Baltic Sea daily surface algae (Sentinel-2 MSI) 2017–/Daily surface algae blooms of the Baltic Sea (Sentinel-2 MSI) 2017—

**[EN]** A four-class interpretative map describing surface algae ferries in the Baltic Sea region and, in particular, in the sea areas around Finland. The Interpretation Map describes the likelihood of surface algae present in a given area. The categories on the map are 1) no surface algae, 2) possible, 3) likely and 4) secure surface algae. Surface algae cannot be observed with optical satellite intrusions through the clouds, so the surface algae map is missing in cloudy areas. Interpretations will be made from the end of June to the beginning of September, and even longer if necessary. Mainly algae ferries are mostly found in July-August, so the number of surface algae observations is the highest for the period in question. The method of interpretation is based on the intensity of the reflection observed in the various wavelengths of the satellite, which is different in blue-green algae than in non-algae waters. The interpretation of surface algae made in the area of individual pixels is made more coarse, so that the algae areas shown on the map are clearer. Cloud areas are removed from the map area by means of an automatic cloud recognition system, supplemented, if necessary, manually before release. The interpretation method has been developed in SYKE and utilises the observations of three different satellite instruments. This data is based on the observations of the Sentinel-2 satellite series MSI instrument since 2017. The interpretation of satellite material is carried out with a precision of 60 m, so that it can also be extended to broken island regions and areas close to shores. This is how accurate observations are obtained from different sea basins every few days, but some of the observations are made from the observations of the Landsat-8 OLI (separate metadata). An annual compilation map of the data is made for algae information purposes. The material is part of SYKE’s open data (CC BY 4.0). *** **[En]** A four-class interpretation map describing surface floating algae blooms in the Baltic Sea region and especially in the sea areas surrounding Finland. The map describes the probability that algae accumulates in the surface layer o the sea in a particular area. The map categories are 1) no surface algae, 2) possible, 3) probable, and 4) certain surface algae. Surface algae cannot be detected by optical satellite instruments through clouds, so a map of surface algae is missing from cloudy areas. Interpretations made annually from late June to early September, and longer if necessary. Algae Rafts mainly occur in July-August, so the number of surface algae observations is the highest during that period. The interpretation method is based on the intensity of the reflection detected by the different wavelength ranges of the satellite, which is different in cyanobacterial areas than in areas without Cyanobacteria. A surface algae estimation by individual pixels is generalised so that the areas of algae displayed on the map are clearer. Cloudy areas are removed from the map area by an automatic cloud detection method, which is also completed manually before publication, if necessary. The interpretation method has been developed at SYKE and utilises the observations of three different satellite instruments. This data is based on observations by Sentinel-2 series MSI instruments starting from year 2017. The interpretation is made with an accuracy of 60 m, so it covers also the fragmented archipelago areas and areas close to the shores. Observations are obtained from different sea areas every few days, but the a part of the observations are made from observations of the NASA Landsat8 satellite (separate metadata). An annual summary map of the data is made for the needs of SYKE’s algae bloom monitoring (separate metadata). *** **WMS server/WMS service endpoint**: https://geoserver2.ymparisto.fi/geoserver/eo/wms **WMS level/WMS layer**: EO_HR_WQ_S2_ALGAE *** **[EN]** Algae ferry interpretation resulting from remote sensing monitoring based on Sentinel-2 MSI satellite observations. Daily algae ferry data from 2017 onwards. Processing history: Algae rafts have been interpreted from the Sentinel-2 MSI satellite instrument material. The original satellite data has been downloaded from the European Space Agency (ESA) download services. In the heartbeat, they've counted algae readings.