The Neogoniolithon brassica-florida - EQEL
Transcription
The Neogoniolithon brassica-florida - EQEL
Journal of Coastal Research 27 2 394–398 West Palm Beach, Florida March 2011 The Neogoniolithon brassica-florida (Harvey) Setchell & L.R. Mason (1943) Reef of Bahiret el Bibane Lagoon (Southeastern Tunisia) Habib Langar{, Meriem Bessibes{, Aslam Djellouli{, Christine Pergent-Martini1{{, and Gérard Pergent{{ { Institut National des Sciences et Technologies de la Mer (INSTM) Laboratoire de Biodiversité et des Biotechnologies Marines 28 rue du 2 mars 1934 2025 Salammbô, Tunisie [email protected] { Faculté des Sciences de Tunis Département de Biologie Campus Universitaire el Manar 1002 Tunis, Tunisie 1 Regional Activity Center for Specially Protected Areas (RAC/SPA) Boulevard du Leader Yasser Arafat, BP 337 1080 Tunis cedex, Tunisie www.cerf-jcr.org {{ Université de Corse UMR CNRS 6134 Faculté des Sciences et Techniques BP 52 20250 Corte, France ABSTRACT LANGAR, H.; BESSIBES, M.; DJELLOULI, A.; PERGENT-MARTINI, C., and PERGENT G., 2011. The Neogoniolithon brassica-florida (Harvey) Setchell & L. R. Mason (1943) Reef of Bahiret el Bibane lagoon (southeastern Tunisia). Journal of Coastal Research, 27(2), 394–398. West Palm Beach (Florida), ISSN 0749-0208. Neogoniolithon brassica-florida (encrusting rhodobionta) is generally reported as a veneering coral community along the wave-beaten rocky coast of the Mediterranean Sea. Its presence in the hyperhaline lagoon of Bahiret el Bibane, situated in SE Tunisia, takes on particular importance because of its extension. It constitutes a reef formation 14 km long developing on both sides of the sea inlet. The building of this ‘‘natural monument’’ seems to be the result of an evolutionary series. The aim of this study was to investigate the current status of this reef, comparing it to 30 years ago. The current extension of the Neogoniolithon brassica-florida reef appears to be much reduced from that reported 30 years ago, indicating a regression of 26%. Four different phases leading to the reef building were also identified. ADDITIONAL INDEX WORDS: Coral reef, biogeography, status, evolutionary series. INTRODUCTION The encrusting rhodobionta, Neogoniolithon brassica-florida (Harvey) Setchell & L.R. Mason (1943), is widely distributed, mainly on the wave-beaten rocky coasts, both in tropical and temperate habitats (John et al., 2004; Silva, Basson, and Moe, 1996; South and Skelton, 2003; Womersley, 1996) in the lower mediolittoral zone (Giaccone et al., 1993; Molinier, 1960). In the Mediterranean Sea, the Neogoniolithon brassicaflorida community appears in the European repertories for the conservation of the biodiversity (EUNIS code: A1.232–Mediterranean Area) and in the list of habitats of interest for conservation of the Barcelona Convention (CAR-ASP code: II.4.2.8; PNUE-PAM-CAR/ASP, 2007). In the hyperhaline lagoon of Bahiret el Bibane (southern Tunisia), the Neogoniolithon brassica-florida builds a spectacular reef. Other localized and less-spectacular reefs have been recorded in Greece and Turkey, but they cannot compare with the reef of Bahiret el Bibane. This reef is unique, with no other similar formation in the entire Mediterranean, and for this reason, must be protected and gain the status of natural monument (Boudouresque, 2004, and references therein). DOI: 10.2112/JCOASTRES-D-10-00082.1 received 2 June 2010; accepted in revision 26 August 2010. Published Pre-print online 8 December 2010. ’ Coastal Education & Research Foundation 2011 The aim of this study was (i) to make an assessment about the current extent of this reef, (ii) to determine its dynamics during the past few decades, and (iii) to identify the phases leading to the reef building. MATERIALS AND METHODS Study Area This work was carried out in Bahiret el Bibane, a hyperhaline lagoon of SE Tunisia (Keer, 1976; Medhioub, 1979) (Figure 1). This lagoon, which