Madrepora oculata forms large frameworks in hypoxic waters off Angola (SE Atlantic)

Abstract

This study aims to map the occurrence and distribution of Madrepora oculata and to quantify density and colony sizes across recently discovered coral mounds off Angola. Despite the fact that the Angolan populations of M. oculata thrive under extreme hypoxic conditions within the local oxygen minimum zone, they reveal colonies with remarkable heights of up to 1250 mm-which are the tallest colonies ever recorded for this species-and average densities of 0.53 ± 0.37 (SD) colonies m^-2. This is particularly noteworthy as these values are comparable to those documented in areas without any oxygen constraints. The results of this study show that the distribution pattern documented for M. oculata appear to be linked to the specific regional environmental conditions off Angola, which have been recorded in the direct vicinity of the thriving coral community. Additionally, an estimated average colony age of 95 ± 76 (SD) years (total estimated age range: 16-369 years) indicates relatively old M. oculata populations colonizing the Angolan coral mounds. Finally, the characteristics of the Angolan populations are benchmarked and discussed in the light of the existing knowledge on M. oculata gained from the North Atlantic and Mediterranean Sea.

Figure 1

Geographical distribution of Madrepora oculata and Madrepora carolina. The global map shows the distribution of the two reef-forming species of the genus Madrepora (sources: World Conservation Monitoring Centre of the United Nation Environmental Programme, UNEP-WCMC; Ocean Biogeographic Information System, OBIS). While a large number of records of M. oculata (yellow dots; live occurrences indicated by a red cross) is reported in the North Atlantic and the West and South Pacific, M. carolina (white dots) is endemic to the NW Atlantic. The red polygon highlights the study area off Angola in the SW Atlantic, where living M. oculata have been discovered for the first time. The map is based on the digital elevation model (DEM) CleanTOPO2 (an edited version of the SRTM30 Plus) published by Patterson (2006; http://www.shadedrelief.com/cleantopo2) and was created with ESRI ArcGIS Desktop 10.7.

Figure 2

Occurrence of Madrepora oculata on the Angolan cold-water coral mounds. (A) Overview map of the Angolan coral mound province. Coral mounds for which ROV video footage was analysed are marked by boxes. Grey boxes indicate Snake mound (1) and Anna ridge (2), for which no Madrepora records have been documented; red boxes indicate the Valentine (B), Buffalo (C), Castle (D) and Scary (E) mounds being colonised by M. oculata. (B–E) Detailed maps of the Valentine, Buffalo, Castle and Scary coral mounds. ROV video transects (white line) crossing the respective mounds and identified locations of living M. oculata (blue dots) are indicated. Bathymetry data was acquired during R/V Meteror cruise M122 (Raw data is available under https://www2.bsh.de/daten/DOD/Bathymetrie/Suedatlantik/m122.htm).

Figure 3

Scaling of large Madrepora oculata framework (Scary mound, Angola). The ROV used for video surveys was equipped with two line lasers. The parallel green lines are the projected laser beams, which are 30 cm apart (the distance between the beams is depicted by a white line) and are used as a scale. Credits: photograph by MARUM ROV SQUID.

Figure 4

Extraordinary large Madrepora oculata framework of 1.2 m height observed on the Scary mound off Angola. (a) Panoramic view of the coral framework. (b) Zenithal view of the coral structure showing the different growing layers of the framework. Blue arrows indicate the Lophelia pertusa colonies in the vicinity of the large M. oculata framework. Credits: photographs by MARUM ROV SQUID.

Figure 5

Coral colony size structure of the Angolan Madrepora oculata population. The number of M. oculata specimens for each defined colony size-class (given in mm) is depicted in this graph. They were documented for the Angolan mounds: (a) Castle, (b) Scary, (c) Buffalo, (d) Valentine, and (e) displays a compilation of all mounds.

Figure 6

Depth distribution of Madrepora oculata and the corresponding environmental envelope off Angola. The water-mass properties (i.e. dissolved oxygen concentrations (DO), temperatures and salinities) were recorded with the ROV-mounted CTD during video surveys across various Angolan coral mounds (see legend for colour code; data for DO and temperatures were originally published in Hebbeln et al.). Live colonies of M. oculata (diamond symbols) were only observed on Castle (n = 30), Scary (n = 8), Buffalo (n = 39), and Valentine (n = 6) mounds. Depth levels of live M. oculata occurrences are highlighted by horizontal grey bars.

Figure 7

Variability of colony dimensions and morphotypes of Madrepora oculata documented for different regions in the Atlantic Ocean and the Mediterranean Sea. (a) off Norway (ca. 20 km southwest of Stornest off mid-Norwegian shelf break), 453 m depth, bushy morphotype, (b) off Ireland (Porcupine Seabight, Belgica coral mound province), 808 m depth, bushy morphotype, (c) Gulf of Mexico (West-Florida slope), 520 m depth, fan-shape morphotype, (d) Gulf of Biscay (Petit Sole canyon), 900 m depth, cauliflower morphotype, (e) off the U.S. (Florida Straits, west off Bimini), 468 m depth, bushy morphotype, (f) off Mauritania (Tamxat coral mound complex), 396 m depth, bushy morphotype, (g) western Mediterranean Sea (Gulf of Lions, Lacaze-Duthiers canyon), 350 m depth, fan-shape morphotype, (h) western Mediterranean Sea (Gulf of Lions, Cap de Creus canyon), 190 m depth, cauliflower morphotype, (i) off Angola (Scary mound), 335 m depth (this study), massive fan-shape morphotype. Credits: (a) Pål Buhl-Mortensen, Institute of Marine Research, Norway; (b,c,e) MARUM ROV CHEROKEE; (d) Ifremer, ROV Victor6000, BobEco2011; (f) Tomas Lundälv, Tjärnö Marine Laboratory, University of Gothenburg; (g) Dierk Hebbeln, MARUM; (h) JAGO, IFM-GEOMAR & ICM-CSIC; (i) MARUM ROV SQUID.