In nutrient rich lakes and fjords, it is normal to see a large growth of microscopic algae that turns the water green, brown or red in July-August. In 2018, this growth happened much earlier due to the warm and calm weather of June and July. The lack of wind in combination with the warm water, led to immobile waters and the algae started to rot. It led to oxygen depletion in many rivers and lakes, which is detrimental to the life in the water. Luckily the weather of August more normal and winds brought some oxygen to the waters.
On the picture: Algae-waters from Furesø
Source: Per Juel Hansen, professor of Marine Biologi at Copenhagen University