Satellite imagery provides extensive information on the state of the Baltic Sea

Satellite images offer a new point of view for examining marine areas. Momentary fluctuations in surface waters, such as resuspension caused by a strong gust of wind or turbid areas occurring during dredging work, can cover large water areas. Resuspension refers to a situation when sunken sediment from the bottom is mixed back into the water due to strong winds. It is visible as turbid surface water as well. Pollen can also accumulate on the surface of the sea. It can be seen in high resolution satellite images as light-coloured swirls on the surface.

Changes in water quality monitored with satellite images

The state of the Baltic Sea is monitored with satellite images that are processed into values of several indicators. These indicators include surface water temperature, blue-green algae blooms, biomass of algae in the water, water turbidity and transparency.

Substances mixed into the water – such as blue-green algae, clay and other inorganic matter transported by rivers from inland areas – affect the colour and intensity of the reflected light. Reflected light can provide very accurate information when interpreted with various mathematical models. Today, most of these models are neural network applications that have been trained to estimate water quality in a variety of different types of water bodies across the globe.

Satellites operated by NASA and the ESA are utilised for research

Satellites observe targets on land and in water with a variety of methods. The whole Baltic Sea can be monitored as a single entity with observations made within a single day. Satellites can also observe more detailed areas, for example a single river’s estuary or an archipelago. SYKE uses satellite images from the Sentinel satellite constellation, belonging to the EU’s Copernicus programme, and NASA’s Landsat 8 satellite. During most weeks, the satellites collect plenty of data on all sea areas.

Observations are made daily and extensively throughout the entirety of the Baltic Sea with an accuracy of 300 metres. Observations are made with accuracy of 10–30 metres as well, but less often and the images cover smaller areas. Even the more accurate observations cover the whole coast of Finland every three days. Sometimes cloud cover prevents observation of the surface. During summertime, observations from a sea area consist of millions of individual pixels.[SK1] These observations can provide up-to-date information about the sea and be used to estimate the status of the sea in a longer period.

Clouds are not shown in the Tarkka service’s remote sensing products, so areas covered by clouds are shown with a background map. Despite the frequent cloud cover, a sufficient amount of observations can be made.