Interannual variability of sulphur plumes off the Namibian coast

Abstract

Coastal sulphur plumes in the upper water layer are frequently observed off the Namibian coast. However, their temporal and spatial development, strength, size and impact on marine life differ at a wide range. This study compares the events of years 2007/2008 to the years 2004/2005 along the Namibian coast, including regional features on the basis of remote sensing satellite data, in situ measurements of a mooring and local observations. The remotely sensed derived intensity of coastal sulphur plumes of year 2008 was very weak compared to 2004 and especially to 2005, the year with the highest activity during the study period. In 2008, the overall maximum of sulphur plumes was observed in February, about 2 months earlier than in 2004 and 2005 when highest intensity was detected in April. Locally, differences in timing and strength of sulphur plumes occurred in the area of the Walvis Bay. There, the maximum intensity was observed in March 2004 and in April 2005, but in 2008 no clear maximum was found. The remotely sensed identified coastal sulphur plumes correlate with periods of low oxygen concentrations in bottom water and high fraction of South Atlantic central water (SACW) at a mooring 20 nautical miles off the central Namibian coast. An exception is the area in the vicinity of the Walvis Bay lagoon that seems to be driven by different local dynamics. The total variability of the sulphur plumes could not be explained by the observed oceanographic conditions alone. Additionally, microbiological and chemical processes in the sediment and the bottom water layer should be included in future investigations related to annual and interannual variability. Disastrous local events were observed in the year 2008 although the averaged intensity was low compared to the years 2004/2005. Therefore, the remotely sensed derived averaged intensity gives probably no real impression of the local impact of sulphur plumes on marine life. A combination of all available methods like the detection by local observations, in situ measurements and remote sensing approaches is required.