Blue-green algae bloom again on the Dyje below Nové Mlýny — fish gasping for oxygen

Since Friday, a thick layer of blue-green algae has covered the entire stretch of the Dyje river from Břeclav to Nové Mlýny, leaving the water almost completely depleted of oxygen. According to Povodí Moravy, the state water management authority, oxygen levels in the river near Bulhary and the Zámecká Dyje branch dropped to zero on Saturday morning, forcing water managers to shut down the small hydroelectric plant in Bulhary on an emergency basis and restrict the inflow of water into the Zámecká Dyje.

Aerators were switched on above the dam in Bulhary and on the Zámecká Dyje to pump oxygen back into the water — by 2 p.m. they could already be turned off. Throughout the day, a wave of thick, oxygen-free foam moved downstream toward Břeclav, so at ten in the morning the decision was made, in coordination with local water authorities, to shut down the small Břeclav hydroelectric plant as well. According to a company spokesperson, no major fish kill had been recorded by Saturday, and dissolved oxygen levels had climbed back above 4 mg/l.

Anglers describe a murkier picture

The Moravian Anglers' Union sees the situation as far more complicated and worrying. "Oxygen levels in the Dyje below Nové Mlýny already began dropping on Friday afternoon. By nine in the evening at the bridge in Bulhary we recorded zero, and it stayed that way until morning," said union chairman Miroslav Laníček.

He is reluctant to estimate the scale of the damage just yet — much depends on how many fish were even left in the river after previous incidents, and whether they had been sheltering near the aerators. "Some fish have likely died, but we won't know how many will surface until Sunday. So far it doesn't look as bad as it did in the first half of July. We're mainly talking about smaller fish — a few have already been spotted upstream of the dam near Bulhary," the union chairman said cautiously. The union continues to monitor both the main channel and the side branches of the Dyje.

Firefighters called in to help

As part of the emergency response, firefighters were setting up a containment barrier in Břeclav on Saturday. According to the Povodí Moravy spokesperson, water spraying above the Břeclav and Poštorná dams is scheduled for around 10 p.m., with oxygen levels in the river at Břeclav expected to drop again by midnight — if the situation unfolds the way it did in mid-July.

Problems with blue-green algae and mass fish die-offs downstream of Nové Mlýny have become a recurring feature of recent summers. Fish suffocate as sprawling algae colonies consume the available oxygen — the algae enter the river from the reservoirs upstream, where conditions are ideal for their growth. The worsening problem, driven by this summer's heat and drought, has reignited debate among ecologists and water managers over whether at least the lowest of the three Nové Mlýny reservoirs should be drained.

Around Prague, based on reporting by novinky.cz