Seasonality and trophic diversity in molluscan assemblages from the Bay of Tunis (southern Mediterranean Sea)

Soft bottom molluscan assemblages from the Bay of Tunis have been studied in order to analyse their seasonality and trophic diversity in relation to environmental variables. A total of 147 species of molluscs was identified, with gastropods displaying the highest species richness and bivalves the highest abundances, and including five non-indigenous species such as the dominant bryozoan grazer Polycerella emertoni. Carnivorous and scavenger gastropods were among the most frequent species, reflecting a diverse benthic community. Seasonal changes were significant, being more acute at 3-4 m than at 10-15 m depth, and were correlated mainly with seawater temperature and percentage of organic matter in the sediment. The high affluence of tourists in summer was coincident with high decreases in species richness and abundance of molluscs, together with a strong siltation of the sediment. Nevertheless, most trophic groups persisted and the trophic diversity was relatively high. Significant relationships were found between the index of trophic diversity and Shannon-Wiener diversity and evenness indices, suggesting that the identity of the species with its particular trophic trait, together with the balanced distribution of the individuals among the species would be the drivers for the maintenance of the molluscan food web under environmental stress. The abundance of P. emertoni altered the trophic structure of the molluscan assemblage, bringing the ectoparasite trophic group to an unusual peak of dominance. Soft bottom molluscan assemblages of the Bay of Tunis should be taken into account in monitoring programs for anthropogenic impacts and for non-indigenous species trends throughout the Mediterranean basin.Versión del editor0,568

Behaviors of the energy of solutions of the wave equation with damping and external force

AbstractIn this paper we study the behavior of the energy of solutions of the wave equation with localized damping and an external force on compact Riemannian manifold with boundary. Under the Geometric Control Condition of Bardos et al. (1992) [4] and certain condition on the force, we prove that the energy goes to zero when the time goes to infinity and we give the rate of decay of the energy functional. More precisely, the behavior of the energy depends on the L2 norm of the force and is determined from a forced differential equation

Temporal variation of the algae-associated molluscan assemblage of artificial substrata in Bay of Tunis (Tunisia)

The molluscan assemblage inhabiting the photophilous algae from artificial rocky shores in theBay of Tunis) was studied from February 2009 to February 2010. Five monthly replicates were taken with a quadrat of 25 × 25 cm .. A total of 16105 individuals of molluscs, belonging to 93 species, were collected. Gastropods were the most species-rich group, with 66 species (70.96%), followed by bivalves with 26 species (27.95%). Gastropods were also the most abundant with 11529 individuals (71.60%), followed by bivalves with 4575 individuals (28.41%). Nevertheless,Mytilaster minimus was the dominant species overall during the studied period (15.6% of total abundance), followed by Tricolia miniata (11.6%), Bittium reticulatum (11.5%), Gibbula racketti (10.4%) and Rissoa similis (9.8%). The high abundance of juveniles of soft (e.g. Venerupis aurea, Nassarius corniculum) and hard bottom species (e.g. M. minimus, Fissurella nubecula) point out the important role   of this habitat as nursery for species from nearby located habitats. The presence of the alien molluscs Mitrella psilla (2.5% dominance) and Arcuatula senhousia highlights the importance of algae as vehicle and/or receptor habitat for alien species. Species richness and abundance displayed significant seasonal changes, with maxima in winter 2009, but the Shannon-Wiener diversity index showed similar values throughout the year. Multivariate analyses also indicated the presence of significant seasonal differences of this molluscan assemblage

Temporal variation of the algae-associated molluscan assemblage of artificial substrata in the Bay of Tunis (Tunisia)

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Dinámica espacio-temporal de la malacofauna asociada a fondos blandos de la bahía de La Goulette (norte de Túnez)

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