Seasonal and annual variability of coastal sulphur plumes in the northern Benguela upwelling system

Abstract

We investigated the seasonal and annual variability of surface sulphur plumes in the northern Benguela upwelling system off Namibia because of their significant impacts on the marine ecosystem, fishing industry, aquaculture farming and tourism due to their toxic properties. We identified the sulphur plumes in ocean colour satellite data of the medium resolution imaging spectrometer (MERIS) for the 2002–2012 time period using the differences in the spectral properties of Namibian Benguela optical water types. The sulphur events have a strong seasonal cycle with pronounced main and off-seasons forced by local and remote-driven processes. The main peak season is in late austral summer and early austral autumn at the beginning of the annual upwelling cycle caused by increasing equatorwards alongshore winds. The sulphur plume activity is high between February and April during the seasonal oxygen minimum associated with the seasonal reduction of cross-shore ventilation of the bottom waters, the seasonal southernmost position of the Angola Benguela Frontal Zone, the seasonal maximum of water mass fractions of South Atlantic and Angola Gyre Central Waters as well as the seasonal arrival of the downwelling coastal trapped waves. The off-season is in austral spring and early austral summer during increased upwelling intensity and enhanced oxygen supply. The annual variability of sulphur events is characterized by very high activities in years 2004, 2005 and 2010 interrupted by periods of lower activity in years 2002 to 2003, 2006 to 2009 and 2011 to 2012. This result can be explained by the relative contributions or adding effects of local and remote-driven forces (from the equatorial area). The probability for the occurrence of sulphur plumes is enhanced in years with a lower annual mean of upwelling intensity, decreased oxygen supply associated with decreased lateral ventilation of bottom waters, more southern position of the Angola Benguela Frontal Zone, increased mass fraction of South Atlantic Central Water and stronger downwelling coastal trapped waves. Understanding of the variability and forcing processes of the toxic sulphur events will help in the future to monitor and forecast them as well as to manage their social and economic consequences in the northern Benguela upwelling system off Namibia.

Coastal Sulfur Plumes off Peru During El Niño, La Niña, and Neutral Phases

“…The algorithm delivers the daily spatial extensions (size) of the sulfur plumes in the upper surface water layer. Sulfur concentrations lower than 631 nM cannot be detected with this algorithm (Ohde & Dadou, ). Note that the subsurface plumes can be overlooked by this method.…”

Natural, incidental, and engineered nanomaterials and their impacts on the Earth system

“…New ocean-based natural nanomaterial-related ocean processes are now being discovered. For example, H 2 S gas is released from sediments off the coast of Namibia on a seasonal basis to surface waters, where it is oxidized by microbes to nanoparticulate S 8 that is distributed over thousands of square kilometers as observed by satellite (63). The water column becomes oxygen deficient, which negatively affects sea life.…”

N2O Emissions From the Northern Benguela Upwelling System

“…However, a drawback of their estimates is the lack of near-coastal data as presented in this study. Given the widespread occurrence of sulphidic events in the BUS (Ohde & Dadou, 2018), it could be that such conditions enhance surface fluxes of N 2 O as bottom waters are advected/mixed toward the surface. Löscher et al (2016) reported extreme N 2 O concentrations at the fringes of bottom H 2 S-containing waters on the Peruvian shelf.…”

“…Löscher et al (2016) reported extreme N 2 O concentrations at the fringes of bottom H 2 S-containing waters on the Peruvian shelf. Given the widespread occurrence of sulphidic events in the BUS (Ohde & Dadou, 2018), it could be that such conditions enhance surface fluxes of N 2 O as bottom waters are advected/mixed toward the surface. Nonetheless, we did not account for the seasonal-intraseasonal variability of upwelling, adding to the uncertainties of our annual efflux of N 2 O.…”