Perturbation des cycles biogéochimiques de l’azote et du phosphore

• Nitrogen is an essential nutrient for plant growth. Reactive nitrogen, released abundantly into the environment, can however constitute a surplus relative to the needs of plants, trees, algae, etc. It then contributes to water pollution through nitrates. Combined with other nutrients such as phosphate, and depending on particular physicochemical conditions, it is also responsible for eutrophication. The main sources of nitrogen emissions into the environment are nitrogen fertilizers and the combustion of fossil fuels and industrial processes. Nitrogen from polluting emissions of nitrogen oxides (NOx) in the atmosphere from transport and industry is not taken into account in the planetary boundary specified below (Steffen et al., 2015b). • Like nitrogen, phosphorus is also an essential nutrient for plant growth. The alteration of its biogeochemical cycle, caused by agriculture (fertilizers, livestock effluent) and urban wastewater (human excreta and detergents), affects the biosphere's capacity to sequester it and leads to eutrophication of freshwater. • Damage caused by nitrogen and phosphorus is generally considered a regional rather than global problem. However, in the context of work on nine planetary boundaries, a global threshold has been defined for each of the two biogeochemical cycles of nitrogen and phosphorus.

The challenge for nitrogen is to prevent excessive discharge of reactive nitrogen into water and natural aquatic environments to avoid their eutrophication. During the revision of the conceptual model (Steffen et al., 2015b), the threshold not to be exceeded was set between 62 and 82 million tonnes (Mt) per year, or 41 to 55 kg of excess nitrogen (surplus) per hectare per year (kg/ha/year) on average at the global scale. In 2015, nitrogen losses into the environment are estimated at 150 Mt.

• For phosphorus, the challenge initially considered was to prevent a major oceanic anoxic event (episode of strong oxygen reduction in the oceans) with impacts on marine ecosystems. During the revision of the conceptual model in 2015, a two-level geographical approach was proposed. • At the global level (ocean asphyxiation), the threshold is estimated at 11 Mt per year of phosphorus released into water (agricultural surplus and insufficiently treated wastewater). In 2015, it is exceeded with 22 Mt of phosphorus actually released into waters. • At a more localized scale, an additional threshold is defined to address eutrophication of continental freshwaters. This threshold concerns phosphorus surpluses resulting from excessive inputs during fertilization of agricultural soils. These phosphorus surpluses must not exceed, each year, a range of between 6.2 and 11.2 Mt (or 4.1 to 7.5 kg/ha/year) to prevent eutrophication of freshwater systems. In 2015, the limit is crossed with approximately 14 Mt.

• In France, excessive nitrogen and phosphorus inputs, mainly from agricultural activity for the former and from urban wastewater for the latter, correspond to the surplus in the environment, releases to sea and freshwater, which can lead locally to eutrophication phenomena and green algae proliferation. Surpluses are calculated at a regional scale without accounting for treatments and possible exports to other regions, particularly in livestock regions. • Nitrogen from agricultural activity is supplied in organic or mineral form. In 2015, organic inputs average 62 kg per hectare (ha) of utilized agricultural area (UAA) and mineral inputs 77 kg/ha. Both types of inputs have decreased since 1990. This decrease is explained notably by the reduction in French livestock numbers and better nitrogen management in agricultural operations. The latter results particularly from implementing nitrogen action programs in vulnerable zones, progressively extended, under the nitrate directive, as well as technological advancement. • Over the 2006-2015 period, the average nitrogen surplus is 45 kg/ha of UAA, compared to 55 kg/ha for 1996-2005. In 2015, regions with the highest nitrogen surpluses are Brittany and Pays de la Loire with over 70 kg/ha. However, these figures do not account for exports to neighboring regions seeking organic nitrogen and livestock effluent treatments. These measures are mandatory beyond a certain annual quantity for some Breton operators. The regions closest to equilibrium are Corsica, Burgundy and Provence Alpes Côte-d'Azur, while Île-de-France shows a negative balance.

• Phosphorus is supplied in organic or mineral fertilizer form. Surpluses are calculated without accounting for treatments and possible exports, particularly in livestock regions. • In 2015, in Brittany, the surplus reaches 20 kg/ha. Intensive pig and poultry farming zones experience total phosphorus inputs (mainly organic) sometimes too high relative to crop and pasture needs. In metropolitan France, phosphorus surplus fell from 9 kg/ha of UAA to 0 kg/ha between 2000 and 2015. This decline is mainly linked to decreased mineral fertilizer inputs. Since 2009, the balance is near equilibrium. • For nitrogen, the average agricultural surplus respects the theoretical planetary boundary (between 41 and 55 kg/ha/year). However, this limit is significantly exceeded in certain regions. For phosphorus, the average agricultural surplus is below the 4 to 7.5 kg/ha/year limit threshold. However, this threshold is exceeded in certain regions. Furthermore, phosphorus residues in wastewater after treatment, representing the main source of phosphorus emissions to water in France, do not represent a sufficiently large mass to exceed the limit across all of France.

