POSEIDON is an operational marine monitoring, forecasting and information system for the Greek Seas. The observing component is a distributed infrastructure made by three coastal buoys (Saronikos buoy-SB, Heraklion Coastal Buoy-HCB and Athos buoy-AB) and one FerryBox (PFB). A calibration laboratory (see Chapter 2) is supporting the observing activities. The Athos buoy is equipped with meteo sensors and T, C/S, DO, fluorescence and turbidity sensors up to 100m in depth, is located in the coastal area in the Northern Aegean, representative of mesotrophic to oligotrophic conditions affected by circulation. Although coastal the depth of the water column allows deeper observations.