The “Corsica Channel Cold-Water Coral Province” (Mediterranean Sea)

Over 25 mounds have been identified in the Corsica Channel (Mediterranean Sea) through multibeam bathymetric mapping at depth of 400–430 m, with dimensions ranging from 70 to 330 m, achieving maximum heights of 25 m. Two mounds have been explored in detail using a remotely operated vehicle, revealing thick coral growth with a predominance of the branching scleractinian Madrepora oculata as main frame builder and subordinate Desmophyllum pertusum. The solitary scleractinians Desmophyllum dianthus and Javania cailleti add to the biodiversity here, which accounts for at least 50 macro- and megabenthic species. In consideration of the remarkable surface (ca. 5.3 km2) covered by living corals, their density and healthy appearance, and discontinuity with other major cold-water coral (CWC) occurrences in the Mediterranean Sea, we propose that this area represents a distinct CWC province in a sector already known for the presence of pre-modern CWC mounds. Noticeably, well-developed contourite drift systems occur in the Corsica Channel, lending support to their strict spatial link with coral establishment at depth. The ecosystemic value of the new CWC province calls for proper conservation measures to ensure their present Good Environmental Status.


INTRODUCTION
The concept of "Cold-Water Coral Province" is not rigorously codified in literature thus far, vaguely referring by convention to geographically discrete areas with important deep-sea scleractinian presence (e.g., Taviani et al., 2011;Hebbeln et al., 2014;Mohn et al., 2014;Wienberg et al., 2018). It is anyhow of practical use because (i) it identifies those situations where the area covered by cold (or deep)-water corals (CWCs) is often many tens square kilometers, far exceeding occasional to sparse CWC occurrences; and (ii) it helps to narrow the areal of seabed meritorious of special attention for management and protection purposes. To date, seven CWC provinces, structured by the colonial scleractinians Madrepora oculata and, subordinately, Desmophyllum pertusum (Lophelia pertusa; Addamo et al., 2016), have been identified in the Mediterranean Sea (Taviani et al., 2017Chimienti et al., 2019). From east to west, the CWC provinces are Bari Canyon (e.g., Freiwald et al., 2009;Angeletti et al., 2014Angeletti et al., , 2020 and Santa Maria di Leuca Vertino et al., 2010) in the Adriatic and Ionian seas, South Malta (Schembri et al., 2007;Freiwald et al., 2009;Knittewis et al., 2019), South Sardinia Nora Canyon (Taviani et al., 2017) in the Tyrrhenian Sea, Cassidaigne (Fabri et al., 2014(Fabri et al., , 2019 and Cap de Creus canyons (Orejas et al., 2009) in the Gulf of Lion, and Alboran Sea (Lo Iacono et al., 2019, with references therein).
There is a consolidated consensus that CWCs establish under a bottom current regime of high intensity, with the Levantine Intermediate Water (LIW) mass as the main driver controlling their distribution in the Mediterranean Basin Taviani et al., 2016Taviani et al., , 2017Orejas and Jiménez, 2019). Along the LIW path, the recent visual surveys have, indeed, led to the discovery of healthy CWC grounds in the Nora and Dorhn canyons in the Tyrrhenian Sea (Taviani et al., 2017. Bathing the Tyrrhenian Italian margin, the LIW reaches the Ligurian Sea by flowing through the Corsica Channel. Besides some anecdotal evidence reporting live Madrepora oculata in this region (F. Serena, M. Borghi, M. Vacchi, personal communication), to date, the presence of CWCs in the Corsica Channel concerns Late Pleistocene buried coral mounds (Remia and Taviani, 2005;McCulloch et al., 2010). Substantial evidence of live CWC populations between Tyrrhenian and Ligurian sites (Tunesi et al., 2001;Fanelli et al., 2017) is, however, still missing.
Exploration of the southern Ligurian and northern Tyrrhenian seas in the frame of the Italian Marine Strategy Framework Directive in 2016 and 2017 documented in the present study proves prolonged CWC regional presence, although not necessarily continuous.
Areal extent and CWC density and richness warrant the identification of the new "Corsica Channel Cold-Water Coral Province" as described below.
The discovery of these healthy and lush coral mounds site bears to the distribution, organization, and connectivity of the Mediterranean CWC communities.

