Monitoring algae from satellite

SMHI has been monitoring some sorts of algae in the Baltic, the Skagerrak and the Kattegat since 2002. Algae can be detected from satellite when they gather on or near the sea surface.

The algae map shows areas where algal blooms have been surveyed from satellite. A suspected bloom that is unclear or difficult to see because it is under the surface is marked in yellow as a “risk for subsurface bloom”, while a “surface bloom” is marked in orange and defines an area where the algal bloom has concentrated on the surface. Clouds are shown in grey and areas of missing data are shown in black.

Since algal blooms cannot be detected through clouds, an overview of the last 7 days is also shown in the form of a diagram indicating the number of days that algae were present over the last week, for each location. This can be very useful if a bloom has been present for a while and then the area becomes cloud-covered. The chances are that there is still a bloom under the cloud cover even though it cannot be detected from satellite.

SMHI analyses the algae situation with the help of satellite images

The algae maps on SMHI’s website are mainly based on data from two satellites - ENVISAT and EOS-AQUA, which have two comparable sensors, MERIS and MODIS. The passage of the satellites over Swedish waters varies in space and time so that there is not always complete coverage of the Baltic. This problem is minimised by using two satellites.


The satellites pass over the Baltic region in the middle of the day. The images become available later in the afternoon and are combined to cover as much of the area as possible. A report of the algae situation in the Baltic area together with an analysis of the images is published on SMHI’s webpage the following morning. If enough satellite data is received earlier in the day, these images are published during the afternoon.

SMHI also has the possibility of using further satellite data from
NOAA-AVHRR if there is no data at all from the other two satellites. However NOAA’s satellites do not give a very satisfactory picture of the algae situation as the sensors have a lower resolution.

New technology improves algae monitoring

This summer, the monitoring of algae in the seas around Sweden will become more detailed. By combining the satellite data from two more suitable satellites the images become more detailed, especially near the coast.

“The new technology also makes it easier for us to see whether the algae lie on the surface or are mixed in the water”, says oceanographer Martin Hansson.

Over the last year, a new method has been developed that combines data from the two European weather satellites ENVISAT and EOS. A complete picture of all the seas around Sweden can be obtained by combining the images from two different satellites. The new satellites have a higher resolution so are better suited to detection of algal blooms.

Ytansamlingar av cyanobakterier i Östersjön
Cyanobacteria on the surface of the Baltic Sea.

Common questions

Which algae can be seen from satellites?

The satellite sensors can only see algae that have gathered on or near the surface. Cyanobacteria on the surface show up as yellow-green, meandering areas. The high reflectance of algal blooms enables them to be seen from satellites.

Can satellites see the algal blooms through clouds or in the dark?

No, at least not the satellites that are used by SMHI.

When do the algal blooms start?

The start of the algal blooming period is controlled by sea temperature, weather and wind, which makes it difficult to define when they will start each year. In general the blooming period starts at the end of June. Calm and warm weather increases the chances of the algae multiplying quickly.

Are algae dangerous? Is it OK to go swimming?

More information is available from the Information Office for the Baltic Proper at the County Administrative Board of Stockholm.

More information about algal blooms from SMHI

Algal blooms in the Baltic Sea

Algal blooms

For questions or more information, please call us or send an e-mail to:

SMHI's oceangrapher
tel. 031- 751 89 05
oceanograf@smhi.se