has an area of about 230 km2, is separated from the sea by a fossil Tyrrhenian oolitic limestone offshore bar (Medhioub, 1979). The central part of this offshore bar is split at approximately 2.5 km into a series of nine small islands separated by passes. On the largest of these islands (1 ha), surrounded by the two deepest passes (the lagoon sea inlet), a fishing company is in charge of managing fishing within the lagoon. Low stone walls, submerged during high-sea levels, block all other passes. The Development Company of Bahiret el Bibane recently built a hotel with 12 bungalows on one of the nine small islands at the entrance of the lagoon (Ramsar, 2007). With a maximum depth of 6 m, the lagoon is characterized by increasing salinity from the sea inlet to the enclosed extremities, reaching more than 50% during the summer (Guelorget, Frisoni, and Perthuisot, 1982; Medhioub, 1979). The bottom of Neogoniolithon brassica-florida Reef of Bahiret el Bibane Lagoon Table 1. Figure 1. Geographical location of Bahiret el Bibane lagoon. the central part of this lagoon is covered by a vast meadow of Cymodocea nodosa (Ucria) Ascherson (1869), mixed locally with the chlorobionta Caulerpa prolifera (Forsskål) Lamouroux (1809) (Zaouali, 1982); the surface is covered by a mixed meadow that is estimated at 19,546 ha, i.e., 84% of the lagoon (Vela et al., 2008). Neogoniolithon brassica-florida, reported in the lagoon since 1929 (Seurat, 1929), develops in the northern part of the lagoon on both sides of the sea inlet, where it establishes a reef parallel to the coast. The reef is about 30 km long, which is comparable to a small fringing reef (Thornton, Pilkey, and Lynts, 1978). Field Activities and Data Analysis In a previous study (Vela et al., 2008), 459 field data points were obtained from the surface of the lagoon using a diving mask, when the depth and turbidity allowed it, or by scuba diving to cover the various types of benthic features represented in the lagoon. These data enabled the identification of bottom types and seabed types in the el Bibane lagoon and the approximate location of a thinly scattered Neogoniolithon brassica-florida reef platform lining the north coast of the lagoon. The limits of this reef were located in this study using a Global Positioning System (GPS) Garmin 276C (accuracy estimated between 5 and 10 m). Reef size, shape, distance from the coast, and vertical structure were investigated by snorkeling. The main reef builders (both algal and animal) were also identified, documenting their vertical position in the reef. Samples of macrophytes were also collected on the reef to produce an inventory of the main species of the benthic multicellular marine algae living on the reef. For different subtidal and intertidal Mediterranean macro- 395 Braun-Blanquet cover-abundance scale. Braun-Blanquet Score Range of Cover (%) 5 4 3 2 1 + 75–100 50–75 25–50 5–25 ,5; numerous individuals ,5; few individuals phyte assemblages, the qualitative minimum sampling area was previously determined to be between 64 and 200 cm2 (Ballesteros, 1992; Boudouresque, 1974; Boudouresque and Belsher, 1979a, 1979b; Cinelli et al., 1977a, 1977b; Coppejans, 1980). In the present study, samples were collected in three replicates within 400 cm2 (20 cm 3 20 cm) metal frame and preserved in a 4% formalin–seawater solution (commercial 37% formaldehyde 5 100%). Identification of algal species was based on microscope studies of formalin-preserved specimens. An exhaustive species list was drawn up. Taxonomic nomenclature was aligned with the nomenclature used in the Algaebase database (Guiry and Guiry, 2009). Dominance and abundance of each species was visually estimated in situ, using the Braun-Blanquet cover-abundance scale (Braun-Blanquet, 1932, 1964), as shown in Table 1, developed for terrestrial phytosociological studies but also being applied by phycologists, especially those