Often it is, but: the greenish shade of water is not always due to cyanobacteria or other algae.

The reason for the greenish color of the water can be, for example, one of the following:

• turbid water deposited in water layers below the surface,

• visibility of benthic vegetation through water in shallow areas,

• pollen in water,

• snow or ice mixed with water,

• decomposition products of dead algae mixed with water,

• cyanobacteria or other algae in water.

Without expertise, it may be difficult to interpret the causes for the greenish hue of the water. At the bottom of the page there are examples of cases where the water appears to turn green due to various reasons.

Please also note that it is not possible to make algal species determinations based on the data in the Tarkka service. Depending on the season, several species of algae occur in Finnish waters. More detailed information on the observed algae species can be found on Syke's algae status pages and  waterinfo.fi pages during the summer.

"Greenery" in the true color image of Sentinel-2: (1) Bottom of the sea in shallow areas through water. (2) Cyanobacterial surface raft at sea. (3) Mixed cyanobacteria and turbidity and macrophyte growth in inland waters. (4) A mixture of turbid water and cyanobacteria mixed with surface water discharged from an estuary. The link to the image in Tarkka.

Another example of a true color image of Sentinel-2. The link to the image in Tarkka.

Examples

Turbid water mixed with water south of the Archipelago Sea (a link to Tarkka)

Visibility of bottom features through water on the Estonian coast

Large bottom feature in the southern Baltic Sea

Sentinel-3 true color image and seabed depth from the corresponding area. The extensive greening in area 1 shown in the true color image is due to the view of the shallows through the surface water layer. In area 2, the greening of the water is due to the algae mixed with the water (link to the picture in the Tarkka service). The smaller image shows the depth of the seabed from that area.

Pollen in Pyhäjärvi, Säkylä

In particular, pine pollen sometimes forms extensive rafts on both lakes and the sea. The link to the image in the Tarkka service.

Algae in water

Resuspension on the west coast

^{Resuspension is a phenomenon in which strong winds mix bottom material with the entire water layer in a shallow coastal area. This can be seen as several greenish-brown 'zones' along the coast. Resuspension is quite common on the west coast and especially in the Oulu region in the Bothnian Bay.}