Every summer for more than 50 years, “green tides” have soiled Breton beaches, releasing a pestilential odor and toxic gas. Back to a pollution that has become almost inevitable.

In 2009, a record volume of nearly 100,000 m³ of green algae was collected in Brittany, compared to an average of 60,000 m³ over the past ten years.

On February 5, 2010, the national preventive and curative plan against green algae was launched.

Each summer, they come back, the first green algae spreads on the Breton coasts… and the exasperation mounts among the natives. Forty years that this lasts. Tens of thousands of tons of seaweed wash up on the sand, covering and soiling entire beaches. The problem is not only aesthetic: their decomposition leads in a few days to the release of hydrogen sulphide with a pestilential odor. Result: in the bays most affected, these green tides affect summer tourism on which depends an important part of the local economy. Not to mention that hydrogen sulfide is also a poisonous gas: it caused the death of a horse in 2009, and was suspected, the same year, of having caused that of a 47-year-old man, victim of a heart attack while handling a cargo of green algae.

In practice, the municipalities must organize the regular collection, transport and management of this waste, without really knowing what to do with it. While there are industrial projects to promote green algae, in particular to transform them into animal meal or biomaterials, none have yet come to fruition. The algae are therefore rejected in landfill or spread as fertilizer on fields. However, this support represents a significant cost for certain small municipalities.

The only real solution would be to go back to the source of the problem, intimately linked to the development of the Breton economy: the intensification, in the 1970s, of agriculture, whose practices lead to massive releases of nitrogen. in the ocean where they promote the multiplication of these green algae. Following the green tides of exceptional magnitude in summer 2009, the State unveiled, in February 2010, a plan to fight against green algae, at a cost of 134 million euros over five years.

It provides for curative actions (increased collection), but also aims to reduce nitrogen discharges through regular checks of the quantities of nitrogen used in farms and the rehabilitation of natural areas. Transforming Breton agriculture, whose economic weight is considerable, will not be easy, however. And, even if all the objectives are met, the first effects should not be felt for at least ten years.

In 2017, the second part of the plan was validated for the period 2017 – 2021. On this occasion, new water quality objectives, both ambitious and realistic, were set according to local specificities (geographical characteristics and hydrographic, types of crops and livestock, etc.). The challenge is to control the proliferation of algae by 2027.

In France, green algae are deposited mainly between April and October on the Breton coasts, although it is also found in the Arcachon basin and in the lagoons of the Midi. The Côtes-d’Armor, with the bays of Saint-Brieuc and Saint-Michel-en-Grève, and Finistère are the two departments most affected, with thousands of hectares of covered beaches.

In a few days, the algae deposited on the beaches dry up and form on their surface a white crust impermeable to air. The decomposition of the algae which, under this crust, are deprived of oxygen, leads to the formation of pockets of hydrogen sulphide, released when the crust is damaged (for example when picking up or walking on it). In high concentrations, this gas can be fatal.

Intensive agriculture in Brittany is the main cause of green tides. Indeed, the droppings of farm animals and the mineral fertilizers deposited on the fields are rich in nitrogen, which the rain carries away in the rivers and the ground water, to finally reach the ocean in the form of nitrates. However, the latter feed the green algae, and thus lead to their multiplication.

The nitrate concentration of Breton rivers has increased sixfold in forty years, reaching an average of 28 mg / l in 2009. The public authorities are aiming to reduce this rate by at least 30% by 2020, and 60% in 2027. If these objectives are reached, the nitrate concentration in waterways will nonetheless remain higher than those measured in the 1970s, yet sufficient to see green tides appear.

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