Automated monitoring of water quality

In the middle region of  the Archipelago Sea, an automated monitoring buoy has been in operation since 2006. The purpose of the monitoring is to collect detailed information on sea water quality, vertical stratification, and seasonal and interannual changes of the sea. At-sea monitoring is important for understanding the state of the sea, and is of fundamental importance to environmental management.  Long-term observations also reveal trends and patterns that can help interpret experimental results or yield new research hypotheses. 

The station is composed of a YSI 6952 buoy base and a multiparameter sonde YSI 6000, which measures salinity, temperature, dissolved oxygen, turbidity, chlorophyll-α, and blue-green algae content. The station makes four depth measuring profiles from the water column per day. The winch moves the measuring device vertically in the water column and the measurements are taken at depth stages of one meter in a depth area extending from 40 m depth to 2 m below the surface. The station is installed to operate in the spring as soon as the ice allows and is removed in early winter before the ice comes. In 2015, The Archipelago Research Institute also added a weather station to the station, which measures air temperature, pressure, humidity, precipitation, wind speed and direction to the buoy.

Here, data from 2011 are provided as graphs. The graphs can be scaled, zoomed and saved, and a desired section of the data can be downloaded as a csv-file. The graph visualizations are created by University of Turku, Research IT.

The monitoring is carried out by the Turku University of Applied Sciences in collaboration with the Archipelago Research Institute of the University of Turku, the Finnish Meterological Institute (FMI) and other FINMARI partners.


Select the year from which the graphs will be displayed.

Temperature [°C] Sea water temperature from the surface to 40 m depth. The temperature begins to rise in May and concurrently thermal stratification begins to develop. The stratification is at its strongest condition in late summer when the surface mean temperature can exceed 20 °C and the deep water is ca 6-7 °C. The transition layer between warmer mixed water at the sea surface and cooler deep water below,  i.e., a layer in which the water temperature drops rapidly, is called a thermocline. In the autumn, the surface water slowly cools down and eventually the thermocline disappears: the whole water column is then mixed by autumn storms and convection.

Salinity [‰] Sea water salinity in parts per thousand (‰). In the Archipelago Sea, the salinity usually varies between 5,7 and 6,2‰. The more saline water is more dense and, therefore, the heavier saline water sinks to the bottom of the water column. In the Baltic Sea, the water column also has a vertical salinity gradient as well as a temperature gradient. In more deeper areas, a transition layer where salinity changes rapidly, i.e. a halocline, exists at ca. 60-80 m depth. However, in the Archipelago Sea, the halocline is very weak or non-existent.

Oxygen [mg/L] Dissolved oxygen content in the water column. Seasonal changes affect dissolved oxygen concentrations. Cold water can hold more dissolved oxygen than warm water, therefore the oxygen content is higher during the cold water period. In summer,due to thermal stratification, the near-bottom oxygen concentrations can be quite low. Warmer temperatures also speed up the photosynthetic rate’s in phytoplankton, which can be observed as elevated oxygen concentrations in the surface especially during phytoplankton blooms in April-May and late summer.

Chlorophyll-a [ug/L] Reflects the biomass of microscopic algae in the water column. Planktonic algae require sunlight for photosynthesis, therefore their abundance is the highest in the surface layer. The highest chlorophyll concentrations are measured during phytoplankton blooms.

BGA [cells/mL] Blue-Green Algae (cyanobacteria) concentration in the water column. BGA also require sunlight for photosynthesis. The concentration of BGA is the highest in late summer. During calm wind conditions, BGA can increase to very high densities.

Turbidity [NTU] Clarity of sea water and the content of material that causes water to be turbid. In general, the turbidity of sea water increases towards the bottom of the water column. Material that causes water to be turbid include clay, silt, finely divided inorganic and organic matter, algae, soluble colored organic compounds, and plankton and other microscopic organisms.

Open data licence: The data at https://saaristomeri.utu.fi website is licenced with The Creative Commons Attribution 4.0 International license (CC BY 4.0). “Seili Environmental monitoring programme, Odas profiling buoy datasets. YEAR. Turku University/ Turku University of Applied sciences, Turku. https://saaristomeri.utu.fi/odas_en/”.