The subject of green algae made a remarkable comeback in the news this year. First, because the Court of Auditors and the Senate have crushed the management of the problem in the region. But above all because the most affected Breton beaches have been covered with it since the start of summer. The observation of very significant strandings was confirmed by the center for the study and valuation of algae (Ceva) which carries out overflights of the coast. “Objectively, we have had a bad year with a very important proliferation. Over the period April to September, we would be at + 40% or + 50% compared to the multi-year average, ”indicates Sylvain Ballu, monitoring project manager at Ceva, specifying that these figures are provisional.
Since 2002, the Ceva has carried out one flight every month between April and October, at low tide and by large tidal coefficient, as part of the monitoring of green tides. Present at sea in its natural state, green algae proliferate in Breton bays, where they are nourished by contributions of nitrates spread by farmers in the fields and the surpluses of which are transported by coastal rivers. When they putrefy, these ulvae generate hydrogen sulphide, a gas dangerous to health.
Three quarters of strandings in Saint-Brieuc and La Fresnaye
This year, two bays in the Côtes d’Armor (Fresnaye and Saint-Brieuc) have concentrated three quarters of the strandings of green algae. In these shallow bays with little exposure to storms, green algae grew strongly in the spring due to exceptional sunshine in April. Then the thunderstorms in June brought new nitrogen flows to the algae when they needed it most to proliferate. “The flows of certain rivers have been multiplied by ten or by fifteen” during this period, explains the member of the Ceva.
Conversely, the bays of Douarnenez and Concarneau (Finistère), which had been cleaned by winter storms, were hardly affected by this phenomenon. Sylvain Ballu recalled that the Breton rivers had known “substantial improvements” for several years with nitrogen concentrations sometimes “divided by two”. But this is still insufficient to prevent the formation of green tides when the weather conditions are unfavorable.
The public plans to fight against the proliferation of green algae in Brittany were “ill-defined” and had only a “limited impact”, according to the Court of Auditors which recommended in July to promote agriculture “low. nitrate leaks ”. “We can see that the dynamic of mobilizing farmers has run out of steam in most watersheds. Thus, the applied nitrogen pressure has stagnated since 2015 ”, write the magistrates who recall that“ the nitrogen (nitrates) present in the berries is more than 90% of agricultural origin ”.