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Abstract

Sporadic occurrences of milky turquoise waters visible in true colour satellite imagery in the Benguela upwelling system off Namibia are characterized by high reflectances due to enhanced scattering properties. Previous opinion was that the features are due to increased reflectances by elemental sulphur (S0) resulting from upwelling or eruptions of hydrogen sulphide-enriched waters.

In March/April 2003, a remotely sensed offshore plume was identified as a coccolithophore bloom for the first time in this region, followed by satellite imagery and intensively investigated using ship-borne measurements. Different methods were applied such as high-performance liquid chromatography (HPLC) pigment analysis, electronic disperse X-ray analysis, scanning electronic microscopy as well as spectral particle absorption and reflectance measurements. Ocean colour satellite data of the SeaWiFS and MERIS sensors were used to investigate the blooms. The SeaWiFS coccolithophore algorithm identified the bloom in March 2003. MERIS-derived spectral reflectances which were in a good agreement with in-situ measurements were used to study the occurrence of blooms in the first half of the year 2004. The observed offshore plumes can be optically divided into two groups. Far offshore the plumes are dominated by coccolithophores Emiliania huxleyi (Type-C) increasing the reflectance by scattering. Directly outside the active upwelling area the spectral shape of reflectance is affected by absorbing phytoplankton, but the magnitude is the result of scattering by coccolithophores and possibly diatom frustules. The coccolithophore bloom water was characterized by high calcite content, the occurrence of the marker pigment 19′- hexanoyloxyfucoxanthin as well as by a double maximum in the blue spectral range of the specific absorption of phytoplankton. The coccolithophore bloom develops as a phytoplankton succession during a low wind period in a stable stratified water body with a shallow top layer under high solar radiation with the starting conditions of low silicate and high N/P ratios. In the area off Namibia the sporadic occurrence of milky turquoise water may have two different causes. The features nearshore are due to sulphur discolouration but the further offshore plumes are caused by coccolithophores. The occurrence of coccolithophores is not as sporadic as the blooms which are visible in satellite data. These results improve the interpretation of milky turquoise water identified by ocean colour satellite data in the SE Atlantic Ocean off Namibia because they verified the occurrence of coccolithophores in that region for the first time with different methods.