The improvement of rhodolith beds mapping and the knowledge of rhodolith growth rate will help understand the latitudinal and depth variation in bed dynamics, and allow quantification of carbonate production of this major benthic community (Foster, 2001; Basso, 2012). Rhodoliths were discovered in the Ras Ghamila lagoon, southern Sinai (27°58,579’N, 34°25,631’E), and their benthic environment was investigated during a two-years survey. Along a W-E profile from the beach to the reef-front, rhodoliths of the praline type and unattached branches were abundant in two facies: 1) a coral-coralline facies in biodetric gravelly sand, at about 50-140 cm of water depth, and 2) a phanerogam meadow (Thalassodendron ciliatum) where coralliths and rhodoliths occur together, at about 50 cm depth. A total of 38 rhodoliths were collected in May 2007 in the coral-coralline facies and have been Alizarin-S stained (0.25g/l for 24h; Rivera et al., 2004). Each rhodolith was tied with nylon wire bearing a plastic ring to mark the sample. After staining, rhodoliths were put back in their original environment, at 60 cm depth. Rhodoliths were recovered after one year (April 2008). The air-dried specimens were incorporated in resin (Araldite) and sectioned radially using a rock saw. The thickening and the elongation of the thallus during the year after staining was then measured as the distance from the Alizarin mark to the living surface of the alga. Coralline species were identified by Scanning Electron Microscopy (SEM) at the Milano-Bicocca facilities and by Optical Microscopy (OM) after decalcification at the Milano University facilities. Rhodoliths of the coral-coralline facies are mostly monospecific or oligospecific, have a fruticose growth form, a spheroidal shape, a protuberance degree of IV and correspond to the unattached branches or praline type (Bosence, 1983; Basso et al. 2009). They are formed by Neogoniolithon brassica-florida (Harvey) Setchell & Mason, Neogoniolithon spp., Porolithon onkodes (Heydrich) Foslie, Lithophyllum kotschyanum Unger, and Titanoderma sp. The observed mean growth rate of L. kotschyanum, the commonest rhodolith-forming coralline species in Ras Ghamila (as unattached branches), was 1268.76 (SE 449.5) µm y-1. The other Lithophylloid, Titanoderma sp. (in coated grains), recorded the maximum growth of 1360 µm y-1. Neogoniolithon spp. (pralines) growth ranged from 566.7 to 698.6 (SE 337.3) µm y-1, reaching 825 µm y-1 in N. brassica-florida. The measured growth in P. onkodes rhodoliths (coral-rubble coatings) was 850 (SE 70.7) µm y-1. In some rhodoliths, corallines grew over the plastic ring used to mark the specimen. In these cases we were able to measure also the elongation of the thallus, which resulted as fast as 5250 to 16450 µm y-1 in L. kotschyanum, 11750 µm y-1 in Titanoderma sp., 9250-9400 µm y-1 in Neogoniolithon spp., and 11350 to 12500 µm y-1 in P. onkodes. The Ras Ghamila rhodoliths are important primary producers and carbonate producers in this very shallow, tropical marine environment. Their formation and coexistence with coralliths is also discussed.

Southern Sinai rhodoliths: facies, species composition, and growth rate (Red Sea, Egypt) / D. Basso, A. Caragnano, F. Benzoni, G. Rodondi. ((Intervento presentato al 4. convegno IV International Rhodolith Workshop tenutosi a Granada nel 2012.

Southern Sinai rhodoliths: facies, species composition, and growth rate (Red Sea, Egypt)

G. Rodondi
2012

Abstract

The improvement of rhodolith beds mapping and the knowledge of rhodolith growth rate will help understand the latitudinal and depth variation in bed dynamics, and allow quantification of carbonate production of this major benthic community (Foster, 2001; Basso, 2012). Rhodoliths were discovered in the Ras Ghamila lagoon, southern Sinai (27°58,579’N, 34°25,631’E), and their benthic environment was investigated during a two-years survey. Along a W-E profile from the beach to the reef-front, rhodoliths of the praline type and unattached branches were abundant in two facies: 1) a coral-coralline facies in biodetric gravelly sand, at about 50-140 cm of water depth, and 2) a phanerogam meadow (Thalassodendron ciliatum) where coralliths and rhodoliths occur together, at about 50 cm depth. A total of 38 rhodoliths were collected in May 2007 in the coral-coralline facies and have been Alizarin-S stained (0.25g/l for 24h; Rivera et al., 2004). Each rhodolith was tied with nylon wire bearing a plastic ring to mark the sample. After staining, rhodoliths were put back in their original environment, at 60 cm depth. Rhodoliths were recovered after one year (April 2008). The air-dried specimens were incorporated in resin (Araldite) and sectioned radially using a rock saw. The thickening and the elongation of the thallus during the year after staining was then measured as the distance from the Alizarin mark to the living surface of the alga. Coralline species were identified by Scanning Electron Microscopy (SEM) at the Milano-Bicocca facilities and by Optical Microscopy (OM) after decalcification at the Milano University facilities. Rhodoliths of the coral-coralline facies are mostly monospecific or oligospecific, have a fruticose growth form, a spheroidal shape, a protuberance degree of IV and correspond to the unattached branches or praline type (Bosence, 1983; Basso et al. 2009). They are formed by Neogoniolithon brassica-florida (Harvey) Setchell & Mason, Neogoniolithon spp., Porolithon onkodes (Heydrich) Foslie, Lithophyllum kotschyanum Unger, and Titanoderma sp. The observed mean growth rate of L. kotschyanum, the commonest rhodolith-forming coralline species in Ras Ghamila (as unattached branches), was 1268.76 (SE 449.5) µm y-1. The other Lithophylloid, Titanoderma sp. (in coated grains), recorded the maximum growth of 1360 µm y-1. Neogoniolithon spp. (pralines) growth ranged from 566.7 to 698.6 (SE 337.3) µm y-1, reaching 825 µm y-1 in N. brassica-florida. The measured growth in P. onkodes rhodoliths (coral-rubble coatings) was 850 (SE 70.7) µm y-1. In some rhodoliths, corallines grew over the plastic ring used to mark the specimen. In these cases we were able to measure also the elongation of the thallus, which resulted as fast as 5250 to 16450 µm y-1 in L. kotschyanum, 11750 µm y-1 in Titanoderma sp., 9250-9400 µm y-1 in Neogoniolithon spp., and 11350 to 12500 µm y-1 in P. onkodes. The Ras Ghamila rhodoliths are important primary producers and carbonate producers in this very shallow, tropical marine environment. Their formation and coexistence with coralliths is also discussed.
set-2012
Settore BIO/02 - Botanica Sistematica
Southern Sinai rhodoliths: facies, species composition, and growth rate (Red Sea, Egypt) / D. Basso, A. Caragnano, F. Benzoni, G. Rodondi. ((Intervento presentato al 4. convegno IV International Rhodolith Workshop tenutosi a Granada nel 2012.
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