Preprint volume - SIBM

Pre-print Volume - Oral presentations
Topic 2: MARINE ORGANISMS AND ECOSYSTEMS AS MODEL SYSTEMS
mountains and photosynthetic communities may develop on their tops. The
megabenthic assemblages zonation along the Vercelli Seamount may represent a study
model to describe, through ROV imaging, seamount ecosystems in the Mediterranean
Sea.
Materials and methods - Vercelli Seamount, rising from the flat muddy bottom of the
Tyrrhenian Sea around 2000 m depth, is located at about 50 miles off Olbia, along the
Sardinian coast (41°06.114’N - 10°53.979’E). The survey was conducted with an
observer class Remotely Operated Vehicle, ROV Pluto (Gaymarine, Switzerland). The
ROV was equipped with an underwater acoustic tracking position system (HDR made
by Gaymarine ultrashort baseline operating with a 30kHz responder), providing records
of its track along the seabed. Additionally it had a depth sensor, a compass, and two
parallel laser beams providing a 10 cm scale for measuring the areas of the frames
(approximately 1-2 m 2 ). The ROV, moving at about 1 m height from the seabed (about
2 m of visual field), was equipped with a digital camera and a high definition video
camera. To confirm the taxonomic determination of the specimens observed in the
frames, we examined samples collected by dredging on the seamount (60-120 m depth)
during an oceanographic campaign on board of the R/V Urania in May 2009.
Results - The geomorphological characteristics of the Vercelli Seamount were
previously studied by underwater photoprofiling (Zhuleva, 1988). The topography of
the structure is characterised by steep rocky or detritic walls rising from the sea bottom
and turning into flat coarse detritic planes gently sloping from 200-250 to 100 m depth
where a rocky peak rises reaching its maximal elevation around 60 m depth (Fig. 1).
The ROV observations indicated that the different portions of the rocky pinnacle
(summit, from 60 to 70 m depth, SW flank and NE flank, from 70 to 100 m depth)
hosted different coralligenous biocoenoses (Fig. 1). The top of the seamount showed a
dense population of the kelp Laminaria rodriguezii covering almost 80% of the
substratum. The substratum free from the algae was colonised by encrusting sponges.
Many specimens of the crinoid Antedon mediterranea (on average 50 individuals m -2 )
occurred on the algal thalli. The two flanks of the peak were characterised by
significantly different assemblages (Fig. 1). The southern side of the ridge was
dominated by gorgonians which sometimes reached very high densities. On the rocky
boundary close to the detritic bottom (90-100 m) the only large coral present was the
gorgonian Eunicella cavolinii, and the more rare Paramuricea clavata. Specimens of
Callogorgia verticillata, Corallium rubrum and the scleractinian Dendrophyllia
cornigera were occasionally observed in the deepest part (90-100 m) of the pinnacle.
Several specimens of Axinella verrucosa and Axinella damicornis were observed.
Between 70 and 90 m the density of P. clavata progressively increased while that of E.
cavolinii decreased. One colony of the arborescent antipatharian Antipathella
subpinnata was observed at about 90 m depth. The northern side of the peak was
characterised by the dominance of suspension feeding organisms. From the bottom to
80 m depth we recorded a very large number of colonies of the blue soft coral
Paralcyonium spinulosum, while Alcyonium palmatum was only occasionally
observed. P. clavata and E. cavolinii were also present on the northern side but with
lower densities. Specimens of Axinella spp. were observed at all depths while the
dominant sponge species from 70 to 80 m depth was Tethya citrina here showing
41 st S.I.B.M. CONGRESS Rapallo (GE), 7-11 June 2010
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