitute of Marine and Coastal Sciences, Rutgers University, Rutgers/NOAA CMER Program, New Brunswick, New Jersey, USA. Netherlands Environmental Assessment Agency, Bilthoven, Netherlands. Environmental Systems Analysis Group, Wageningen University, Wageningen, Netherlands. School of Science, Open University of Netherlands, Heerlen, Netherlands.

Millennium Ecosystem Assessment scenario drivers (1970–2050): Climate and hydrological alterations

This study was carried out to support and enhance a series of global studies assessing contemporary and future changes in nutrient export from watersheds (Global Nutrient Export from Watersheds (NEWS)). Because hydrography is one of the most important drivers in nutrient transport, it was essential to establish how climatic changes and direct human activities (primarily irrigation and reservoir operations) affect the hydrological cycle. Contemporary and future hydrography was established by applying a modified version of a global water balance and transport model (WBMplus) driven by present and future climate forcing, as described in the Millennium Ecosystem Assessment scenarios (1970–2050). WBMplus represents a major upgrade to previous WBM implementations by incorporating irrigational water uptake and reservoir operations in a single modeling framework. Contemporary simulations were carried out by using both observed climate forcings from the Climate Research Unit of East Anglia (CRU) data sets and from Global Circulation Model (GCM) simulations that are comparable to the future simulations from the same GCM forcings. Future trends in three key human activities (land use, irrigation, and reservoirs operation for hydropower) were taken from the Integrated Model to Assess the Global Environment (IMAGE). The reservoir operation required establishing a realistic distribution of future reservoirs since the IMAGE model provided only the hydropower potentials for the different future scenarios.

Anthropogenic nitrogen autotrophy and heterotrophy of the world's watersheds: Past, present, and future trends

Anthropogenic nitrogen autotrophy of a territory is defined as the nitrogen flux associated with local production of harvested crops and grass consumed by livestock grazing (in kg N/km2/yr). Nitrogen heterotrophy is the nitrogen flux associated with local food and feed consumption by humans and domestic animals. These two summarizing characteristics (anthropogenic nitrogen autotrophy and heterotrophy (ANAH)) indicate the degree of anthropogenic perturbation of the nitrogen cycle by agriculture and human consumption: their balance value provides information on either the potential for commercial export or the need to import agricultural goods; in a watershed, their vector sum is related to the nitrogen flux delivered to the sea. These indicators were calculated for all the watersheds in the Global Nutrient Export from Watersheds (NEWS) database for 1970 and 2000, as well as for 2030 and 2050, according to Millennium Ecosystem Assessment scenarios. During this 30 year period, many watersheds shifted from relatively balanced situations toward either more autotrophic or more heterotrophic conditions. This trend is predicted to become more pronounced over the next 50 years.

Subregional and downscaled global scenarios of nutrient transfer in river basins: Seine‐Somme‐Scheldt case study

In an attempt to downscale the global prospective scenarios established by the Millennium Ecosystem Assessment to the level of three individual watersheds (the Seine, Somme, and Scheldt rivers), we examined the application of the regional RIVERSTRAHLER model, based on a mechanistic representation of in‐stream processes, in tandem with the semiempirical Global Nutrient Export from Watersheds (NEWS) model, by downscaling the input data of the latter into information required by the former. Overall, the model simulates the major trends of the changes that occurred in 1970–2000, although with some discrepancies revealing the weakness of certain hypothesis in the global approach. For the future, the prediction is a significant decrease in total nitrogen and phosphorus fluxes into the sea compared to those of 2000. We showed the benefits of combining a process‐based approach of nutrient transfer at the local scale with the use of global‐scale models for integrating the development of socioeconomic driving forces acting at the global level.

