Being ecosystem engineers, framework-forming Scleractinian Cold-water Corals provide habitat for thousands of deep-sea species, revealing equally remarkable levels of biodiversity as found in tropical Coral Reefs. Lophelia pertusa is the dominant reef-forming Cold-water Coral in the Atlantic, and based on its distribution correlated with ocean conditions, upper and lower tolerable limits for basic oceanographic parameters were proposed for this species. Among them, dissolved oxygen concentrations can exert control on its biogeographic distribution. However, lowest dissolved oxygen concentrations inhabited by this species apparently differs between the northeast Atlanatic, where it can tollerate levels as low as about 2 millilitres of oxygen per litre of seawater, and the northwest Atlanatic, where it can tollerate levels as low as about 3.7 millilitres of oxygen per litre of seawater. These observations are corroborated by laboratory experiments, revealing that Lophelia pertusa individuals collected from waters on the the Scottish margin in the northeast Atlantic, where there is a dissolved oxygen concentration of about 6 milligrams per litre of seawater, Atlantic, were unable to maintain normal aerobic functions at dissolved oxygen concentrations of less than 3.2 millilitres per litre of seawater. Moreover, for Lophelia pertusa specimens collected from areas of the Gulf of Mexico with dissolved oxygen concentrations of about 2.8 millilitres per litre of seawater, a 7-day exposure to a dissolved oxygen concentration of 1.5 millilitres per litre of seawater proved fatal. However, discoveries of Lophelia pertusa in the oxygen minimum zones of the subtropical eastern Atlantic have hinted at an even wider tolerance of Lophelia pertusa to low dissolved oxygen concentrations. Nevertheless, the limited capability of Lophelia pertusa to thrive under dissolved oxygen concentrations (artificially) reduced below those of their natural environment questions its ability to cope with the global change-induced ocean deoxygenation expected for the coming century.
In a paper published in the journal Coral Reefs on 6 April 2020, Dierk Hebbeln and Claudia Wienberg of the MARUM Center for Marine Environmental Sciences at the University of Bremen, Wolf-Christian Dullo of the GEOMAR Helmholtz Centre for Ocean Research, André Freiwald of the Marine Research Department at Senckenberg am Meer, Furu Mienis of the NIOZ Royal Netherlands Institute for Sea Research and Utrecht University, Covadonga Orejas of the Centro Oceanogra´fico de Baleares of the Instituto Español de Oceanografía, and Jürgen Titschack, also of the MARUM Center for Marine Environmental Sciences at the University of Bremen, and the Marine Research Department at Senckenberg am Meer, present the discovery of Lophelia pertusa-dominated Cold-water Coral reefs thriving in the hypoxic oxygen minimum zone off Angola in the southeast Atlantic. The regional adaptation of the Angolan Cold-water Corals to such extreme conditions sheds new light on their potential capability to cope with expected future environmental changes in the ocean.
During RV Meteor expedition M122 in January 2016, in situ oceanographic parameters such as dissolved oxygen concentrations and temperature were recorded off Angola. Data were collected during eight dives with the Remotely Operated Vehicle Marum Squid, carried out three benthic lander deployments, and 17 conventional conductivity, temperature, and depth instrument casts. The conductivity, temperature, and depth instrument was additionally equipped with a non-calibrated fluorescence sensor only providing relative values shown as means per water depth averaged from all conductivity, temperature, and depth instrument casts.
Multibeam bathymetry map showing the distribution of coldwater Coral reefs off Angola. Locations of conductivity, temperature, and depth (CTD) casts, benthic lander deployments, and remotely operated vehicle (ROV) dives are indicated. Hebbeln et al. (2020).
Remotely operated vehicle video observations revealed the presence of Cold-water Coral Reefs dominated by Lophelia pertusa, which colonise the slopes and summits of up to 100 m high Coral mounds. While dispersed Cold-water Coral colonies were found in a depth range of 250–500 m, large aggregates of healthy colonies were restricted to 330-470 m water depth. The observation of over 50 cm high colonies clearly evidenced the continuous proliferation of Cold-water Coral off Angola for many years.
Thriving Cold-water Corals observed in the oxygen minimum zone off Angola. (a), (b) Lophelia pertusa reefs in the center of the oxygen minimum zone (350 m water depth). (c) Transported but alive Lophelia pertusa colony in the lower oxygen minimum zone (500 m depth). (d) Lophelia pertusa colony with many living polyps (439 m depth) (ROV images). Hebbeln et al. (2020).
The available oceanographic data revealed water temperatures of 6.8–14.2°C around the Cold-water Corals at depths of 250–500 m. The corresponding dissolved oxygen concentrations of 0.6-1.5 millilitres per litre of seawater are the lowest ever obtained from waters bathing flourishing Lophelia pertusa colonies.
