Abstract
Abstract. Thriving benthic communities were observed in the oxygen minimum zones along the southwestern African margin. On the Namibian margin, fossil cold-water coral mounds were overgrown by sponges and bryozoans, while the Angolan margin was characterized by cold-water coral mounds covered by a living coral reef. To explore why benthic communities differ in both areas, present-day environmental conditions were assessed, using conductivity–temperature–depth (CTD) transects and bottom landers to investigate spatial and temporal variations of environmental properties. Near-bottom measurements recorded low dissolved oxygen concentrations on the Namibian margin of 0–0.15 mL L−1 (≜0 %–9 % saturation) and on the Angolan margin of 0.5–1.5 mL L−1 (≜7 %–18 % saturation), which were associated with relatively high temperatures (11.8–13.2 ∘C and 6.4–12.6 ∘C, respectively). Semidiurnal barotropic tides were found to interact with the margin topography producing internal waves. These tidal movements deliver water with more suitable characteristics to the benthic communities from below and above the zone of low oxygen. Concurrently, the delivery of a high quantity and quality of organic matter was observed, being an important food source for the benthic fauna. On the Namibian margin, organic matter originated directly from the surface productive zone, whereas on the Angolan margin the geochemical signature of organic matter suggested an additional mechanism of food supply. A nepheloid layer observed above the cold-water corals may constitute a reservoir of organic matter, facilitating a constant supply of food particles by tidal mixing. Our data suggest that the benthic fauna on the Namibian margin, as well as the cold-water coral communities on the Angolan margin, may compensate for unfavorable conditions of low oxygen levels and high temperatures with enhanced availability of food, while anoxic conditions on the Namibian margin are at present a limiting factor for cold-water coral growth. This study provides an example of how benthic ecosystems cope with such extreme environmental conditions since it is expected that oxygen minimum zones will expand in the future due to anthropogenic activities.
Highlights
Cold-water corals (CWCs) form 3-D structures in the deep sea, providing important habitats for dense aggregations of sessile and mobile organisms ranging from mega- to macrofauna (Henry and Roberts, 2007; van Soest et al, 2007) and fish (Costello et al, 2005)
Different environmental properties explain the present conditions of the benthic communities on the southwestern African margin including temperature, dissolved oxygen (DO) concentration, food supply and tidal movements
The DO concentrations probably define the limits of a suitable habitat for CWCs along the Namibian and the Angolan margin, whereas high temperatures constitute additional stress by increasing the respiration rate and energy demand
Summary
Cold-water corals (CWCs) form 3-D structures in the deep sea, providing important habitats for dense aggregations of sessile and mobile organisms ranging from mega- to macrofauna (Henry and Roberts, 2007; van Soest et al, 2007) and fish (Costello et al, 2005). Some framework-forming scleractinian species, with Lophelia pertusa and Madrepora oculata being the most common species in the Atlantic Ocean (Freiwald et al, 2004; White et al, 2005; Roberts et al, 2006; Cairns, 2007), are capable of forming large elevated seabed structures, so-called coral mounds (Wilson, 1979; Wienberg and Titschack, 2017; Titschack et al, 2015; De Haas et al, 2009) These coral mounds, consisting of coral debris and hemipelagic sediments, commonly reach heights between 20 and 100 m and can be several kilometers in diameter. They are widely distributed along the North Atlantic margins, being mainly restricted to water depths between 200 and 1000 m, while records of single colonies of L. pertusa are reported from a broader depth range of 50–4000 m depth (Roberts et al, 2006; Hebbeln et al, 2014; Davies et al, 2008; Mortensen et al, 2001; Freiwald et al, 2004; Freiwald, 2002; Grasmueck et al, 2006; Wheeler et al, 2007)
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