ORIGINAL ARTICLE Spatial scale variability in shoot density and epiphytic leaves of Posidonia oceanica on Kerkennah Island (Tunisia) in relation to current tide effects Mounir Ben Brahim 1,2 , Lotfi Mabrouk 1,2 , Asma Hamza 2 , Mabrouka Mahfoudhi 2 , Abderahmen Bouain 1 & Lotfi Aleya 3 1 Facult e des Sciences Sfax, Universit e de Sfax, Sfax, Tunisia 2 Centre de Sfax, Institut National des Sciences et Technologies de la Mer, Sfax, Tunisia 3 Laboratoire de Chrono-Environnement, Universit e de Franche-Comt e, Besanc ßon Cedex, France Keywords Current tide; epiphytes; Kerkennah Island; Mediterranean; Posidonia oceanica; spatial scale; Tunisia. Correspondence Lotfi Aleya, Universit e de Franche-Comt e, Laboratoire de Chrono-Environnement, UMR CNRS 6249, 1, La Bouloie, F-25030 Besanc ßon Cedex, France. E-mail: lotfi.aleya@univ-fcomte.fr Accepted: 1 September 2014 doi: 10.1111/maec.12231 Abstract A study was undertaken of the patterns of spatial variability, epiphytic biomass and distribution of epiphytic fauna and flora of Posidonia oceanica. Samples were taken at four stations located approximately 4 km apart, exposed to differ- ent current conditions. Stations A and B, situated near the Oued Mimoun tidal channel with its relatively strong bi-directional flows, were affected by high cur- rent tide. The other two stations, North Oued Mimoun (L1) and South Oued Mimoun (L2), were located further from the channel, in low current tide condi- tions. Sampling was conducted in the Attaya area of Kerkennah Island (Tunisia) in August 2009 at depths between 2 and 3 m, with the results indicating differ- ences among the stations. Shoot density decreased when exposed to high levels of hydrodynamic activity generated by current tides whereas the epiphytic bio- mass of P. oceanica leaves decreased at sheltered stations located far from the channel. Epiphytic algae such as Heterokontophyta, Rhodophyta and Chloro- phyta, and epiphytic fauna represented by Bryozoa, Hydrozoa, Annelida, Porif- era and Tunicata, dominated the epiphytic assemblages and were abundant at the station most exposed to high current tide hydrodynamics. Cyanobacteria, however, were dominant in stations exposed to low current tide. Introduction Seagrasses are the dominant plant species of sandy and muddy bottoms and their communities play an impor- tant role in the structure and dynamics of many coastal and estuarine benthic systems (La Loggia et al. 2004; Infantes et al. 2011). Their ecological importance lies mainly in their structural stability, nutrient recycling, effects on the coast, the food and shelter that they pro- vide to many organisms and their high productivity. Seagrasses provide a nursery area for many valuable fish species (Wood et al. 1969; Mazzella et al. 1992) and har- bor epiphytic assemblages, contributing to system biodi- versity and production and thus facilitating the provision of an important food resource for many invertebrates and fish (Fong et al. 2000; Borowitzka et al. 2006; Russell & Connell 2007). However, the structural stability of sea- grass may be subject to a range of physical and biological pressures (Den Hartog 1971; Fonseca et al. 1983; Kirk- man & Kuo 1990; Marb a & Duarte 1994, 1995; Marb a et al. 1994). For example, the disruption of seagrass due to major storms or pollution events may be more severe and result in greater community instability (Marb a& Duarte 1995; Fonseca & Bell 1996) or even long-term deterioration of the ecosystem (Cambridge 1979; Kirkman 1987; Fonseca & Bell 1996). Both seagrasses and their epiphytes are influenced by water movement dynamics, which affect the functional and structural features of seagrass beds and also regulate the availability of organic matter (Fonseca et al. 1983; Folkard 2005). Hydrodynamics also control other physical and chemical factors such as temperature and salinity, Marine Ecology (2014) 1–21 ª 2014 Blackwell Verlag GmbH 1 Marine Ecology. ISSN 0173-9565