In early 2016, during the ANNA expedition with the German research vessel METEOR, video observations from the seabed made by the remotely operated vehicle (ROV) SQUID revealed the presence of thriving cold-water coral reefs in 250-500 m water depth in the south-east Atlantic. Perhaps not an unusual occurrence for a continental margin setting, but a study recently published in the scientific journal Coral Reefs reports that these reefs are thriving within the very strong oxygen minimum zone off Angola that develops due to the very high productivity in the surface waters – conditions that traditionally would have been dismissed as unsuitable for coral growth.
Despite this setting, the cold-water coral reefs host an impressive biodiversity, as is observed for many reefs in more hospitable conditions in the Atlantic. Mainly built up by Lophelia pertusa, the dominant reef-forming cold-water coral in the Atlantic, these reefs thrive in a rather warm, strongly oxygen depleted setting. According to textbook knowledge, the oxygen concentrations of <1 mL L-1 observed in this area should preclude any coral occurrences in this region – proving once again that our knowledge about the secrets of the deep sea is still very limited.
Apparently well adapted to these conditions, the Angolan corals obviously have different preferences for ambient oxygen conditions compared to other known Lophelia occurrences in the North Atlantic and contradicts our knowledge from laboratory experiments to date. The Angolan corals’ ability to cope with the low oxygen conditions may be bolstered by a very high food supply provided by the south-east Atlantic upwelling system, but there are obviously regional differences in the lower range of oxygen levels that the corals can withstand.
Upper and lower tolerable limits of basic oceanographic boundary conditions for Lophelia are usually determined based on presence/absence observations correlated to ocean conditions and laboratory data, and then applied at a global scale. Such global ecophysiological ranges are frequently applied in habitat suitability models to identify suitable environmental settings for corals in the present-day ocean, and under future ocean conditions, taking into account the expected changes to relevant oceanographic boundary conditions. However, putting all presence observations of Lophelia with respect to oxygen together reveals a global range of <1 to >6 mL L-1, which would not apply to all regional populations. Thus, for future studies applying such models, regional adaptations to oxygen levels, as well as the relationships between stressors (as documented here for oxygen levels and food supply) should be considered. This work has significant implications for iAtlantic’s work on stressors and tipping points.
The paper summarising the results of this study is open access and available online:
Hebbeln, D., Wienberg, C., Dullo, W. et al. Cold-water coral reefs thriving under hypoxia. Coral Reefs (2020). DOI: 10.1007/s00338-020-01934-6