SETTING
The area is located in the Corsica Channel between the Corsica Island to the west and the Tuscan Archipelago to the east. The Corsica Channel or (Trough) is a narrow (10-35 km wide), shallow (maximum depth of ca. 900 m in the south), and northsouth oriented feature, separating the Corsica from the Tuscan shelves, and connecting the Tyrrhenian and Ligurian basins in the Western Mediterranean Sea (Figure 1). The narrowest and shallowest zone is off Capraia Island, where the Tuscan and Corsica shelves (at 200 m) are separated by the Corsica Sill that is 9.5 km wide. Modified Atlantic Water [(MAW) from 0 to 200 m], northward flowing, is the main water mass circulating in the shallow (i.e., shelf) Corsica Channel (e.g., Astraldi and Gasparini, 1992;Millot, 1999;Millot and Taupier-Letage, 2005). Secluded in the eastern side, the LIW flows northward at depths between ∼250 and 600-700 m (e.g., Artale and Gasparini, 1990;Vetrano et al., 2010;Toucanne et al., 2012). Alongslope water mass movements are responsible for long-lasting welldeveloped contourite drifts from the south (off Montecristo Island) to the north near the Gorgona Island, which extend almost continuously for more than 100 km (e.g., Cattaneo et al., 2017;Rebesco and Taviani, 2019, with references therein). These contourite depositional processes started in the middle Pliocene under the anticlockwise influence of the Coriolis force and as a response to the topographic setting (Stanley et al., 1980;Marani et al., 1993;Roveri, 2002). Morphology and size (maximum width of 10 km: Miramontes et al., 2016) of the contourite drifts change remarkably due to the differences in the basin morphology and depth (Roveri, 2002;Miramontes et al., 2016). Turbiditic systems characterize, on the contrary, the western side of the Corsica Trough (Bellaiche et al., 1994;Gervais et al., 2004;Calvès et al., 2013), among which the Golo Turbidite System is active at least since the Late Pleistocene (e.g., Deptuck et al., 2008;Sømme et al., 2011). Finally, the strong hydrodynamics marking this area is also responsible for sediment starvation and hardground formation along the slope (Remia et al., 2004).

MATERIALS AND METHODS
The study area has been surveyed in the last 20 years by different technologies during cruises LM99 (December 1999-January 2000 and CORTI (December 2003-January 2004) onboard R/V Urania and during cruises Strategia Marina-Ligure/Tirreno (July-August 2016) and MSFD-IT-1-2017 (July 2017) onboard R/V Minerva Uno.
Swath bathymetry and backscatter data considered in this study have been acquired using a hull-mounted Reson Seabat 7160 Multibeam Echosounder with a nominal frequency of 44 kHz, swath coverage of ca. 4 × greater than water depth and 512 beams per second acquired with Reson PDS2000 software. All data were plotted in the Transverse Mercator-UTM 33N-WGS84 Coordinate System. Morphobathymetric map, with a cell size of 5 × 5 m 2 , was obtained using CARIS SIS and HIPS software (Figure 2). Seabottom sampling was performed with a large volume (60 l) modified Van Veen grab, epibenthic and heavy-chained dredges. Samples were stored at the ISMAR-CNR Repository in Bologna.
Seawater physical attributes of the studied area were obtained from 20 depth profiles (Figure 3) sourced from the World Ocean Atlas 2018 (WOA2018) database and were visualized using Ocean Data View software, version 5.3.0 (Schlitzer, 2020). Metadata on bottom sampling and oceanographic data are reported in Table 1.
Two remotely operated vehicle (ROV) dives (Stations MS17-I-54 and MS17-I-135) were conducted using a Pollux III (Global Electric Italiana) equipped with a low-resolution CCD video camera for navigation and a high-resolution (2,304 × 1,296 pixels) video camera in order to visually groundtruth the mounds identified by swath bathymetry. Three laser beams at 20 cm of each other provided the scale bar on the videos. The ROV was equipped with an underwater acoustic tracking system that gives position and depth every 1 s. Still-photo footage, one frame every 10 s, was georeferenced using Adelie, and ESRI ArcGIS software, provided taxonomic information. Macro-(>2 cm) and mega-benthic organisms were identified to the lowest possible taxonomic rank. Taxa unidentifiable at the species level from images alone (e.g., most sponges) were categorized only as morpho-species or morphological categories (e.g., Bell and Barnes, 2001;Bo et al., 2012;Cau et al., 2017;Foglini et al., 2019). Taxonomic identifications follow the World Register of Marine Species database (WoRMS Editorial Board, 2020 1 ).

