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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-52
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-52
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 25 Feb 2019

Submitted as: research article | 25 Feb 2019

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Biogeosciences (BG) and is expected to appear here in due course.

Environmental factors influencing cold-water coral ecosystems in the oxygen minimum zones on the Angolan and Namibian margins

Ulrike Hanz1, Claudia Wienberg2, Dierk Hebbeln2, Gerard Duineveld1, Marc Lavaleye1, Katriina Juva3, Wolf-Christian Dullo3, André Freiwald4, Leonardo Tamborrino2, Gert-Jan Reichart1,5, Sascha Flögel3, and Furu Mienis1 Ulrike Hanz et al.
  • 1NIOZ-Royal Netherlands Institute for Sea Research and Utrecht University, Department of Ocean Systems, Texel, 1797SH, the Netherlands
  • 2MARUM–Center for Marine Environmental Sciences, University of Bremen, Bremen,28359, Germany
  • 3GEOMAR Helmholtz Centre for Ocean Research, Kiel, 24148, Germany
  • 4Department for Marine Research, Senckenberg Institute, Wilhelmshaven,26382, Germany
  • 5Faculty of Geosciences, Earth Sciences Department, Utrecht University, Utrecht,3512JE, the Netherlands

Abstract. Fossil cold-water coral mounds overgrown by sponges and bryozoans were observed in anoxic conditions on the Namibian margin, while mounds colonized by thriving cold-water coral reefs were found in hypoxic conditions on the Angolan margin. These low oxygen conditions do not meet known environmental ranges favoring cold-water corals and hence are expected to provide unsuitable habitats for cold-water coral growth and therefore reef formation. To explain why the living fauna can nevertheless thrive in both areas, present day environmental conditions at the southwestern African margin were assessed. Downslope CTD transects and the deployment of bottom landers were used to investigate spatial and temporal variations of environmental properties. Temporal measurements in the mound areas recorded oscillating low dissolved oxygen concentrations of 0–0.17 ml l−1 (≙ 0–9 % saturation) on the Namibian and 0.5–1.5 ml l−1 (≙ 7–18 % saturation) on the Angolan margin, which were associated with relatively high temperatures (11.8 13.2 °C and 6.4–12.6 °C respectively). Semi-diurnal barotrophic tides were found to interact with the margin topography producing internal waves with excursions of up to 70 and 130 m for the Namibian and Angolan margins, respectively. These tidal movements temporarily deliver water with more suitable characteristics to the coral mounds from below and above the hypoxic zone. Concurrently, the delivery of high quantity and quality of suspended particulate organic matter was observed, which serves as a food source for cold-water corals. On the Namibian slope organic matter indicates a completely marine source and originates directly from the surface productive zone, whereas on the Angolan margin the geochemical signature of organic material suggest an additional mechanisms of food supply. A nepheloid layer observed above the cold-water coral mound area on the Angolan margin may constitutes a reservoir of fresh organic matter, facilitating a constant supply of food particles by tidal mixing. This suggests that the cold-water coral communities as well as the associated fauna may compensate unfavorable conditions induced by low oxygen levels and high temperatures with an enhanced availability of food. With the expected expansion of oxygen minimum zones in the future due to anthropogenic activities, this study provides an example on how ecosystems could cope with such extreme environmental conditions.

Ulrike Hanz et al.
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AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Interactive discussion
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement
Ulrike Hanz et al.
Ulrike Hanz et al.
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Short summary
Along the Namibian and Angolan margins low oxygen conditions do not meet environmental ranges for cold-water corals and hence are expected to be unsuitable habitats. Environmental conditions show that tidal movements deliver water with more oxygen and high quality organic matter, suggesting that corals compensate unfavorable conditions with availability of food. With the expected expansion of oxygen minimum zones in the future, this study provides an example how ecosystems cope with extremes.
Along the Namibian and Angolan margins low oxygen conditions do not meet environmental ranges...
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