working in the Mediterranean (e.g., Boudouresque, 1971a, 1971b). Areas used for estimating dominance and abundance were those selected for sampling macrophytes. RESULTS The Neogoniolithon brassica-florida reef extends 12.5 km to the west and for about 1.5 km to the east of the sea inlet (Figure 2). This reef is 0.5 m to 2.0 m wide and is continuous in the western side of the sea inlet and delimits a small channel parallel to the coast (Figure 3), whereas on the eastern side, it is discontinuous and less developed (Figure 4). The reef distance to the coast varies between 1 m on each side and nearly 20 m in the central part, especially along the western coast. Taking into account the reef vertical structure, the upper part is composed of the rhodobionta Neogoniolithon brassicaflorida, whereas the lower part was composed of Sabellariidae. This latter part, bioconstructed by the Sabellariidae, is more fragile than the rhodobionta section and has tended to deteriorate with time (bioerosion) generating structures like mushroom (Figure 5) or overhanging along the reef (Figure 6). On the Sabellariidae, several macrophytes are observed (Table 2). Between the reef and the coast, other structures bioconstructed by the Sabellariidae (platforms) and covered by a few macrophytes and some individuals of Neogoniolithon brassica-florida were also observed. Samples of macrophytes taken from the base of the reef and from the bioconstructed platforms were very homogeneous from a specific point of view. However, the relative abundance of each species was variable between the sampling from the base of the reef and that of the bioconstructed platforms. Journal of Coastal Research, Vol. 27, No. 2, 2011 396 Langar et al. Figure 2. Extension of the Neogoniolithon brassica-florida reef in the Bahiret el Bibane lagoon. The main species of macrophytes recorded at the base of Neogoniolithon brassica-florida reef includes 6 Heterokontophyta (class Phaeophyceae), 3 Chlorophyta (2 Ulvophyceae and 1 Bryopsidophyceae), and 5 Rhodophyta (class Florideophyceae). The most abundant species was Cystoseira compressa, and the least abundant one was Padina pavonica. On the bioconstructed platforms, the most abundant species was Padina pavonica, and all other species were weakly represented. DISCUSSION The current extension of the Neogoniolithon brassica-florida reef (14 km) appears much reduced from that last reported in 1978 (31 km) by Thornton, Pilkey, and Lynts (1978). However, the less-detailed method adopted by Thornton, Pilkey, and Lynts (1978) to estimate the extension differed from that used in this work (measuring the coastal line); using our method on the 1978 observations would reduce the last extension of the reef to approximately 19 km. Figure 3. Neogoniolithon brassica-florida reef, continuous appearance. Figure 4. Neogoniolithon brassica-florida reef, discontinuous appearance. Despite this correction, regression of the coral reef seems to have occurred during the past 30 years, primarily affecting the areas located on both sides of the sea inlet, where the hydrodynamism is the most important in the lagoon (Medhioub, 1979). To the east of the sea inlet, the totality of the reef observed in 1978 seems to have disappeared, and a new reef, with a discontinuous appearance, is now observed more to the east (Figure 2). However, because the fastest growth rate reported for any of the corals is the staghorn species Acropora cervicornis (Roth, 1979), with a maximum growth rate of 264 mm/y (Lewis et al., 1968), it seems unlikely that our observations correspond to a new bioconstructed structure, achieved during the past 30 years, but they could correspond to what remains of the previously described reef. Our current method of geolocation, being more precise than that of yesteryear, we can consider that our geographical location is more accurate. The regression observed on both sides of the sea