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Systematic investigation of the spatiotemporal variability of TOXIC hydrogen sulphide events and their potential RISKs for the Namibian fishing and aquaculture industry

Periodic Reporting for period 1 - TOXIC-RISK (Systematic investigation of the spatiotemporal variability of TOXIC hydrogen sulphide events and their potential RISKs for the Namibian fishing and aquaculture industry)

Reporting period: 2016-07-01 to 2018-06-30

The Benguela upwelling system sustains a very high primary production due to the upwelling of nutriment rich waters forced by trade winds. Due to the subsequent export production and the intense remineralisation, an oxygen minimum zone is present in the northern Benguela upwelling system. Hydrogen sulphide (H2S) outbreaks and their sulphur (S0) plumes are unique events in this region (Fig. 1). They influence the marine ecosystem due to their toxic effects, have direct impacts on the biogeochemical cycles and are able to affect the human life. The local fish and shellfish industry, an important Namibian economic factor, is threatened. H2S-outbreaks can cause the mass mortality of commercially important fishes, oysters, shrimps and prawns.

The TOXIC-RISK project contributed to the understanding of the H2S-outbreaks and their S0-plumes. The overall objective was the investigation of the variability of S0-plumes.

Conclusions (Ohde & Dadou, 2018, PLoSONE, e0192140. https://doi.org/10.1371/journal.pone.0192140): “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 (ABFZ), the seasonal maximum of water mass fractions of South Atlantic and Angola Gyre Central Waters (SACW and AGCW) 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 ABFZ, increased mass fraction of South Atlantic Central Water and stronger downwelling coastal trapped waves.”
In the initial phase the identification tool for S0-plumes was developed and validated. In the middle and end phase all data sets were processed and evaluated. One student from the University of Toulouse was trained in programming, numerical methods, and remote sensing. A field campaign with students took place at the Mediterranean coast. They were taught to measure different environmental variables. All results of the project were presented at the Benguela Symposium in Cape Town in 2016 (www.ma-re.uct.ac.za/benguela2016) the 3rd International Ocean Colour Science Meeting in Lisbon in 2017 (iocs.ioccg.org/iocs-2017-meeting) the Ocean Science Meeting in Portland in 2018 (osm.agu.org/2018) and the European Geosciences Union in Vienna in 2018 (www.egu2018.eu). The results were published in PlosONE (Ohde & Dadou, 2018).

Main results (Ohde & Dadou, 2018): “Our study illustrates that the main season is at the same time of increasing equatorward alongshore winds, so before the main upwelling season. Consequently, the coastal upwelling and offshore Ekman transport are increased which enhances the probability of the upwelling of H2S enriched bottom water. Furthermore, the activity of sulphur plumes is high during the seasonal oxygen minimum time period supporting the accumulation of H2S in the bottom water layer. The oxygen level in the bottom water is controlled by different local and remote-driven processes. Therefore, the main season of sulphur plumes is observed at the same time of the seasonal reduction of cross-shore ventilation of the bottom waters, the seasonal southernmost position of the ABFZ, the seasonal maximum of mass fractions of SACW and AGCW (sub-type of SACW) as well as the seasonal arrival of the downwelling coastal trapped waves. In contrast, the off-season is found during the upwelling season with maximum upwelling intensity and enhanced oxygen supply on the Namibian shelf. This higher oxygen level is associated with an increased ventilation of the bottom water by local processes (Ekman recirculation, mixing) as well as remote driven processes with the equatorward position of the ABFZ, the seasonal minimum of SACW mass fraction and upwelling coastal trapped waves.
… We conclude that the annual variability of sulphur plumes is characterized by years with very high activity interrupted by periods of lower activities ... For the first time this study highlights that the annual variability can be explained by the superposition of local and remote-driven processes. The sulphur plume activity is high in years with decreased yearly mean of the equatorward alongshore winds and increased coastal SST compared to normal years. It means years with a lower yearly mean of upwelling intensity probably supporting the deceleration of the depletion of the yearly H2S pool. There is a higher probability for an enhanced sulphur plume activity in years with a decreased oxygen supply promoting the formation of H2S. The years with higher sulphur plume activity are associated with decreased lateral ventilation of bottom waters by local processes as mixing and Ekman recirculation. Furthermore, the remote-driven processes like the displacement of the ABFZ, the interaction of SACW with the ESACW and the coastal trapped waves play also a strong role for the oxygen water content on the Namibian Benguela shelf. Therefore, the probability of the occurrence of strong sulphur plume activity is enhanced in years with a more southern position of the ABFZ, an increased mass fraction of SACW and stronger downwelling coastal trapped waves.”

The TOXIC-RISK project guarantees the exploitation of the results to the end-users by their open-access publication:
1. PANGAEA, doi.pangaea.de/10.1594/PANGAEA.887243
2. PlosONE, doi.org/10.1371/journal.pone.0192140

The results were disseminated in:
1. PlosONE, doi.org/10.1371/journal.pone.0192140
2. HAL, hal.archives-ouvertes.fr/hal-01713215
3. ZENODO, www.zenodo.org
4. ResearchGate, www.researchgate.net/project/Systematic-investigation-of-the-spatiotemporal-variability-of-TOXIC-hydrogen-sulphide-events-and-their-potential-RISKs-for-the-Namibian-fishing-and-aquaculture-industry-TOXIC-RISK
5. IOCCG, ioccg.org/2018/05/may-201
6. CNRS, www.insu.cnrs.fr/node/9051
7. LEGOS, www.legos.obs-mip.fr/actualites/actualites-scientifiques/emissions-toxiques-de-sulfure-dhydrogene-detectees-par-satellite-et-leur-forcage-en-milieu-cotier www.legos.obs-mip.fr/members/ohde/toxic_risk_project/overview
For the first time the seasonal and annual variability of S0-plumes were determined. The dominant role of local forcing as well as remote forcing were demonstrated. Their complex interplay provides the variability of S0-plumes. The TOXIC-RISK results are a major step forward to forecast such events and to develop a warning system.