First geo-marine survey of living cold-water Lophelia reefs in the Ionian Sea (Mediterranean basin)
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- Taviani, M., Remia, A., Corselli, C. et al. Facies (2005) 50: 409. doi:10.1007/s10347-004-0039-0
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Prosperous deep coral mounds including living colonies of Lophelia pertusa together with Madrepora oculata and Desmophyllum dianthus (= D. cristagalli) have been discovered in 2000, by fishery operations on the eastern side of the Ionian Sea. The living coral mounds are located between ca. 300 and 1,100 m on a gently dipping shelf off Apulia at Santa Maria di Leuca (SML), and characterized by a complex seabed topography. Side scan sonar, shallow high-resolution seismics and sampling indicate that these Lophelia-bearing coral mounds colonize quasi-indurate (firmground) Pleistocene sediment. At places live corals were found on Pleistocene coral-hardgrounds. The fauna associated with these Ionian modern coral mounds is less diversified than modern Eastern Atlantic counterparts. The core of living coral mounds colonies is at present located in 500–700 m and is tentatively suggested that their survival is mostly controlled by oceanographic factors. The SML coral banks represent so far a unique example of living Lophelia-bearing coral mounds in the Mediterranean basin.
KeywordsDeep coralLiving Lophelia reefsIonian SeaMediterraneanRecent
The importance of this unexpected Mediterranean finding in the context of the on-going research on deep coral banks worldwide (Freiwald 2002; Freiwald et al. 2004) prompted the organization of the first full-scale oceanographic mission in this area. Mission CORAL (COR-2) of RV Urania took place in summer 2002 and the preliminary results of this expedition are presented here.
Material and methods
Main characteristics of COR-2 sampling stations offshore SML (G Grab, RD Rock Dredge, ED Epibenthic Dredge, SD Square dredge, CTD rosette Conductivity-Temperature-Density probe with rosette water sampler, GC Gravity Corer, SW Gravity Sediment Water Corer, I Ingegno)
SML coral mounds
We have measured T and S above one site of active coral growth; at surface we recorded T=29°C and S=38.88‰; at 500 m, T=13.8°C and S=38.78‰.
Core and grab samples taken from the coral mounds and near-coral sea bottom indicate that the substrate settled by corals is consistently a sandy-clay firmground (Fig. 8). Its age is late Pleistocene (<70 ka) as documented by nannoplankton indicating the Emiliana huxleyi acme (A. Negri, personal communication). The planktonic foraminifer assemblage includes Neogloboquadrina pachyderma associated with the benthic foraminifer Hyalinea balthica. This Pleistocene sediment is capped by an extremely thin Holocene veneer containing abundant planktonic foraminiferans (e.g. Globigerinoides ruber pink) and Hyalinea balthica. As for macro organisms, the occurrence of valves of the Pleistocene pectinid Pseudamussium septemradiatum was noted.
A first account of the biodiversity of SML coral mounds has been recently presented by Tursi et al. (2004), which incorporate also results from the CORAL cruise. From this study it is evident that SML biodiversity is comparatively lower than that of most Recent Atlantic counterparts (e.g., Jensen and Frederiksen 1992; Mortensen et al. 1995; Rogers 1999). Beside the scleractinian triad mentioned above, two benthic taxa are considered as characteristic (exclusive) of the Lophelia mound by Tursi et al. (2004), i.e., the solitary coral Stenocyathus vermiformis and the commensal polychaete Eunice norvegica. The latter induces extracalcification by both Madrepora and Lophelia. Associated benthos comprehends both non- or poorly-skeletonized epifauna (gorgonians). Common associated species with calcareous exoskeleton include the bivalves Asperarca nodulosa, Bathyarca philippiana, Delectopecten vitreus and Spondylus gussonii, the brachiopod Megerlia truncata, and the serpulid polychaetes Vermiliopsis sp. and Filogranula sp. A preliminary evaluation of bryozoans from dead Madrepora branches documented the existence of three species of living bryozoans, Copidozoum exiguum, Smittina crystallina, Schizomavella sp. (Rosso 2003). These taxa, however, are neither exclusively linked to living corals nor to dead coral frames, being known to colonize various types of hard substrata in the deep-sea such as rocky bedrocks, hardgrounds, cables etc. Acesta excavata was not found. This large limid is frequently, although not exclusively, associated with deep coral banks in the NE Atlantic; furthermore, it has been reported from deep coral assemblages of late Pliocene to late Pleistocene age in the Mediterranean basin where it reached as far east as Rhodes (López Correa et al. 2005). In principle a general situation suitable for large suspension feeding coral communities should be suitable, too, for this bivalve within the biocoenosis. Absence, if really confirmed, may be explained by either the lack of viable parental stocks in this region of the Mediterranean (López Correa et al. 2005) or lack of proper substrata for that byssate bivalve. We cannot discard the alternative hypothesis of water temperature (>13°C) possibly being too warm for Acesta. Coral colonies are affected by Cliona-endolithic activity (Tursi et al. 2004).
Many records of deep-sea coral mounds refer to indurated or rocky bottoms (e.g., Neumann et al. 1977; Newton et al. 1987; Messing et al. 1990). Precipitous submarine topographies are often considered most suitable sites for deep coral growth. However, some extant and subfossil deep coral mounds have been documented to develop on a somewhat gently-sloping, non-rocky seabed (e.g., Allen and Wells 1962; Mullins et al. 1981; Roberts et al. 2003; Remia and Taviani 2004). This is partly the case of SML living Lophelia-bearing coral mounds, although it is possible that coral hardgrounds forming individual elevated spots may act as significant settling ground for the Recent coral community. Since no visual documentation has yet been obtained by ROV or submersible, it remains uncertain to which extent older coral hardgrounds form the substrate of live corals.