inlet, therefore, concerns nearly 5 km (26% of the reef extent), according to our method of measurement. Figure 5. Vertical structure of the mushroom-like Neogoniolithon brassica-florida reef. Journal of Coastal Research, Vol. 27, No. 2, 2011 Neogoniolithon brassica-florida Reef of Bahiret el Bibane Lagoon 397 Table 2. Main macrophytes present at the base of Neogoniolithon brassica-florida reef and on the bioconstructed platforms and their respective affected Braun-Blanquet score according to Table 1. Braun-Blanquet Score Class, Order, and Species At the base of the reef On the platform Florideophyceae Figure 6. Neogoniolithon brassica-florida reef overhanging structure. Ceramiales Laurencia obtusa (Hudson) Lamouroux 1813 Corallinales Jania rubens (Linnaeus) Lamouroux 1816 Gigartinales Hypnea musciformis (Wulfen) Lamouroux 1813 Peyssonnelia dubyi Crouan & Crouan 1844 Peyssonnelia squamaria (Gmelin) Decaisne 1842 3 + 2 1 2 1 2 + 1 + 1 + 1 + 1 + 2 + 2 + 2 2 1 + + 2 4 + Bryopsidophyceae The observed macrophytes species are typical of lagoonal environments (Djellouli, Verlaque, and Rais, 2000; Pérez-Ruzafa et al., 2008). Their presence on the reef and on its base suggests that the reef constitutes a favorable substratum for the installation of an algal assemblage dominated by Cystoseira compressa and combining several species of the upper infralittoral. It is difficult to identify exactly the phases leading to the reef building, but it is possible to propose the following hypothesis: Phase 1: Installation of Sabellariidae on shell debris, creating small patches of hard substrate Phase 2: Development of a colony of Sabellariidae, which is at the origin of the bioconstruction of platforms on which an association with Cystoseira compressa builds on. Phase 3: Individuals of Neogoniolithon brassica-florida settle on the platforms and start to develop both horizontally and vertically. Phase 4: Isolated colonies of Neogoniolithon brassicaflorida join and form the reef. The vertical structure of the Neogoniolithon brassica-florida formation shows this superposition, with a ‘‘layer’’ of Sabellariidae at the base, on which the macrophytes grow, surmounted by the encrusting rhodobionta (Figure 5). CONCLUSION The decline of the Neogoniolithon brassica-florida reef in the past few decades and the recent construction of infrastructures on the small islands located at the entrance of the lagoon, which are likely to degrade the environmental quality, justify implementation of measures allowing for the preservation of this natural monument. The inclusion of the lagoon on the list of sensitive sites in Tunisia, by the Coastal Protection and Planning Agency (APAL) (Decree No. 98-2092 of October 28, 1998) and its recent inclusion in 2007 on the Ramsar List of Wetlands of International Importance (Ramsar site No. 1697) (Ramsar–Iran, February 2, 1971, United Nations Treaty Series No. 14583) are first steps in this direction and should be continued. Bryopsidales Halimeda tuna (J. Ellis & Solander) J.V. Lamouroux 1816 Ulvophyceae Cladophorales Anadyomene stellata (Wulfen ) C. Agardh 1823 Dasycladales Acetabularia acetabulum (Linnaeus) P. C. Silva 1952 Phaeophyceae Cutleriales Cutleria multifida (Turner) Greville 1830 Dictyotales Dictyota dichotoma (Hudson) Lamouroux 1809 Dictyota dichotoma var. intricata (C. Agardh) Greville 1830 Dilophus fasciola (Roth) M.A. Howe 1914 Padina pavonica (Linnaeus) Thivy in W.R. Taylor 1960 Fucales Cystoseira compressa (Esper) Gerloff & Nizamuddin 1975 ACKNOWLEDGMENTS This work was carried out and funded through the framework of the decentralized cooperation program between the territorial collectivity of Corsica (France) and the Medenine governorate (Tunisia), which brings together the University of Corsica, the I.N.S.T.M. (National Institute of Science and Technology of the Sea), and the University of Tunis. 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