Hydrography
The local intermediate water mass close to the CWC occurrences in the Corsica Channel has a mean potential temperature of 13.4 • C, salinity of 38.56 psu, and potential density of 29.07 kg/m 3 (Figure 3), which are typical values of the LIW in this area of the Mediterranean Sea (Vetrano et al., 2010). The LIW flows through the Mediterranean Sea westward, entering the Tyrrhenian Sea from the Strait of Sicily and separates in two branches: one crossing northward the Corsica Channel and one flowing toward the eastern Sardinia (Millot and Taupier-Letage, 2005;Vetrano et al., 2010;Ozer et al., 2017).

Seafloor Bathymetry
Morphobathymetry and backscatter data have been combined with high-resolution Chirp profiles to reconstruct the origin of the elevated and elongated seafloor features observed. We identified 28 mounded features in a 5.3-km 2 area at a depth of 400-430 m, ranging in length from 70 to 330 m and up to 25 m high. All these structures are north-south oriented, suggesting a preferential growth strongly influenced by currents. Two main patches of mounded features occur (Figures 2A,E), one ca. 20 km south of Gorgona including eight mounds (Site A) and another ca. 12 km east of Capraia clustering 20 mounds (Site B). Slope     (Figures 2C,G). High intensity (>15 dB) typifies the major mounds, characterized by hard and firm substrates, while low intensity (<15 dB) characterizes the surrounding areas dominated by mobile sediments (i.e., muds). To better identify the mounds, we applied a selection algorithm, developed in ArcGIS 10.5, integrating features elevation, represented by the Bathymetric Position Index (BPI), with the seabed reflectivity value. The BPI was computed from the high-resolution bathymetry using the Benthic Terrain Modeler tool (BTM, V. 3.0) in the ArcGIS software, selecting an inner radius of 5 pixels and an outer of 20 pixels, corresponding to 25 and 100 m, respectively. The algorithm considers and selects all positive BPI as potential mounds (features elevating from the seafloor). This selection was then filtered by extracting a reflectivity intensity value in correspondence of georeferenced living coral occurrences, obtaining a 15-dB value as a threshold for cnidarian-dominated mounds (Figures 2D,H); Focal Statistic tool of ArcGIS (radius of 5 pixels) was used to exclude false positive.
Additionally, smooth rounded features, ranging from 30 to 250 m in length and from 20 to 80 m in width and achieving a maximum height of ca. 5 m covering an area of 1.94 km 2 , could represent buried mounds.

