Variability in the structure of epiphyte assemblages on leaves and rhizomes of Posidonia oceanica in relation to human disturbances in a seagrass meadow off Tunisia

• Biometric parameters of Posidonia decreased in the sewage disturbed station.
• Epiphyte species composition varied between control and disturbed stations.
• Epiphyte biomass increased in the disturbed station compared to control stations.
• Water pollution deteriorated seagrass meadow vitality in Eastern Tunisia.

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.