The Seili Index describes the current surface-water chlorophyll a concentration (chl-a, µg/L) and provides a forecast up to 10 days ahead. The index values indicate the total amount of phytoplankton in the immediate surroundings of Seili Island and generally reflect the level of marine eutrophication.
In the index visualization, a larger symbol and greener color indicate a higher level of eutrophication and more turbid seawater, while a smaller, bluer symbol represents clearer seawater. An increase in index values may therefore signal, for example, the onset of a phytoplankton bloom, as well as more generally reflect a deterioration in seawater quality.
The forecast is based on a generalized additive model (GAM; Generalised Additive Models), which is used to identify the relationship between chlorophyll concentration and the temperature of surface water and the overlying air layer. Chlorophyll concentration data are derived from the Seili automatic profiling monitoring buoy, and temperature data are based on a 10-day long-term forecast by the Norwegian Meteorological Institute. The GAM model for the index was fitted by PhD Joni Virta from the University of Turku/Aalto University, and the index visualization was implemented by the IT team at the University of Turku.
Note! Visualization of the Seili Index was restarted in October 2025. Data from previous years are not available. The buoy was placed in winter storage on 27 November 2025, and measurements will resume in spring 2026.
Open data license: The data on the https://saaristomeri.utu.fi website are licensed under the Creative Commons Attribution 4.0 International license (CC BY 4.0). To cite this data, please use the following format: ‘Seili Environmental Monitoring Programme, Seili-Index Portal. YEAR. Turku University/ Turku University of Applied Sciences, Turku. https://saaristomeri.utu.fi/seili-index/‘.”