• Information on pollutant fluxes reaching the sea confirms the results stated above. Nitrate releases remain stable, while phosphorus releases have decreased significantly since the early 2000s. • Nitrogen reaches the sea via waterways, essentially in the form of nitrates of agricultural origin and ammonium of domestic origin. For example, nitrogen surpluses applied to crops partly migrate to groundwater and waterways before reaching the sea [2]. From 1990 to 2017, total nitrogen flux transported by waterways averaged 408,000 tonnes per year. • Total phosphorus flux to sea averaged 20,000 tonnes per year between 1990 and 2017, with a declining trend. This decline is notably linked to improved wastewater treatment plant performance, the ban on phosphate use in detergents since 2007, increased connection of residents to collective sanitation, and reduced use of phosphate fertilizers in agriculture. • Significant year-to-year variations occur for both nitrogen and phosphorus, and can be explained by variations in rainfall and waterway flows.

Eutrophication phenomena disrupt the condition of rivers, lakes, estuaries and marine waters. They are present on the metropolitan coast but do not have the same intensity across maritime facades. Overall, in the Channel, most coastal zones experience eutrophication-related problems, but these remain limited near estuaries (Seine, Somme) and confined bay bottoms in Brittany (Saint-Brieuc, Lannion, Morlaix). On the Atlantic facade, early nitrogen and phosphorus enrichment is limited north of the Gironde. In the Mediterranean, eutrophication problems mainly concern highly localized sites in certain Occitanie lagoons.

• Excess nutrients lead to eutrophication and significant algae development on the metropolitan coast, called algal bloom (green algae). Two types of algae are involved: macroalgae (ulva), which produce green tides, and microscopic algae (phytoplankton), which cause colored waters, with possible toxicity risk to marine organisms and food supply. Green algae are mainly present on Breton coasts and extend to Central Atlantic and Lower Normandy. Microscopic algae are localized, from Flanders to the Arcachon basin and Mediterranean lagoons. • Over 2007-2012, at sea, the number of algal bloom events (excessive growth of marine algae) varies year to year but remains globally stable, while episodes with toxin proliferation increase very slightly. These toxins can be dangerous for wildlife and humans. These episodes therefore also have economic and health impacts.

Due to strong currents dispersing nitrate flows, Brittany experiences green tide phenomena in shallow bays with limited seawater renewal. To address this problem, the State and Region are implementing a plan to combat green algae proliferation in the eight affected bays. A second plan was signed for 2017-2021. It aims notably to reduce nitrogen flows, the source of green algae proliferation, toward coastal waters. Eight territorial charters, specific to the eight bays affected by green algae strandings on beaches, were also developed in 2017.

• Sargassum are brown algae living in tropical waters, in the open sea, with high concentrations in the Sargasso Sea, off the East Coast of the United States. Sargassum banks at sea play an important ecological role, serving as refuge for many species. • Since 2011, sargassum have episodically stranded more or less massively on the coasts of the Caribbean and French Guiana, peaking in 2014-2015 and recurring since 2017. Massive sargassum arrivals have multiple consequences: • health: emission of toxic gases such as hydrogen sulfide and ammonia; • economic: penalty to tourism, fishing, and aquaculture activities; • environmental: water acidification from sulfuric acid production, coral death, increased nitrate and phosphate concentration causing eutrophication, disruption or prevention of sea turtle nesting, beach erosion during collection, etc. • According to the interministerial mission report (July 2016), between 2014 and 2015, approximately one million m3 of sargassum stranded on beaches or exposed bays of Martinique and the Guadeloupe archipelago. • The origin of algae development is under study. Initial results from two sea campaigns conducted in 2017 by a scientific consortium coordinated by the Research and Development Institute identify several factors, including climate change. Responsible for marine water temperature rise (favorable to algae development), it could influence sea currents and facilitate sargassum movement. Nutrient input to the marine environment could also promote algae multiplication. • End of 2018, a national plan for sargassum prevention and control was implemented, aiming to define the organization of resources and actions in areas affected by strandings. Early 2019, the State launched a project call on sargassum strandings "Sargassum". This project call aims to provide pragmatic solutions to sargassum strandings and enhance knowledge of this phenomenon affecting particularly Caribbean basin islands.