Simposio Ibérico de Estudios de Biología Marina (San Sebastià)

Com ja us informàvem al juny, els resultats i conclusions que s'han desprès de l'estudi dels Jardins Submergits de La Selva han estat acceptats per ser presentats en un comunicat oral al Simposio de Bentos Ibérico a San Sebastià.

http://www.siebm.org/2012/

Estem contents de la rebuda que ha tingut i sobretot de l'interès mostrat pels investigadors experts en grans profunditats. Si bé nosaltres ens hem quedat quasi al límit superior dels seus estudis, ja que ells baixen amb submarins, sí que ha estat interessant l'intercanvi d'idees ja que no han treballat per la costa de La Selva però sí del Cap de Creus.

Esperem que aquest sigui l'inici de noves col·laboracions! (Per demanar que no quedi ;D)

Bé i pels que tingueu ganes de dades més tècniques, aquí teniu el resum de la presentació:

The sea fan (Paramuricea clavata) ‘forests’ near the Blanes Canyon (Spain, Northwestern Mediterranean)

Carlo Tidu^a,b, Carolina Campillo-Campbell^b, Joan Mora Crespo^c, Sergi Taboada Moreno^c,d

^a Tecnoambiente (carlo.tidu@tecnoambiente.com), C/Industria 550-552, 08918 Badalona (Barcelona)

^b Centre Esportiu Subaquàtic de Calella – C.E.S.C.,  Rierany Valldenguli s/n 08370 Calella (Barcelona)

^c S’Agulla, Entitat medioambiental. Carrer Abat Escarre, 28, 2º, 17300, Blanes (Girona)

^d Universitat de Barcelona, Departament de Biologia Animal, Avinguda Diagonal 645, 08028, Barcelona

The Costa Brava (Spain, Northwestern Mediterranean) is environmentally considered one of the most important areas of the Spanish Mediterranean coast due to the uniqueness and diversity of the marine natural communities that in many cases still presents an excellent state of preservation. The coast between Blanes and Tossa de Mar is emblematically considered as the beginning of the Costa Brava, being distinctive for its abrupt rocky shore line. Its sea bottom, between 50-70m deep, on the hard substrata is colonised by the Mediterranean coralligenous assemblage. This marine biocenosis is characterized by long living species, slow population dynamics and a low rate of population increase. This kind of biocenosis is included as priority habitat (EUR27:1170) in the European Community Habitat Directive.

Mediterranean coralligenous assemblages are considered amongst the Mediterranean benthic habitats of high preservation interest for their biogeographic uniqueness, in which the highly diversified physical structure leads to the presence of a high species biodiversity (Ballesteros, 2006).

Although the majority of studies and cartographies on the coralligenous biocenosis are located in the western Mediterranean subregion (Agnesi et al., 2009), no data exists for this area. Furthermore, the information available for this biocenosis over 40 meters deep is very poor, due to the inherent problems of deep diving.

In order to preserve and manage such a unique and fragile environment of high ecological importance, deep knowledge of the habitat is required. The first step to achieve such goal is to characterize this habitat by means of a detailed cartography with acoustic survey with side-scan sonar, and direct sampling with scuba divers. Secondly we focused our attention on the red sea fan Paramuricea clavata, a keystone and engineering species, which its conservation is crucial to maintain the biodiversity of the marine communities (Linares et al., 2008). For this keystone specie there are very few data available for populations located below 40 meters deep. This implies for example that it is not very well known if the high mortality that affects cyclically shallower P. clavata populations (Cerrano et. al., 2000; Harmelin & Garrabou, 2005; Linares et al. ,2008), also affects deeper populations.

The presence of the red sea fan Paramuricea clavata and the coralligenous biocenosis in general, in the sea bottom between Blanes and Tossa de Mar, are poorly documented and directly known only by some experienced divers from the Club Esportiu Subaquàtic de Calella (C.E.S.C.), that regularly for the last 30 years perform their dives in the area known with the name of "Gorgonias”. With the information provided by the C.E.S.C. divers, a rectangular area of 5 km² between 50 and 70 meters depth was defined (fig. 1).

