Time series of in situ optical data and concurrent SeaWiFS observations in the Cretan Sea are used to assess the phytoplankton dynamics in an area with Case 1 waters. The in situ data were collected during the pilot operation of the multi-parametric M3A buoy system in the Cretan Sea during 2000 and 2001. The synergy and limitations of the two different data sets are discussed. The optical properties in the area of interest are found to be highly correlated with local circulation dynamics. Finally, the value of these observa¬tions for the calibration and validation of ecosystem models is demonstrated.