Environmental impact of water exchange blocking in a strait – a multidisciplinary study in the Baltic Sea

•Water exchange is blocked by a 3 km road dam in the shallow strait in the Baltic.•Ecological conditions in the strait have considerably deteriorated.•Water exchange in the area close to the dam has decreased 10–12-fold.•Extreme temperature, oxygen and total nitrogen occur near the dam.•Bottom sediment structure, benthos and fish are impacted by the dam.