Variability in the structure of epiphytic assemblages of Posidonia oceanica in relation to human interferences in the Gulf of Gabes, Tunisia

Abstract

In this study we evaluate whether the pattern of spatial variability of the macro-epiphyte assemblages of leaves of Posidonia oceanica differed in relation to anthropogenic interference in the Gulf of Gabes (southern coast of Tunisia). A hierarchical sampling design was used to compare epiphytic assemblages at 5 m depth in terms of abundance and spatial variability at disturbed and control locations. The results indicate that the biomass and mean percentage cover decreased at locations near the point of sewage outlet in comparison to control locations. These losses were related to the distance from the source of disturbance. This study revealed that the diversity is reduced in disturbed locations by the loss of biomass and the mean percentage cover, explained by means of a multiple-stressor model which plays an important role in the macro-epiphytes' setting. It is urgent to propose the best management plans to save the remaining P. oceanica meadow in the Gulf of Gabes and its associated epiphytes.

Variability in the structure of epiphytic microalgae assemblages on the leaves of <em>Posidonia oceanica</em> in relation to human disturbance in a meadow off Tunisia

“…The number of leaves per shoot was not different between stations, a finding which is consistent with previous studies (Capiomont et al 2000. A decline in shoot density, leaf area index, leaf surface and leaf length at disturbed stations compared with control sites has been found by several authors (Leriche et al 2004, Ben Brahim et al 2010). These results cannot be assigned only to nutrient enrichment; several factors may also explain this decline: first, the decrease in water transparency at the disturbed station (Table 1) involves the reduction of light intensity necessary for photosynthesis.…”

“…It was clear that the effect of nutrient loading and turbidity could be depicted in the community structure and diversities of seagrass-associated epiphytes , Ben Brahim et al 2010 …”

“…Epiphytic microalgae composition, in particular, is influenced by abiotic factors such as temperature (Aligizaki and Nikolaidis 2006;Turki 2005), light, salinity, and nutrient availability (Armitage et al 2006) and biotic factors such as grazing (Mazzella and Russo 1989), shoot length, density and morphology of the host plant (Sirota and Hovel 2006, Chung and Lee 2008, Mabrouk et al 2011. Nutrient over-enrichment caused by anthropogenic activities has been associated with the shift of the structure of microepiphytic assemblages (Armitage et al 2006) and the decline of seagrass (Green and Short 2003, Orth et al 2006, Ben Brahim et al 2010. For example, a reduction of leaf growth and a decrease in shoot density were observed around a sewage outfall (Pergent-Martini 1994) as a consequence of epiphyte overgrowth (Cambridge et al 1986).…”

“…It is widely known that the distribution of epiphytic assemblages of P. oceanica both on leaves ) and rhizomes (Piazzi et al 2004;Balata et al 2008) can change, but few studies analysing changes in epiphyte assemblages on leaves of P. oceanica in Tunisia has been published (Ben Brahim et al 2010, and to our knowledge no studies on microepiphytes have been conducted up to now. Many authors have bemoaned the lack of data in some Mediterranean regions, particularly North Africa (Ruiz et al 2009).…”

Variability patterns of epibenthic microalgae in eastern Tunisian coasts

“…The concentration of P. lima on the outer surface of the Posidonia leaf, explained by the behaviour it uses to escape predators (Ben Brahim et al 2010), is in opposition to the general behaviour of other epiphytic species, particularly Ostreopsis cf. ovata, which has been shown to prefer the inner face of Posidonia leaves (Alcoverro et al 2004, Peirano et al 2011).…”