Fluorophores in the Arabian Sea and their relationship to hydrographic conditions

 Water mass distributions, upwelling and filament formation are of complex significance for biological and chemical processes. Temperature and salinity are routinely used to characterize these hydrographic conditions. In situ profiles and sample analyses from the cruise So119 of RV Sonne in May 1997 into the Arabian Sea indicate that optical parameters such as the fluorescence of phytoplankton pigments, gelbstoff and proteins reflect hydrographic processes as well, and are useful to describe and explain biochemical processes. The dependence of bio-optical parameters to DOC and chlorophyll a concentrations is examined. While the relation between extracted chlorophyll and chlorophyll fluorescence is quite reasonable, DOC and gelbstoff fluorescence do not show a functional dependency. This is mainly due to sunlight-induced degradation of fluorophores in the surface layer. Enhanced gelbstoff signals in intermediate and deep waters are related to Persian Gulf Water. The optical data reveal a relation between the depth of the chlorophyll maximum, the depth of 1% surface light and the geographical latitude. In the context of remote sensing of phytoplankton biomass from ocean colour images, the depth of the chlorophyll maximum is a crucial parameter. Thus, the revealed dependency on latitude is useful for the evaluation of primary production with satellite remote sensing. Received: 22 May 2002 / Accepted: 18 February 2003 Responsible Editor: Andreas Oschlies Acknowledgements. This work was supported by a grant from the Federal Minister of Education and Technology, Bonn, within the framework of the JGOFS Arabian Sea program. We are grateful to the captain and the crew of RV Sonne for their support. Special thanks are given to the chief scientist of the cruise, Pr. V. Ittekkot, for helpful discussions. We are indebted to Mr. Rüdiger Heuermann and Mr. Nils Bürckel for their participation in the cruise and their support in the data analysis.