Seawater is most turbid close to the shoreline

The more organic matter and soil materials leached into a water body from the catchment area, the more turbid or cloudy the water will be. For example, organic particles include algae, while leached soil materials contain clay.

The unit of turbidity is known as the Formazin Nephelometric Unit (FNU). On the high seas, turbidity ranges from 1 to 3 FNU. In the outermost parts of coastal waters, it is typically 2 to 6 FNU. At its peak, turbidity can be as high as 80 FNU in some estuaries. On shallow coasts, turbidity increases due to the resuspension caused by waves, i.e. the particles that have already sunk to the bottom are once again mixed into the water column.

Turbidity varies according to seasons and weather. Coastal waters are most turbid when rivers carry a lot of soil materials into estuaries or strong winds cause resuspension. In the open sea areas, water turbidity is mainly affected by algal abundance.

Take a closer look at the long-term trends and seasonal fluctuations in turbidity in the Baltic Sea!