One of the major design goals of the Medium Resolution Imaging Spectrometer (MERIS) was the capability to use the signal from chlorophyll fluorescence stimulated by ambient sunlight to detect and map phytoplankton. This is considered tobe especially useful in coastal waters, where the determination of chlorophyll from water-leaving radiance spectra using the conventional blue/green ratio method is often complicated by high concentrations of gelbstoff and suspended matter. Based on a variety of studies, three spectral channels centred at 665, 681.25 and 705nm were included in the design of MERIS for retrieving the fluorescence signal. This paper presents observations with highresolution spectrometers which demonstrate the main factors affecting the observed signal in the red part of the spectrum. These factors are absorption by pure water, scattering by suspended particles, absorption and fluorescence of chlorophyll and the influence of submerged macrophyta. The influence of exceptional blooms such as 'red tides' on radiance spectra is also discussed. The paper shows how the combination of these effects can be understood using simple and easy-to-use radiative transfer models, and can be exploited by MERIS for improved mapping of phytoplankton, red tides and coastal, submerged and tidal flat vegetation.