Bio-optical properties of gelbstoff in the Arabian Sea at the onset of the southwest monsoon

As part of the German JGOFS Indian Ocean Programme, a cruise occurred in the Arabian Sea in May 1997 at the onset of the southwest monsoon. Data on gelbstoff, chlorophyll o and tryptophan-like fluorescence as well as gelbstoff absorption were obtained. They indicate, that these optical parameters support the interpretation of hydrographie and biogeochemical conditions. Tryptophan-like fluorescence seems to be a useful indicator of changes in the constitution of the biomass. A comparison of gelbstoff absorption and fluorescence data from the upper 100 m reveals only a weak covariance. Special emphasis is given to the fit procedure used for retrieving the exponential slope S of the spectral absorption coefficient. S values with a mean of 0.016 nm-1 are found in the 350 to 480 nm wavelength range. A mean slope S set to the frequently reported value of 0.014 nm-1 in the exponential description of gelbstoff absorption might lead to a systematic over/underestimation, and thus to systematic errors, if single-wavelength absorption values are extrapolated to other spectral regions on the basis of this parameter.