![]() ![]() Disruption of the N:P ratio away from the Redfield molar ratio of 16:1 ( Redfield, 1934) is one of the major causes of harmful algal bloom (HAB) proliferation ( Glibert, 2019), even in a situation of declining nutrient loads. In addition to nutrient loading per se, the coastal research community has become increasingly aware that the stoichiometric ratio of nutrients is essential ( Billen et al., 1991). Initial eutrophication research focussed on linking riverine nutrient loading with responses such as chlorophyll (as a proxy for algal biomass), primary production and occurrence of anoxic conditions ( Cloern, 2001). In the 1960s these nutrient-induced environmental problems were recognized in freshwater ecosystems, but it took another two decades before coastal eutrophication became a topic of societal, economic and scientific concern. from macrophytes to fast growing macro- and microalgae, from diatoms to flagellates), reduced water clarity, oxygen depletion and the formation of dead zones, benthic habitat loss, and the proliferation of harmful or toxic algal blooms ( Duarte, 1995 Conley et al., 2009). Loads and concentrations of N and P have been increasing in nearly every water body across the globe with multiple effects on ecosystem structure and functioning, such as changes in the community structure (e.g. This has resulted in increasing nutrient loading of freshwaters through losses from agricultural land by leaching and runoff, discharge of wastewater in urbanized areas, and atmospheric N deposition. This has led to dramatic growth of emissions from fossil fuel combustion, use of fertilizer N and P and animal manure. ![]() Increasing population and affluence have boosted energy, crop, livestock, and aquaculture production. Human activities have markedly altered the earth's cycles of the plant nutrients nitrogen (N) and phosphorus (P) since pre-industrial times ( Galloway et al., 1995 Smil, 2000). Integrated strategies for both N and P considering all relevant trade-offs and societal sectors are urgently needed to reduce the nutrient pressure on surface waters. The human-dominated river N and P export with elevated N:P ratios will increase in all SSPs, except in SSP1 where it stabilizes. In Brazil, India and China, however, a decline of N:P is only the case in a scenario oriented toward sustainable development (SSP1). Future nutrient export is projected to decline in high-income countries (with N:P ratios exceeding Redfield). The five Shared Socioeconomic Pathways (SSP) were used in combination with the Representative Concentration Pathways climate scenarios to project river nutrient loadings for 2050. Here we show that rivers, which have more than 50% anthropogenic sources and at the same time elevated N:P ratios (> 25) contributed 36% to the total global N export to coastal waters in 2015. Not only nutrient loading is important but also the molar ratio and its deviation from the “natural” Redfield ratio. As a result, these nutrients can be found in nearly every water body across the globe. Nitrogen (N) and phosphorus (P) from anthropogenic sources are needed to produce food for the growing world population. ![]() 2PBL Netherlands Environmental Assessment Agency, The Hague, Netherlands.1Department of Earth Sciences – Geochemistry, Faculty of Geosciences, Utrecht University, Utrecht, Netherlands. ![]()
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