Epiphytic microalgae were monitored on various substrates of seagrass and macroalgae and in the water column for one year (from March 2013 to March 2014) in Oued Lafrann along the eastern coast of Chebba (Tunisia) with a focus on the distribution patterns of the epibenthic toxic dinoflagellates Ostreopsis cf. ovata, Prorocentrum lima and Coolia monotis. Microalgae assemblages were dominated by diatoms and dinoflagellates both in the water column and on vegetation. High concentrations of epiphytic toxic and potentially toxic dinoflagellates were preferentially hosted by Posidonia leaves, mainly in the apical and middle regions of the leaves, and P. lima was the dominant species. A significant positive correlation was found between P. lima concentrations on Posidonia and in the water column, suggesting that macrophytes should be sampled in the framework of harmful algal species monitoring. Ostreopsis cf. ovata, exhibited low concentrations and was mainly present on the inner surface of the Posidonia leaf, whereas P. lima was mainly present on the outer surface of the leaf, suggesting a likely space competition.