To gain insight into the seasonal variability of dissolved oxygen concentrationsoff Angola, as the M122 data only represent an 8.5-day snapshot from January 2016, Hebbeln et al. included further 21 conductivity, temperature, and depth instrumentcasts obtained within the mapped area off Angola between 1995 and 2013. These data, spanning from March to September, almost completely correspond to the M122 data or reveal even lower dissolved oxygen concentrations. Interestingly, even in this hypoxic environment, most prolific Cold-water Coral Reefs are bound to the center of the Angolan oxygen minimum zone where lowest dissolved oxygen concentrations prevail, which coincide with enhanced water-column fluorescence pointing to an increased availability of relatively fresh organic matter.
Based on field observations in the northwest and northeast Atlantic, the assumed lower limit of Lophelia pertusa’s oxygen tolerance ranges around 2-3.7 millilitres per litre of seawater. This has recently been challenged by very low dissolved oxygen concentrations of 1.1-1.4 millilitres per litre of seawater reported from Cold-water Coral sites off Mauritania, which, however, are associated with only sporadic occurrences of small Lophelia pertusa colonies. The new Angolan data documented for the first time Lophelia pertusa’s ability to develop thriving reefs even under dissolved oxygen concentrations of under 1 milligram per litre of seawater.
In addition, off Angola Lophelia pertusa lives at temperatures of up to 14.2° C, which are among the highest temperatures ever observed for this species. Thus, off Angola, the partly high temperatures could act as a second stressor since respiration rates of Lophelia pertusa increase with increasing temperature.
Stress induced by low dissolved oxygen concentrations and relatively high temperatures is energetically a challenge for the metabolism of most marine species, but can be compensated by the availability of large quantities of high-quality organic matter. The Angolan and Mauritanian margins belong to highly productive upwelling systems triggering extensive oxygen minimum zones. Also at many other Atlantic reef sites, Lophelia pertusa is most abundant at depth intervals with highest oxygen depletion, most likely linked to highest concentrations of suspended food particles in this layer, which also applies to Angola. Comparing ambient dissolved oxygen concentrations and temperature with site-specific net primary productivity, used as a food supply indicator, for several Atlantic Cold-water Coral sites, it appears plausible that the negative effects of hypoxia and high temperatures on Lophelia pertusa seemingly could be compensated by significantly enhanced food supply.
With respect to Lophelia pertusa preferring regional oxygen minima, ambient dissolved oxygen concentrations cannot provide any information about its capability to also cope with lower dissolved oxygen concentrations. However, some information is provided by the aforementioned laboratory experiments. Lophelia pertusa collected in the northeast Atlantic and the Gulf of Mexico could not withstand dissolved oxygen concentrations of less than 40–50% of the ambient values. Consequently, the range of low dissolved oxygen concentrations tolerable by Lophelia pertusa, also beyond its natural environment, might depend on the conditions the corals are acclimated to, thus pointing to a possible genotypic adaptive capacity of Lophelia pertusa. Thus, although on a global scale the tolerable dissolved oxygen concentration limits for Lophelia pertusa range from less than 1 to more than 6 millilitres per litre of seawater, smaller ranges define these limits on regional scales.
Cold-water Coral Reefs are vulnerable marine ecosystems that are partly protected within marine protected areas. These can safeguard Cold-water Corals from destructive Human impacts (e.g., bottom trawling, hydrocarbon exploration), but offer no sustainable protection against global change-induced threats. In concert with ocean acidification and warming of intermediate waters, eoxygenation is expected to become a major stressor for Cold-water Corals. However, Lophelia pertusa’s general capacity to thrive under well-oxygenated as well as hypoxic bottom waters reveals a rather high oxygen tolerance, although individual Lophelia pertusa populations appear to have limited adaptive capabilities to cope with reductions of 40–50% of ambient dissolved oxygen values. Consequently, the expected decrease in oxygenation of about 2% along the Atlantic continental margins by 2100 by itself might not exert a serious threat to Lophelia pertusa, except for already hypoxic settings like the Angolan margin. However, palaeotological studies revealed that during the last approximately 20 000 years regional changes in water column structure caused the collapse of Lophelia pertusa dominated ecosystems due to decreasing dissolved oxygen concentrations. Thus, unlike a small overall decrease in dissolved oxygen concentrations, major regional reductions in dissolved oxygen concentration driven by global change-induced changes in ocean circulation have the potential to eradicate regional Lophelia pertusa populations.
Even if smaller decreases in dissolved oxygen concentration alone might not pose a serious threat to Lophelia pertusa reefs, these have to be considered in concert with other changing environmental parameters that might form additional stressors (e.g. temperature and pH) with largely unknown consequences for the Coral’s biological functions. Moreover, the flux of particulate organic carbon from the surface ocean might decline by about 30% by 2100 along the Atlantic margins, resulting in a lower food supply to the Cold-water Corals and deep-sea organisms in general, thus reducing their capacity to cope with increasing stress.
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