Identification and investigation of sulphur plumes along the Namibian coast using the MERIS sensor

• Published in: Continental shelf research Vol. 27; no. 6; pp. 744 - 756
• Main Authors: Ohde, Thomas, Siegel, Herbert, Reißmann, Jan, Gerth, Monika
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.03.2007
• Subjects: Colloidal sulphur; Hydrogen sulphide; MERIS; Ocean colour remote sensing; Upwelling area off Namibia; PSW, Namibia; Hydrogen sulphide; MERIS; Colloidal sulphur; Upwelling area off Namibia; Ocean colour remote sensing
• ISSN: 0278-4343; 1873-6955
• DOI: 10.1016/j.csr.2006.11.016

In the upwelling area along the Namibian coast of SW-Africa sulphur discolorations were investigated to study the impact of hydrogen sulphide on the ecosystem using satellite imagery. The formation of colloidal sulphur in the upper water layer results from the oxidation of hydrogen sulphide. The occurrence of sulphur plumes as well as their temporal and spatial development was investigated in relation to the driving meteorological and oceanographic conditions. Because of the sporadic occurrence of sulphur events and the limited number of ship-borne investigations in that area remote sensing of ocean colour is the only method to follow these phenomena continuously and synoptically. In the past the sulphur plumes were studied by true colour images derived from ocean colour satellite data like sea-viewing wide field of view sensor or moderate resolution imaging spectroradiometer and identified by their typical milky turquoise discoloration. For the first time, medium resolution imaging spectrometer (MERIS) reduced resolution Level-2 products were applied to identify sulphur discoloration in the surface water off Namibia. Based on their high spectral resolution typical spectral water-leaving reflectances were identified for sulphur enriched waters and distinguished from other optical water types dominated by absorbing or scattering phytoplankton groups and suspended matter. This was the basis for the development of a classification algorithm for the identification of sulphur plumes. This algorithm was derived on available MERIS scenes from the first half of the year 2004 and extended to summer 2005 to study the occurrence, the temporal and spatial development, the extension, and the strength of such events as well as inter-annual differences in these years. Only near-shore sulphur occurrences were identified in the considered time period compared to other studies. A lifetime of sulphur patches between 1 and 6 days and a zonal extent of up to 21 km were determined. The patches propagate in north-westerly direction with a current velocity of approximately 14 cm s −1. The strongest events were detected in the Meob- and Saint Francis Bay in April 2004 and in April 2005.