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 12 m 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.

No abstract

The aim of this survey was to study the cover and the composition of macroepiphytic species on the leaves of Posidonia oceanica in the east of Tunisia. Surveys were conducted in December 2009 (winter period), March (spring period) and August 2010 (summer period) in a fringing reef located in Chebba. At each sampling date, 15 adult leaves were randomly collected and divided into basal and apical parts. The inner face of each part was examined with ocular glasses and microscope to estimate species cover by orthogonal projection. PERMANOVA and ANOSIM were used to test for differences of cover between sampling dates and leaf parts. SIMPER, cluster analysis, and PCA were used to ordinate species assemblages. Comparison of epiphytic cover along leaf blades showed significant differences for all groups, except cyanophycea, with high cover of hydrozoans and bryozoans in the basal part and high cover of algae in the apical part. The species composition and cover also vary with sampling date; minimum values were detected in December and the epiphytic community was composed of a few pioneer species, whereas maximum epiphytic cover values were registered in August, with the epiphytic community being composed of a more mature and more diverse community, termed 'climax'. The main regulatory factors for this distribution are discussed.

No abstract

We studied spatial patterns in assemblages of epiphytic microalgae on the leaves of two seagrass species with different morphologies and longevity, Cymodocea nodosa and Posidonia oceanica, which cooccur in Chebba in Eastern Tunisia. Epiphyte assemblages were described for each species in summer. Epiphyte microalgal assemblages were more abundant on the leaves of C. nodosa but more diversified on the leaves of P. oceanica. We suggest that the differences in species composition and abundance between those seagrass species may reflect an interaction of timescales of seagrass longevity with timescales of algal reproductive biology. Short-lived C. nodosa was dominated by fast growing species such as the cyanobacteria species Oscillatoria sp., while P. oceanica leaves were colonized by more mature and diversified species such as Prorocentrales. Local environmental conditions (hydrodynamics, light penetration), host characteristics (meadow type, shapes forms of leaves, life span, and growth rate), and grazing effect seem also to be responsible for these dissimilarities in epiphytic microalgae communities.