In a exclusively designed experiment optical data from the POSEIDON system Cretan Sea station, above water surface reflectance measurements and in situ chlorophyll-a profiles were collected and combined in order to evaluate the existing remote sensing chlorophyll retrieval algorithms. It was found that both SeaWiFS and MODIS global algorithms overestimate the chlorophyll concentration by >35%, but another more interesting finding was that the algorithms developed specifically for the Mediterranean, underestimate the concentration by a similar factor. Further investigation is under way.