Biological and Habitat Characteristics Based Upon Video Surveys
The ROV dive MS17-I-54 explored Site A whereas ROV MS17-I-135 surveyed Site B between 430-415 m and 450-425 m water depths, respectively, for a total coverage of 978 m (Figures 2I,J and Table 1). ROV surveys revealed the existence of lush and healthy coral mounds dominated by the colonial scleractinian Madrepora oculata (Figure 4-7).
A sparse benthic fauna inhabits muddy sediments, including single occurrences of the cnidarian Kophobelemnon stelliferum and of the rare pennatulacea Protoptilum carpenteri (e.g., Prampolini et al., 2020). The macro foraminifera Pelosina sp. is abundant (up to 25 ind. 0.3 m −2 ) at Site A (Figure 5A), while it occurs rarely on the highly bioturbated muddy bottoms of Site B (Figure 5B). Ceriantids (cf. Pachycerianthus and Arachnanthus oligopodus) and sparse presence of Bonellia viridis characterize the sessile fauna inhabiting muddy sediments on both sites. Rare decapods (Plesionika martia) and the demersal fish Lepidorhombus boscii represent vagrant fauna. The echinoid Spatangus purpureus represents the commonest vagile organism at Site B, further testified by the abundance of its skeletal remains; pagurids and other crustaceans are also common presences.
The Corsica Channel hosts at least 20 cnidarian species (Table 2), all known from the Mediterranean Basin (e.g.,

Mound Morphology and Genesis
Coral mounds of the Corsica Channel are roughly parallel to each other, displaying oval to elongated shapes, mean length of ca. 200 m, width of 110 m, heights of up to 25 m, and very steep flanks up to 35 • . Mounds develop at present, adjacent spatially to contouritic deposits, similar to other Mediterranean localities (e.g., off Pantelleria: Martorelli et al., 2011;east and west Melilla: Corbera et al., 2019;Hebbeln et al., 2019;Rebesco and Taviani, 2019, with references therein). Active mounds of the sites A and B seem to develop where the bottom currents decelerate but are still sufficiently intense to sustain coral growth, provide nutrients, and prevent excess silting. Between sites A and B, seismic and bathymetric records show the occurrence of north-southaligned morphological features with elongated shapes and dimensions comparable to the active mounds under scrutiny. These structures cover an area of ca. 2 km 2 and might represent buried CWC mounds. This is also consistent with the documented existence of Madrepora-dominated buried mounds previously described in this region (Remia and Taviani, 2005). In response to climatic changes, the past regime might have been somehow different in direction and strength from today, as for turbidity and nutrient supply. This might account for the demise of such Pleistocene CWC mounds in the study area as recorded by U/Th dated coral mounds  ( McCulloch et al., 2010), and the newly observed buried features here discussed. Anyhow, morphological similarities and comparable alignments of active (modern) and buried (past) mounds in the Corsica Channel suggest a similar environmental control on coral mound development, at least since the latest Pleistocene, with the LIW acting as the main driver.

Cnidarian Assemblage and Associate Fauna
The new "Corsica Channel CWC Province" presents biological characteristics similar to Atlantic and Mediterranean counterparts. Differently from their Atlantic equivalent, where the main frame builder is Desmophyllum pertusum, here, Madrepora oculata shows the highest abundance, contributing primarily to form CWC mounds (e.g., Buhl-Mortensen et al., 2017;Corbera et al., 2019). In terms of biodiversity richness, faunal data compare well with other sites in the Tyrrhenian Sea such as the Nora Canyon (78 species, see Taviani et al., 2017), northeastern Sardinian margin (58 species, see Moccia et al., 2019), and the Dohrn Canyon (64 species, see Taviani et al., 2019). Overall, more than 50 mega-and macro-benthic taxa have been identified by the ROV surveys plus four different demersal species (Table 2). Sessile megafauna observed in the explored mounds shows a preferential spatial distribution, appearing more dense at the crest of the mounds, in particular M. oculata and the alcyonarians A. hirsuta, V. bebrycoides, M. lepida, and S. dubia. However, when dense aggregations of M. oculata cover the crest of the mounds, the associated species occur in areas with more exposed dead framework. Moreover, increasing of cnidarian density and colonies' size have been observed on mound crests. Similar situations have been reported in other Mediterranean sites (Orejas and Jiménez, 2019, and references therein) where the presence of healthy and lush CWC is linked to the flowing of the LIW.
The new "Corsica Channel CWC Province" fits in the path of the LIW, confirming this water mass as the main controlling factor of the distribution of CWC provinces in the Mediterranean Sea (e.g., Taviani et al., 2017Taviani et al., , 2019. Our findings are biogeographically relevant also in terms of connectivity and connectance with other coral sites and provinces in the Mediterranean Basin (Figure 8).