The lack of information about the spatial extension, at regional and local scale, of the Mediterranean coralligenous biocenosis is partially due to the complexity involved in studying deep rocky habitats. Therefore the first step of the study has been to obtain very detailed information of the geomorphology of the sea bottom by means of a side-scan sonar (SSS). The SSS used in the survey was a Klein 3000 (double frequency: 100, 500 kHz) with a digital topside unit for the recording of the acoustic data. The acquisition software of the topside recording unit applied the geocoding to the SSS digital records, using navigation and vessel speed data supplied from the differential GPS system. A 100 kHz range signal with a range of 150 m on each side of the towfish was used. The six navigation lines performed, of 5 km long, were 160 meters apart to allow a sonographs overlapping. The total survey area was of 5 km², with a mean navigation speed of 3 knots. SonarWiz software (Chesapeake Technology, Inc.) was used to generate a sea bottom mosaic, with 1 m resolution, which was afterwards exported as GeoTiff, to be implemented in a geographic information system (GIS). Analysis of the SSS sonograph and sea bottom mosaic allowed the identification of rocky areas in which the coralligenous biocenosis is potentially present.

The study area was divided into 6 identical zones. The number of dives in each zone was determined by the extent of the rocky substrate detected from the SSS data. 30 dives in total were performed between 49 – 64 meters deep. In each dive a photographic inventory (by means of Canon G11 with underwater housing Ikelite) was carried out in order to characterise the community found. Secondly, if the coverage of P. clavata was at least of 50m long to be considered of environmental structural importance (Linares et al. 2008), characterisation of the population was performed: a belt of 1 x 2m (Harmelin & Garrabou, 2005) was deployed and data regarding density, height, degree of colonisation and degree of necrosed branches were gathered.  

The investigation by means of the SSS, identified mainly 3 types of substrate: the first two associated with rocky bottoms: consolidated bottom and outcrops with positive well-defined contours; and the third one is associated to homogeneous sedimentary bottoms, which occupied little more than 60% of the studied area. The consolidated bottom was found to be more frequent rocky bottom, with two different morphologies: one flat or sub-horizontal surface with few edges and discontinuities and the other one with very irregular flat surface, with many edges and discontinuities occupied by detritic sediment.

The 30 dives carried out in the study area have allowed the identification of four different facies of the coralligenous biocenosis: the P. clavata facies, the large sponges with Eunicella singularis facies, the E. singularis with Eunicella sp. facies, and coralligenous platforms. In small caves or in the lower sides of coralligenous platforms facies, the biocenosis of semi-dark caves with the red coral (Corallium rubrum) facies have been found twice. Lastly the sedimentary bottom was classified as detritic bottom with rhodoliths (maerl).

In 10 out of 30 dives a continuous surface of at least 50 meters colonised by the red sea fan P. clavata were detected. This facies was mainly found in the consolidated bottom with flat or sub-horizontal surfaces with few edges and discontinuities. A total of 252 colonies of P. clavata were measured. The height of the colonies ranged between 6 - 112 cm, the mean height varied from 16.2 cm (± 9,5 SD) to 56.3 cm (±17,6 SD) and the density varied between 8 - 20 colonies per square meter.

The 10 dives in which P. clavata was found are part of four different rocky structures of the study area. The analysis of the population structure indicate that in three out of the four the distribution does not differ significantly from normality, which means that there is not a prevalence of small or large classes. Moreover a negative kurtosis was observed indicating a flat distribution, without a clear predominance of any classes.

The colonies showed a rate of epiphyte colonization that varied from 6 - 30%. The percentage of the total population of colonies with epiphyte varied from 10 - 40%. In the present study the most frequently epiphyte species found (between 30 and 60% of the epiphyted colonies) is the equinoderm Astrospartus mediterraneus. In the study area only in once were observed dead colonies that mounted up to 3.

