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Mabrouk L.,University of Sfax | Mabrouk L.,Institute National Des Science Et Des Technology Of La Mer Of Sfax | Hamza A.,Institute National Des Science Et Des Technology Of La Mer Of Sfax | Mahfoudi M.,Institute National Des Science Et Des Technology Of La Mer Of Sfax | Bradai M.-N.,Institute National Des Science Et Des Technology Of La Mer Of Sfax
Cahiers de Biologie Marine | Year: 2012

In order to study the spatial and temporal variation in abundance of the potentially toxic species Ostreopsis siamensis epiphytic on the leaves of Posidonia oceanica, two dates of sampling were conducted in the region of Mahdia (Eastern Tunisia) in July 2008 and January 2009. Two sites within each of four stations were selected at depths of 3, 5, 8 and 12 m, P. oceanica shoots and seawater samples at two different levels of the water column (bottom and surface) were collected. Utermöhl sedimentation method was used to quantify the abundance of O. siamensis epiphyte on the leaves of P. oceanica and living in the water column. Our results showed that the abundance of this epiphytic species is lower than those found in other areas of Tunisia. Covariance analysis (ANCOVA) of abundance of epiphytic O. siamensis on the leaves of P. oceanica between time, station and site nested within station with depth as covariate showed that spatial differences of this abundance depended on the time of sampling and the highest value are recorded in July. Temperature and life cycle of the host plant seemed to be the important factors responsible for this temporal variation. Linear regression showed a significant relationship between the abundance of this species and Leaf Area Index of P. oceanica. We also found positive significant correlations between the abundance of epiphytic O. siamensis and its abundance in the water column (bottom and surface). These results could indicate a transfer of this species from P. oceanica leaves to the water column. Source


Mabrouk L.,University of Sfax | Hamza A.,Institute National Des Science Et Des Technology Of La Mer Of Sfax | Brahim M.B.,University of Sfax | Bradai M.-N.,Institute National Des Science Et Des Technology Of La Mer Of Sfax
Aquatic Botany | Year: 2013

We investigated the impact of sewage discharge on Posidonia oceanica meadows in Eastern Tunisia (Mahdia). We specifically addressed changes in plant growth and epiphyte composition on leaves and rhizomes caused by this anthropogenic interference. A hierarchical sampling design was used to compare epiphyte biomass and community structure between one disturbed and two control stations. Samples were collected by SCUBA diving at 8. m depth in August 2009. A total of 44 taxa, macroinvertebrates and macroalgae, were identified on rhizomes, and 27 taxa on leaves. In the disturbed station, leaf length, leaf surface area and the leaf area index decreased, whereas epiphyte biomass increased compared to the control stations. Rhizome epiphyte coverage differed significantly between the control stations and the disturbed station, mainly due to changes in green, red and brown algae, bryozoans, and ascidians. Leaf epiphyte coverage differed between stations, with more red algae and ascidians on leaves of the disturbed station and more, green and brown algae at the control stations. SIMPER and CCA analyses showed that differences were due to the absence of some species at the control stations, which exhibited a high coverage at the polluted station.In summary, waste water pollution caused a severe deterioration of seagrass meadow vitality, possibly due to an increased epiphyte cover and associated changes in epiphyte community composition on leaves and rhizomes of P. oceanica. © 2013 Elsevier B.V. Source


Mabrouk L.,University of Sfax | Hamza A.,Institute National Des Science Et Des Technology Of La Mer Of Sfax | Brahim M.B.,University of Sfax | Bradai M.-N.,Institute National Des Science Et Des Technology Of La Mer Of Sfax
Marine Ecology | Year: 2011

A survey on the epiphytic microorganisms growing on Posidonia oceanica leaves was conducted along a depth transect along the coast of Eastern Tunisia (Mahdia). Samples were collected by SCUBA diving at depths of 3, 5, 10 and 12m in July 2008 and January 2009. A total of 58 microepiphyte taxa were identified. Multivariate analyses revealed temporal and spatial variation of the abundance of epiphytic microalgae. Water motion, light availability, temperature and motility of species seem to be responsible of temporal and bathymetric variations of epiphytic microalgae. Unlike diatoms, dinoflagellates were more abundant in deep waters, suggesting that they are more vulnerable to hydrodynamics. The significant correlation between leaf area index and abundance of epiphytic species indicates that the phenological parameters of the host plant influence the abundance of the epiphytic microorganisms. Among the epiphytic dinoflagellates, our data showed a great number of potentially toxic species (Alexandrium minitum, Amphidinium carterae, Karenia selliformis, Coolia monatis, Karlodinium veneficum, Ostreopsis siamensis, Prorocentrum concavum, Prorocentrum minimum, Prorocentrum rathymum and Prorocentrum lima). These species were more abundant in the deep station under more sheltered conditions than found at inshore shallower stations, which are subjected to high water motion. © 2011 Blackwell Verlag GmbH. Source

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