Future trends in nutrient export to the coastal waters of South America: Implications for occurrence of eutrophication

We analyze future trends in nutrient export to the coastal waters of South America, with a special focus on the causes of nutrient export and their potential effects. Nutrient Export from Watersheds (NEWS) model results for South America are presented, including trends in human activities and the associated river export of nutrients for the period 1970–2050. For 25 areas in coastal waters of South America where eutrophication or hypoxia has been observed, we investigate how these relate to NEWS model output. For selected watersheds we discuss the causes of increased nutrient loadings of rivers and future trends as projected by the NEWS models.

Magnitudes and sources of dissolved inorganic phosphorus inputs to surface fresh waters and the coastal zone: A new global model

As a limiting nutrient in aquatic systems, phosphorus (P) plays an important role in controlling freshwater and coastal primary productivity and ecosystem dynamics, increasing frequency and severity of harmful and nuisance algae blooms and hypoxia, as well as contributing to loss of biodiversity. Although dissolved inorganic P (DIP) often constitutes a relatively small fraction of the total P pool in aquatic systems, its bioavailability makes it an important determinant of ecosystem function. Here we describe, apply, evaluate, and interpret an enhanced version of the Global Nutrient Export from Watersheds (NEWS)–DIP model: NEWS‐DIP–Half Degree (NEWS‐DIP‐HD). Improvements to NEWS‐DIP‐HD over the original NEWS DIP model include (1) the preservation of spatial resolution of input data sets at the 0.5 degree level and (2) explicit downstream routing of water and DIP from half‐degree cell to half‐degree cell using a global flow‐direction representation. NEWS‐DIP explains 78% and 62% of the variability in per‐basin DIP export (DIP load) for U.S. Geological Survey (USGS) and global stations, respectively, similar to the original NEWS‐DIP model and somewhat more than other global models of DIP loading and export. NEWS‐DIP‐HD output suggests that hot spots for DIP loading tend to occur in urban centers, with the highest per‐area rate of DIP loading predicted for the half‐degree grid cell containing Tokyo (6366 kg P km−2 yr−1). Furthermore, cities with populations >100,000 accounted for 35% of global surface water DIP loading while covering less than 2% of global land surface area. NEWS‐DIP‐HD also indicates that humans supply more DIP to surface waters than natural weathering over the majority (53%) of the Earth's land surface, with a much larger area dominated by DIP point sources than nonpoint sources (52% versus 1% of the global land surface, respectively). NEWS‐DIP‐HD also suggests that while humans had increased DIP input to surface waters more than fourfold globally by the year 2000, human activities such as dam construction and consumptive water use have somewhat moderated the effect of humans on P transport by preventing (conservatively) 0.35 Tg P yr−1 (∼20% of P inputs to surface waters) from reaching coastal zones globally.

Sources and delivery of carbon, nitrogen, and phosphorus to the coastal zone: An overview of Global Nutrient Export from Watersheds (NEWS) models and their application

An overview of the first spatially explicit, multielement (N, P, and C), multiform (dissolved inorganic: DIN, DIP; dissolved organic: DOC, DON, DOP; and particulate: POC, PN, PP) predictive model system of river nutrient export from watersheds (Global Nutrient Export from Watersheds (NEWS)) is presented. NEWS models estimate export from 5761 watersheds globally as a function of land use, nutrient inputs, hydrology, and other factors; regional and global scale patterns as of 1995 are presented here. Watershed sources and their relative magnitudes differ by element and form. For example, anthropogenic sources dominate the export of DIN and DIP at the global scale, although their anthropogenic sources differ significantly (diffuse and point, respectively). Natural sources dominate DON and DOP export globally, although diffuse anthropogenic sources dominate in several regions in Asia, Europe and N. America. “Hot spots” where yield (kg km−2 yr−1) is high for several elements and forms were identified, including parts of Indonesia, Japan, southern Asia, and Central America, due to anthropogenic N and P inputs in some regions and high water runoff in others. NEWS models provide a tool to examine past, current and future river export of nutrients, and how humans might impact element ratios and forms, and thereby affect estuaries and coastal seas.