Cyanobacteria bloom threatens fish in the Dyje River near Nové Mlýny, Czech Republic

High concentrations of cyanobacteria in the lower reservoir of the Nové Mlýny dam on the Dyje River have degraded water quality, creating a foam that the Morava River Basin Authority is collecting on two weir walls. The bloom can become toxic at night as cyanobacteria consume oxygen, raising concerns for fish.

In early July, a sudden drop in dissolved oxygen under the dam led to the death of about one tonne of fish, far fewer than the 30 tonnes that died at the same site a year earlier. Authorities, including the Moravian Fishermen’s Union, continue to monitor oxygen levels and cyanobacteria concentrations to prevent further losses. The situation is described as the worst in recent years, though current oxygen levels in the river downstream remain sufficient.