The satellite data in red and near infrared light (1st and 2nd channels of AVHRR) were processed and compared with chlorophyll a concentration in the surface layer of the Gulf of Gdansk. Taking into account Stumpf and Tyler (1988) proposal, quantitative relation between this concentration and the value of Cij=R2/R1 (where R2 and R1 denotes reflectance coefficients in AVHRR's channel 1 and 2 respectively) has been calculated.