The water look greenish. Is it cyanobacteria?

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 the 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

“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. Observation in the Tarkka

Another example of a true color image of Sentinel-2. 8.3.2020 Curonian Lagoon at the coast of Lithuania. The observation shows combination of mixed algae and turbidity as well as macrophyte growth in the freshwater. Observation in the Tarkka

Examples

Turbid water mixed with water south of the Archipelago Sea. Observation in the Tarkka

Visibility of bottom features through water on the Estonian coast.

Figure 1: 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. Observation in the Tarkka. The right side image shows the depth of the seabed from that area.

Pollen in Pyhäjärvi, Säkylä. In particular, the pollen from pine forms extensive rafts on both the lakes and on the sea. Observation in the Tarkka

Resuspension on the west coast

Resuspension is a phenomenon where strong winds and waves lift substances from the seabed, such as gyttja, silt, and clay, even sand. These substances rise to the upper water layers and appear in satellite images as a greenish-brown ‘zone’ along the coast. Within a few days, the substances settle back to the bottom, and the water recovers its clarity.

Resuspension is common in the Bothnian Sea, on the western coast of Finland, in the inner archipelago, and especially around the Oulu region in the Bothnian Bay.