Contrary to some theories propagated on the internet, nitrogen, generated by agriculture and intensive breeding, plays a predominant role in the development of these harmful plants in the region.
These tides are a scourge that has disfigured the Breton coasts for more than half a century. While the movie Green Algae, adapted from the comic strip by Inès Léraud and Pierre Van Hove, released on Wednesday July 12, false information is still circulating about the origin of the massive strandings of these plants with harmful effects for the environment and health. Some of the allegations point to the responsibility of phosphorus, generated by human activities, released into the sea and which would promote the growth and proliferation of these algae.
Wastewater treatment plants (Step) are thus singled out. “The phosphorus brought by the rivers would come mainly from domestic discharges (STEP), and only in a small way from agricultural land“, said another Internet user on Twitter in mid-May. Several defenders of the agricultural world relayed his remarks. “So we’ve been lied to for years, denounced in a tweet a farmer from Cantal. “We point the finger at farmers for nothing.” “It was so tempting to blame agribusiness and its wicked productivist breeders”added on the same social network a Breton consultant.
In his tweet, the Internet user refers to an opinion, co-written by several research organizations, at the request of the prefect of the Brittany region, in 2011. This is “scientific responses to various claims made about the phenomenon of green tides, its causes and the treatment of algae”, it is specified there. Experts, who represent different organizations (theFrench research institute for the exploitation of the seaAgrocampus Ouest, the Center for the Study and Valorization of Algae and theNational Institute of Agronomic Research) screened 14 claims about the phenomenon.
Among these assertions is the statement relayed by the author of the tweet, on page 8 of the document: “Phosphorus brought by rivers and stored in coastal sediments would come mainly from domestic discharges, and only in a small way from agricultural land.” But, far from being a scientific finding, it is an erroneous assertion that scientists have dissected.
Nitrogen, the main culprit
“Accusing the excessive contributions of phosphorus to be the cause of the tides by not speaking of those of nitrogen is contrary to biological reality: the ulva is a nitrophilous algae [poussant dans un milieu riche en nitrate, une des formes de l’azote] which mostly needs nitrogen, and much less phosphorus“, reacts Alain Ménesguen, formerly an oceanographer and biologist at Ifremer, who is one of the scientists who contributed to the 2011 opinion. The researcher had moreover established in 2003 in a report the link between the increase in concentrations nitrogen in seawater and the proliferation of algae. “the responsibility of nitrate”, and more precisely its high rate in rivers. The researcher observed less algae growth in summer, a period during which the rivers feed less marine waters.
A “natural” nitrate rate is between 2 and 3 mg per liter of water, according to a summary note compiling scientific knowledge on the proliferation of green algae, submitted to the Ministries of Ecology and Agriculture in 2012. However, this concentration exceeded 20 mg in Breton rivers in the early 1980s, “with some rivers much higher than 50 mg”can we read in the document.
After a peak reached in 1998 (52.6 mg / l on average), it was 32.7 mg / l in 2021, according to the Observatory of the environment in Brittany. However, 9% of monitoring stations were still above 50 mg/l in 2021, against 47% in 1998, when the European Union recommended not to exceed this threshold for water intended for food production, in a 1975 directive.
Pollution mainly of agricultural origin
How to explain such a concentration? “Nitrogen sources come either from fertilization (nitrogen fertilizers), or from organic residues from livestock, pig manure“, explains Philippe Potin, researcher at the CNRS at the Biological Station of Roscoff (Finistère). will contribute to the growth of algae, hence their proliferation and massive strandings in coastal areas.
“The agricultural part represents at least 90% of the nitrogenous contributions” coming to create the green tides, underlines the 2012 report submitted to the Ministries of Ecology and Agriculture. The reduction of these contributions “linked to agricultural and livestock activities remains the most relevant objective to limit the proliferation of green algae”, conclude the authors of the document. The scientific consensus is therefore clear on the responsibility of intensive agriculture and the use of fertilizers in green tides.
At a time when Europe, bruised by the Second World War, wanted to ensure its food self-sufficiency, Brittany intensified animal production from the end of the 1950s. In 2018, these predominated there, representing 65% of the raw and processed products. It is the third French agricultural region in terms of production, according to INSEE. According to Agreste, the statistical service of the Ministry of Agriculture, 58% of French pigs are Breton, which makes the region the leading national producer of pigs. This share was four times lower in 1950, with “13% of the national herd”according to a scientific article published in 1984 in a journal of the Ecole Nationale Supérieure Agronomique.
Wastewater, a pollution factor with green algae?
The same goes for phosphorus which contributes, on a much smaller scale than nitrogen, to the development of green algae. To say that it emanates mostly from domestic activities is therefore incorrect. ATToday in Brittany, he “approximately two-thirds comes from agriculture, and one-third from urban waste”, explains Patrick Durand, researcher at I’national research institute foragricultureI’feed and theenvironment. The 2012 summary note sent to the two ministries estimates that the agricultural share of this phosphorus contribution is “from 50 to 60%”.
But could urban discharges also be incriminated, because of their nitrogen contribution? In reality, the role of wastewater in algal blooms is not significant. Scientists already pointed this out in 2011 in their response to the prefect of the Brittany region. “Even if domestic discharges deserve to be better controlled (…) they are not in Brittany the main source of nitrogen responsible for green tides”, they wrote. Wastewater treatment networks have also improved. “Nitrogen discharges from urban wastewater treatment plants have dropped enormously since the adoption of a last denitrifying treatment in the main Breton stations”explains Alain Ménesguen.
According to the latter, there is nothing new in this kind of untruths, propagated on social networks about green algae. “It has been a habit for 50 years among a certain number of farmers and people working in the Breton food industry.“, analyzes the former researcher. However, the Breton agricultural sector is becoming aware of the phenomenon: “All farmers seek to reduce nitrogen leakage. Moreover, it is not possible to do otherwise, given the regulations”provided in 2021 on a daily basis West France Didier Lucas, President of the Agricultural Chamber of Côtes-d’Armor.