Physical description of the area
Data collected from the vicinity of the L. pertusa collection site (Fig. 1) coincided with an area of systematically high temperature (4.13–5.03 °C) and salinity (34.90–34.98) (Fig. 4), which is the known signature of the Atlantic Water and its modifications. Water density was between 27.62 and 27.71 (Fig. 4). In December, the data are spread through the full range of the variables (Fig. 4). The range of variability shown in the temperature–salinity (T–S) plots was particularly wide in the upper layer as expected due to the presence of both Atlantic and Polar Waters and significantly narrowed below 800 m. When viewed by year, there is a strong warming trend in the T–S plots (Fig. 5) with warmer and more saline water appearing in recent years.
There was a strong seasonal cycle through the water column that was most pronounced in temperature (Fig. 6). The temperature minimum was observed at the end of winter to early spring (March–April), right at the end of the cooling season or weeks following it (Fig. 6), suggesting that water was vertically mixed locally through winter-time convection and penetrated to the bottom or stopped not far from it. This mixing will ventilate the environment while the water stays fairly warm—although slightly cooled as a result of vertical/convective mixing, it was still 4.5 °C in winter, providing oxygen, nutrients and chlorophyll (that are known to peak following winter mixing). In June, there was a second dip in temperature below 600 m (Figs. 6 and 7) that was paralleled in salinity (Fig. 7, 700–900 m), creating colder fresher water at depth (Fig. 7, 700–900 m). This is cold intermediate water that was horizontally transported (it did not appear in the upper layers) possibly coming from as far as Cape Farewell where tip-jet wind is known to cause cooling (Moore and Renfrew 2005). The time required for the water to travel from the southern tip of Greenland to the site at the speed of 5–10 cm s−1 was consistent with this conclusion (Yashayaev and Seidov 2015). The water properties in the vicinity of the L. pertusa are summarized in Fig. 7. At this depth, there has been strong inter-annual change as well as seasonal variability. However, the range of seasonal variation is similar to the inter-annual variation, putting the latter into perspective (Fig. 7).
Inter-annual variation in temperature was more pronounced in the deeper waters than at the surface with average temperature varying within a degree over the time period (Fig. 8). In the 1990s, the water over the reef area was colder (Fig. 8) and fresher (not shown) than in recent decades. Despite annual and inter-annual changes, both temperature and salinity at 600 m and deeper appeared very stable and consistently high throughout the record (Fig. 9), which is likely associated with continual feed of water of Atlantic origin, ISW in particular.
The displacements between consecutive positions of individual Argo floats from a 13-year period, between 2001 and 2013, provided a detailed view of mid-depth (~1000 m) circulation around the southern tip of Greenland (Fig. 10). The resulting velocity fields revealed the intensified boundary current along the continental slope and recirculation of this water toward the Irminger Sea. The current at the location of the L. pertusa reef was remarkably strong (10–15 cm s−1) and persistent (standard errors much smaller than mean currents).
The topography of the photographic transect was dominated by the very steep continental slope, and it was not possible for the vessel sounder to make a clear picture of the slope area. The current was very strong, and the camera was nearly lost due to entanglement. Pictures were obtained in the depth interval from 670 to 1047 m ascertained through matching the time stamp on the photographs with that of the depth reading from the Sea-Bird recorder. A total of 96 photographs were taken, of these 36 were without information, 20 contained some information and 40 photographs were considered good (containing information in approximately 50 % or more of the image). The rough and steep nature of the slope made the interval between photographs rather arbitrary, ranging from 0 to 59 m. The average temperature recorded from the Sea-Bird recorder attached to the camera housing was 4.86 °C with a minimum temperature of 4.83 °C and a maximum of 4.92 °C. Those temperatures were consistent with those of the separately collected CTD profiles analyzed above (Fig. 4).
Live L. pertusa colonies forming a reef-like structure (Fig. 11) were observed at 4 locations on one camera transect (CON135; Fig. 3) at depths of 911, 922, 931 and 932 m. Dead L. pertusa were noted at two other locations along the same transect, at 886 and 907 m, respectively. The habitat surrounding the reef was dominated by sponges and other corals (Fig. 11). L. pertusa colonies were not seen in photographs from the second transect, CON134 (Fig. 3).
The reefs had the form of rounded, protruding structures with extensive development of the interior dead parts and with living L. pertusa on the crust. They seemed to reach sizes of at least 1 m, consistent with the presence of a bioherm.
The single sample obtained was a 25-cm large fragment from the surface of a L. pertusa colony (Fig. 2). One side was covered by numerous (>100) living, light reddish polyps (Fig. 2). They were of different diameters with all stages being represented, from initial intra-tentacular buds to separated small polyps still recognizable as daughter polyps, and further to fully grown polyps. In some parts of the sample, up to five daughter polyps were distinguished. Below the layer of living polyps, branches of the colony were well developed carrying calices from dead polyps and had open spaces of different sizes between the branches (Fig. 2).
