The transparency is measured by slowly lowering a Secchi disc, i.e. a round, white disc of approximately 25 cm in diameter, from the surface towards the bottom. The visibility depth is that at which the disc disappears from view.

In the open sea, the Secchi depth typically ranges from three to six metres in summer. In winter, when algae are scarce, visibility depths of up to ten metres are measured. In open seas, the water transparency is particularly affected by the concentration of algae in the water.

In coastal waters, the transparency is affected by river water and winds. In estuaries, the minimum Secchi depth can be only a few tens of centimetres at its lowest. In strong winds, the water transparency decreases as mud and clay deposits from the seabed are mixed in the water.

Find out more about the long-term trends and seasonal variations in the Secchi depths of the Baltic Sea!