The Upwelling Area Off Namibia, the Northern Part of the Benguela Current System

Abstract

In the area off Namibia satellite remote sensing data of the visible and infrared spectral range were used to investigate the upwelling processes and the biological response. Satellite derived Sea Surface Temperature was applied to study upwelling processes in relation to the driving trade winds. The investigations were focussed on the intensity and horizontal extent, the temporal and spatial variability including inter-annual and climate scales. Ocean colour satellite data allow investigation of the response of surface water to the nutrient input into the euphotic zone by upwelling processes. Observations in the area revealed, that the phytoplankton development starts with species absorbing light in the visible spectral range (diatoms and dinoflagellates) and may end in persistent shallow surface filaments with light scattering algae blooms. These blooms of coccolithophores identified by different in situ methods changed the water colour by strong particle scattering to milky turquoise discolorations. Further milky turquoise discolorations were identified as sulphur plumes. Low wind periods support the degradation of organic matter and the development of hydrogen sulphide in the bottom layer. After the onset of the trade winds and the offshore transport of surface water hydrogen sulphide enriched waters reaches with the near-bottom counter current the coast. Upwelling transports the water to the surface where the hydrogen sulphide will be oxidised to elemental sulphur. An algorithm was developed to identify and separate the sulphur plumes from algae blooms and other features on the basis of highly spectrally resolved satellite data from the MERIS sensor. The algorithm identified only coastal plumes as sulphur and that offshore plumes are formed by coccolithophores. The sulphur season is the boreal spring starting in February and reaching the maximum in April.