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Depth profiles of fluorescence at several excitation and emission wavelengths were measured along with CTD data during the cruise So119 of RV Sonne in the Arabian Sea from 12 May to 10 June 1997. In addition to chlorophyll fluorescence from phytoplankton in the near-surface layer, the profiles in the oxygen minimum region well below the euphotic zone show enhanced red fluorescence. Red fluorescence intensity is inversely related to the oxygen concentration in intermediate and deep waters. A relationship to characteristic water masses of the region cannot be found in the data, and this holds also with chemical data such as DOC. Absorbance spectra of water samples taken in the oxygen minimum zone show an absorption band at 420 nm wavelength with about 50 nm bandwidth, much weaker than gelbstoff absorbance in the same wavelength range. The absorption band remains stable after sample filtration with 0.45 lm glass fibre filters. Hence, the size of the absorbing matter is in the range of dissolved molecules or particles much smaller than 1 lm. Fluorescence spectra of unfiltered samples with 420 nm excitation show a weak emission band at 600 nm and a more pronounced one at 660 nm wavelength. The trailing edge of the 660 nm band falls into the range of chlorophyll emission, thus giving rise to the observed depth profiles of red fluorescence in the oxygen minimum zone. Red fluorescence measured on samples remain stable during a few hours after sampling even in the presence of oxygen. It is not detectable after several weeks of sample storage in the dark and cannot be reproduced even after depletion of dissolved oxygen.

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Solar radiation is the energy source for all photosynthetic life-forms. Due to their individual pigment compositions only a specific part of the spectrum can be used by the single species for photosynthesis. Therefore multispectral radiation measurements are helpful for their investigation and monitoring. With the background of increasing ultraviolet radiation due to the decreasing ozone layer in the atmosphere, the measurement of the UV-A and UV-B part of the spectrum is of increasing interest to people. In the RAMSES project (Radiation Measurement Sensor with Enhanced Spectral Resolution), founded by the German Ministry of Research and Technology, a new subminiature multispectral radiometer is under development. This new instrument allows spectral measurements in the range from 250 to 720 urn. A modular concept of the instrument, combined with very low power consumption, allows to use the instrument in profiling systems as well as in stand-alone applications, such as moorings, monitoring systems or weather stations. Sensor heads with different detection characteristics are available, and up to 16 of them can be combined to a single instrument for simultaneous measurement, e.g for up-welling and down-welling light. Typical applications of this instrument are satellite calibration and validation, UV measurements, biology, ecosystem modelling, water quality and even climatology.

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