Anthropic Impact
Insubstantial or null anthropic impact on the CWC grounds in the Corsica Channel was evidenced by ROV images. Differently from other Mediterranean CWC sites (e.g., D'Onghia et al., 2017;Taviani et al., 2017Taviani et al., , 2019Giusti et al., 2019;Enrichetti et al., 2020, with references therein), the limited visual information available shows that the coral grounds here seem less impacted by anthropic activities in terms of macrolitter and derelict fishing gears. The paucity of obvious anthropic macroimpact could imply a rather pristine status of the CWC habitat there. We cannot exclude, however, the occurrence of other forms of direct and indirect impact, including the pervasive microplastics (e.g., Danovaro et al., 2020).

Governance and Protection
International organizations have been taking several actions and recommendations to preserve CWC ecosystems. However, the application and eventual implementation of such conservation measures and policies in the Mediterranean Sea still remain limited (see Rees et al., 2018;Otero and Marin, 2019, with references therein).
Due to its ecosystemic importance in the Mediterranean panorama, pristine condition, presence of several threatened and target species of Vulnerable Marine Ecosystem (VME), and the potential exposure to anthropic impact, the "Corsica Channel CWC Province" surely deserves earning proper governance and protection, in line with current views (e.g., Armstrong et al., 2009Armstrong et al., , 2014Fabri et al., 2014;Taviani et al., 2017Taviani et al., , 2019Danovaro et al., 2020). Although fisheries do not seem to represent a threat to date, and littering appears scant here as documented by video images, a principle of caution should be enforced in the future for ensuring the Good Environmental Status for the area. In fact, the "Corsica Channel CWC Province" hosts a number of species listed in protection protocols, such as RAC/SPA/BD, IUCN Red List of Threatened Species and EU-Habitats Directive. The designation of such a new CWC province in a protection program is highly desirable, also in light of its role within the Mediterranean CWC province network. As a first propositive step, we suggest two suitable areas to be considered for new transnational high-seas marine protected areas, with an extension of 1.47 km 2 and 3.86 km 2 for sites A and B, respectively, and including most of the best CWC "reefs" identified thus far (Figure 9 and Table 3).

CONCLUSION
(1) In consideration of the remarkable surface covered by living corals (ca. 5.3 km 2 ), their density, healthy appearance, and discontinuity with other major CWC occurrences in the Mediterranean Sea, this area in the Corsica Channel is erected as a distinct and new CWC province for this basin. The high levels of biodiversity documented advise for the consideration of proper management and protection of these CWC grounds. (2) This new "Corsica Channel Cold-Water Coral Province" is located off the Tuscan Archipelago in the Corsica Channel, in a sector previously known for the presence of premodern CWC mounds and presence of well-developed contourite drift systems. (3) In line with the main Mediterranean CWC sites, the new province is aligned along the main path of the LIW and represents a potential connection with other coral sites and provinces throughout the Mediterranean Basin.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/Supplementary Material.

AUTHOR CONTRIBUTIONS
LA, AR, GC, and MT analyzed the data and wrote the manuscript together with PM. All authors contributed to the discussion and preparation of the manuscript.

ACKNOWLEDGMENTS
This paper is part of the keynote presentation by MT at the 7th International Symposium on Deep-Sea Corals held in Cartagena, Colombia, July 29-August 2, 2019. We are grateful to the organizers Juan Armando Sánchez, Santiago Herrera, and Luisa Dueñas for their kind invitation to contribute an article to the Symposium's special issue. We thank captain, crew, and scientific staff of RR/VV Urania and Minerva Uno cruises for their skillful and efficient cooperation during operations at sea. Manuscript finalized and revised at the time of the COVID-19 pandemic. This is ISMAR-Bologna scientific contribution no. 1998.