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

Abstract

<p><strong>Abstract.</strong> 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<sup>−1</sup> (≙ 0–9 % saturation) on the Namibian and 0.5–1.5 ml l<sup>−1</sup> (≙ 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.</p>

Climate‐induced changes in the suitable habitat of cold‐water corals and commercially important deep‐sea fishes in the North Atlantic

“…However, cold‐water corals may occur in undersaturated waters of high productivity, leading to the hypothesis that increased food supply may compensate to some degree for undersaturation by providing the additional energy necessary to survive (Baco et al, ; Ross, unpublished data; Thresher et al, ). Elevated food supply may also compensate for low dissolved oxygen concentrations (Hanz et al, ). However, in an environment with consistently scarce food or low oxygen concentration, the metabolic costs of calcifying in extremely low carbonate conditions may become prohibitively expensive, thus compromising coral survival (Carreiro‐Silva et al, ; Hennige et al, ; Maier et al, ).…”

Benthic foraminiferal faunas associated with cold‐water coral environments in the North Atlantic realm

“…The lower DO conc limit of D. pertusum in the North-west and North-east Atlantic has been assumed to be situated around 2-3.7 ml/l Dullo et al, 2008;Freiwald et al, 2009). However, D. pertusum has been observed living in the oxygen minimum zones of the subtropical eastern Atlantic (Colman et al, 2005;Le Guilloux et al, 2009), while thriving reefs develop within rather warm waters (6.8-14.2°C) and under hypoxic conditions (0.6-1.5 ml/l) in the oxygen minimum zone off Angola (Hanz et al, 2019;Hebbeln et al, 2020). These discoveries also contrast with laboratory experiments, where individual D. pertusum corals from the North-east Atlantic and the Gulf of Mexico could not survive DO conc lower than F I G U R E 1 3 Carbon flux (green line), bottom-water current strength (black dashed line) and bottom-water oxygenation (full red line) reconstructions based on the relative abundance of indicator species (Annex S5) for Lopphavet Reef (Stalder et al, 2014), Propeller Mound (Rüggeberg et al, 2007, the East Melilla Coral Mounds (Fentimen et al, 2020a) and the Santa Maria di Leuca Coral Mounds (Fink et al, 2012).…”

Influence of Water Masses on the Biodiversity and Biogeography of Deep-Sea Benthic Ecosystems in the North Atlantic

“…These different temporal scales drive different adaptations and tolerance levels in benthic taxa. Extensive permanent hypoxic zones, where oxygen is limiting for living organisms (Levin et al, 2001;Stramma et al, 2010), occur at midwater regions (100-1200 m depth), being present in the Atlantic only in the south basin, off Namibia and Angola (Hebbeln et al, 2017;Hanz et al, 2019). These oxygen minimum zones (OMZs; 0.2-0.5 ml•l −1 ) are formed by microbial degradation of organic matter beneath highly productive waters (Levin et al, 2001).…”

“…However, living colonies of calcified organisms such as the CWCs L. pertusa and M. oculata have been found in South Atlantic OMZs at DO levels of 0.6 and 1.1 ml•l −1 , respectively (Hebbeln et al, 2017;Hanz et al, 2019), drastically reducing the previous limiting DO levels obtained by modeling and experimentation (2.37 ml•l −1 ; Dodds et al, 2007;Davies et al, 2008). The effect of hypoxia on the occurrence of L. pertusa when food supply is high enough is difficult to be determined (Levin and Gage, 1998;Freiwald, 2002;White et al, 2005;Roberts et al, 2006).…”

“…The effect of hypoxia on the occurrence of L. pertusa when food supply is high enough is difficult to be determined (Levin and Gage, 1998;Freiwald, 2002;White et al, 2005;Roberts et al, 2006). Nevertheless, recent studies in these OMZs suggest that CWCs and the associated fauna may compensate unfavorable conditions induced by low DO and high temperatures with an enhanced food availability (Hanz et al, 2019). Experimental research revealed that L. pertusa maintains constant rates of oxygen consumption over a range of DO concentrations, and colonies are able to survive short-time periods of anoxia (1 h) and hypoxia (up to 96 h; Dodds et al, 2007).…”