Fauna associated with the fragment was found mainly on and in the dead lower parts of the specimen. More than 30 species belonging to the phyla Porifera, Cnidaria, Annelida, Mollusca, Crustacea, Bryozoa and Echinodermata were found (Table 1). Boring sponges were not found.
Associated fauna viewed in the photographs could not be identified to species with the same degree of certainty. Some species such as the gorgonian corals Paragorgia
arborea and Primnoa resedaeformis (Fig. 11, Online Resource 1) were identified by their morphology and their known distribution; however, identification of the sponges required an examination of their spicules which was not possible without specimens. Only one sponge species was identified with confidence. Therefore, only those megafauna that could be identified with some degree of certainty were reported here (Table 1).
Annotated species list of associated fauna
Foraminifera Several species of calcareous Foraminifera were found on the dead branches. No attempt was made to identify them.
Porifera The hexactinellid Asconema setubalense was seen in several in situ photographs; it is known from the area. Forcepia forcipis was the best represented of the species attached to the specimen. On the photographs of the fresh sample, it was seen as a porous sponge filling many of the spaces between the branches (Fig. 2); it was new to the Greenland fauna. Iophon piceum was identified and is also a well-known sponge from the area. The Hymedesmia species were thinly growing and very little material was found when dried; there were enough spicules of each to recognize them as different species, but not for a final identification—18 species of the genus being known from Greenland waters. In comparison, there are >25 species of Porifera reported from Norwegian reefs (Burdon-Jones and Tambs-Lyche 1960; Mortensen and Fosså 2006), >65 species from Swedish reefs (Alander 1942; Tendal unpublished), 95 species from Rockall Bank (van Soest and Lavaleye 2005) and >35 species from Faroese reefs (Jensen and Frederiksen 1992; Tendal unpublished).
Boring sponges were not found in the sample and have not been recorded previously from Greenland waters. As they are elsewhere known as the most important bioeroders of L. pertusa corals (Beuck et al. 2007; van Soest and Beglinger 2009), this means that provided the growth of the corals is not especially slow, the bioherms in Greenland may reach a substantial size in a shorter span of time than elsewhere in the distribution area.
Halecium labrosum and Lafoea dumosa are known from West Greenland (Schuchert 2001). The large octocorals P. resedaeformis and P. arborea are both known from the area (Madsen 1944; Jørgensen et al. 2013; Tendal et al. 2013; Buhl-Mortensen et al. 2014). While P. resedaeformis has been known for many years from West Greenland fjords (Jungersen 1915), this is not the case with P. arborea. Despite being large (Greenland specimens become 2 m wide) and readily identified, it was not recorded by the early major Danish expeditions working in West Greenland, viz. “Ingolf” 1895–1896, “Tjalfe” 1908–1909 and “Godthaab” 1928, the first find being from 1967 (Tendal 1992). In comparison, 32 species of hydroids and 7 species of octocorals were identified from Norwegian reefs (Burdon-Jones and Tambs-Lyche 1960; Mortensen and Fosså 2006), and 15 and 2, respectively, from Faroese reefs (Jensen and Frederiksen 1992; Tendal unpublished).
Annelida Polychaetes were represented by fragments that were only identified to Family; at least 5 species were present. In comparison, 28 species are known from Norwegian (Burdon-Jones and Tambs-Lyche 1960) and 72 from Faroese (Jensen and Frederiksen 1992) reefs. Eunice norvegica, which is generally found in L. pertusa reefs (Mueller et al. 2013) and regarded as an indirect reef builder, was not recorded, and the species is not known from Greenland.
Mollusca The gastropod Mohnia simplex is rare in Greenland, being found only once before.
Janira maculosa is known from West Greenland. Caprella rinki has only been taken once before, on the type locality in South Greenland (Stephensen 1933). Gitana cf. rostrata was new to Greenland, provided the identification is correct. In comparison, 14 species of isopods and 8 species of amphipods were identified from Norwegian (Burdon-Jones and Tambs-Lyche 1960; Mortensen and Fosså 2006), and 13 and 10, respectively, from Faroese (Jensen and Frederiksen 1992) reefs.
Reteporella beaniana, Disporella hispida, Entalophoroecia deflexa and Tubulipora aperta were all known from West Greenland, the last mentioned with some doubt about the identification (Bille Hansen 1962). Exidmonea atlantica and Turbicellepora boreale were both new to the Greenland fauna. Turbicellepora bathyalis and T. groenlandica were both new to science (Denisenko 2016). In comparison, 40 species were identified from Norwegian (Burdon-Jones and Tambs-Lyche 1960; Mortensen and Fosså 2006) and 45 from Faroese (Jensen and Frederiksen 1992) reefs.
Echinodermata Specimens of a Gorgonocephalus were seen on some photographs. Further identification was not possible, four species having been reported from Greenland. Ophiactis
abyssicola and Ophiacantha anomala were known from Greenland. In comparison, 15 species of ophiuroids were identified from Norwegian (Burdon-Jones and Tambs-Lyche 1960; Mortensen and Fosså 2006) and 8 from Faroese (Jensen and Frederiksen 1992) reefs.