The results of the present study indicate that on the seabed located between Blanes and Tossa de Mar between 49 and 64 meters deep is located an important coralligenous biocenosis, in which the facies with the red sea fan Paramuricea clavata is the most important. The results indicate that this species within the study area massively colonises flat or sub-horizontal surfaces resulting in a small "forest". The P. clavata population structure in the study area indicates that this coralligenous facies is possibly one of the most important of the western Mediterranean, since parameters such as average height of the colonies are the highest described so far. The results also show that in the study area there have been no sudden mortalities which have been observed for shallower populations, as it can be deduced from the low numbers of dead colonies found. The presence of this important coralligenous facies can be related to the proximity of the Blanes canyon and to the strong hydrodynamic flows that characterise these interruptions of the northwestern Mediterranean continental shelf (Canals et al., 2006; Flexas et al., 2008), which increase the amount of food availability (Mistri & Ceccherelli, 1994; Ribes et al., 1999). The results of this study are of great significance because, as recognised by the scientific community, the presence of P. clavata is considered critical in maintaining the organization and biodiversity in the coralligenous biocenosis.

Agnesi, S.; Annunziatelli, A.; Cassese, M.L.; La Mesa, G.; Mo, G.; Tunesi, L. (2009). State of knowledge of the geographical distribution of the coralligenous and other calcareous bio-concretions in the Mediterranean. MEDITERRANEAN ACTION PLAN. Ninth Meeting of Focal Points for SPAs. Floriana, Malta, 3-6 June 2009

Ballesteros, E. (2006). Mediterranean coralligenous assemblages: a synthesis of present knowledge. Oceanographic Marine Biology Annual Review, 44: 123-195

Canals, M.; Puig, P.; Durrieu de Madron, X.; Heussner, S.; Palanques, A.; Fabres, J. (2006). Flushing submarine canyons. Nature, doi:10.1038

Cerrano, C.; Bavestrello, G.; Nike Bianchi, C; Cattaneo-Vietti, R.; Bava, S.; Morganti, C.; Morri, C.; Picco, P.; Sarà, G.; Schiapparelli, S.; Siccardi, A.; Sponga, F. (2000). A catastrophic mass-mortality episode of gorgonians and other organisms in the Ligurian Sea (North-western Mediterranean), summer 1999. Ecology Letters, 3: 284-293 

Ribes, M.; Coma, R.; Gili, J.M. (1999). Heterogeneous feeding in benthic suspension feeders: the natural diet and grazing rate of the temperate gorgonian Paramuricea clavata (Cnidaria: Octocorallia) over a year cycle. Marine Ecology Progress Series, 183: 125-137

Flexas, M. M., Boyer D. L., Espino, M.; Puigdefàbregas, J.; Rubio, A., Company, J.B. (2008), Circulation over a submarine canyon in the NW Mediterranean. J. Geophys. Res., 113, C12002, doi:10.1029/2006JC003998.

Harmelin, J.G. & Garrabou, J. (2005). Suivi d'une population de Paramuricea clavata (Risso, 1826) (Cnidaria, Octocorallia, Gorgonacea) dans le parc national de Port-Cros (Méditerranée, France) : Comparaison des états 1992 et 2004 sur le site de la Galère. Sci. Rep. Port-Cros natl. Park, Fr., 21: 175-191

Linares, C.; Coma, R.; Garrabou, J.; Diaz, D.; Zabala, M. (2008). Size distribution, density and disturbance in two Mediterranean gorgonians: Paramuricea clavata and Eunicella singularis. Journal of Applied Ecology, 45: 688-699

Mistri, M., Ceccherelli, V.U. (1994). Growth and secondary production of the Mediterranean gorgonian Paramuricea clavata. Marine Ecology Progress Series, 103: 291-296

NOTA: La Figura 1 és l'àrea d'estudi que aquí no s'ha pogut posar per limitacions tècniques però que trobareu als primers posts d'aquest blog.