Mediterranean records of living (with polyps) Madrepora, Desmophyllum and especially Lophelia are exceedingly rare (Zibrowius 1980). In fact, most dated Mediterranean deep-sea scleractinians are of last glacial Pleistocene ages throughout the basin (e.g. Zibrowius 1980, 1981; Delibrias and Taviani 1985; Taviani and Remia 2003; Remia and Taviani 2004; Taviani, unpublished). These radiometric data confirmed a previous suggestion by Blanc et al. (1959) that deep-sea corals populations of the Mediterranean Sea were thriving during the Pleistocene although incorrectly attributed to the last interglacial.
The western rim of the Ionian basin is marked by the presence of submerged Pleistocene deep coral deposits (Cita et al. 1980; Taviani and Colantoni 1984); dead specimens of Desmophyllum cristagalli from the Malta Escarpment provided uncalibrated C14 ages between ca. 15 and 30 ka (Taviani and Colantoni 1984). To the west of the Malta Escarpment, similar ages were obtained from Lophelia corals obtained from submerged assemblages in the Strait of Sicily (Delibrias and Taviani 1985; Schröder-Ritzrau et al. 2005; Freiwald, unpublished). To the east, Desmophyllum from the Hellenic Trench were equally documented to have lived during the glacial Pleistocene at about 18 ka BP (Zibrowius 1981). Subfossil Madrepora and Lophelia are also known from the Adriatic Sea (Broch 1953; Zupranovic 1969).
The presence of Madrepora and Lophelia in the Ionian Sea off Santa Maria di Leuca is well known since more than a century now due to the collections made by the research ship Pola in 1891. In fact, Madrepora oculata and Lophelia pertusa were dredged at 39°41′05″N and 18°36′18″E, at 760 m (Steindachner 1891; Marenzeller 1893).
The occurrence of Holocene deep-sea scleractinians on the Apulian margin was for the first time proved by Delibrias and Taviani (1985) who published a C14 age of 4350±100 ka (uncalibrated) for a specimen of Desmophyllum cristagalli from station CJ72-25 of RV Bannock (39°32.7N, 18°05.0E; 950 m) hosted in the redepository of the Laboratorio di Geologia Marina-CNR (Bologna). Therefore, such an “anomalous” postglacial age was predictably announcing the presence of extant deep-sea coral banks in the eastern rim of the Ionian Sea. However, the extent and lushness of these Lophelia-bearing formations was unsuspected in light of their observed general decline in the Mediterranean basin (Zibrowius 1980; Delibrias and Taviani 1985; Taviani et al. 2005).
So, the major question is why do we have such an exceptional deep coral site at this particular locality? Information gathered so far is not yet adequate to conclusively address this problem and additional geophysical and oceanographic data are required. Nevertheless, we can offer some speculative hints.
Current opinions about controlling mechanisms of Lophelia bank onset and growth in the NE Atlantic may be referred to two major situations. One perspective privileges oceanographic contour conditions to start and control coral growth (Freiwald 2002; Roberts et al. 2003). The second favors rather the role of light hydrocarbon seepage as a triggering forcing (Hovland and Thomsen 1997; Hovland et al. 1998). A somewhat intermediate view accepts the indirect role played by hydrocarbon seepage in creating mound topography suitable to deep coral growth (Masson et al. 2003).
The core of the SML coral banks (500–700 m) seems to be bathed by warm and salty Levantine water as also suggested by our CTD measurements. The existence of significant bottom currents is well documented by sediment drifts and erosional topographies. On the contrary, no evidence of significant hydrocarbon seepage on this sector of the Ionian margin has been reported so far.
In short, we speculate that oceanographic conditions (strong bottom currents) coupled with the margin’s peculiar seabed topography may explain the spectacular (and so far unique in the Mediterranean Sea) deep coral growth at SML. Further multidisciplinary geophysical and hydrological investigation planned within the frame of the on-going project APLABES will hopefully clarify this issue.
The SML coral mounds represent a (so far) unique example of living Lophelia-bearing coral mounds in the Mediterranean basin, although its biodiversity is low with respect to Atlantic counterparts.
Coral colonies display a patchy distribution on eroded firmground of Pleistocene age, at places characterized by elevated spots of coral-hardground differently from most known Mediterranean examples that are related to bedrock hard substrata.
The SML coral mounds thrive at a temperature slightly in excess to 13°C which is the highest temperature known to date for the occurrence of living Lophelia (see Freiwald 2002).
The core of living colonies seems at present located between 500–700 m; as a working hypothesis we propose that coral survival is guaranteed by oceanographic conditions (strong and predictable bottom currents) while a link with hydrocarbon seepage is not evident.
We thank Captain, Officers, Crew and Colleagues onboard r/v Urania during cruise CORAL for their cooperation. The mission was funded by CNR and ISMAR-Bologna with additional support provided by the Universities of Milano, Bari, Bologna, Trieste and Erlangen. We are indebted with Fabio Trincardi for valuable discussions and the critical reading of the manuscript. Thanks are due to Helmut Zibrowius and to an unknown referee for useful comments that helped to improve this text. This article is a contribution to SINAPSI, ESF Euromargins “MOUNDFORCE” and FIRB “APLABES” programmes. IGM scientific contribution no. 1417