The Sun-stimulated chlorophyll fluorescence is a small but significant property of phytoplankton which can be detected using remote-sensing techniques. Besides the influence of oceanic properties, chlorophyll fluorescence is masked by atmospheric extinction. While an increase in chlorophyll concentration of 1 mg/m3causes an increase in the upwelling radiances of about 0·03Wm−2sr−1μm−1just above the water surface and due to the chlorophyll fluorescence, the upward radiances measured at λF= 685nm and at the top of the atmosphere ranges from 8 to 20Wm−2sr−1μm−1for realistic atmospheric turbidity variations and a solar zenith distance of Θs= 50·7°. Additionally, the fluorescence, peaking at λF= 685nm with a half-width of about 10 nm, is reduced by the absorption of O2and H2O. However, the fluorescence signal is nearly unaffected, when wavelengths λ≥686nm are exluded and a spectral interval of ΔλF= 5nm is used for the radiance measurements.