Variation along the year of trace metal levels in the compartments of the seagrass Posidonia oceanica in Port El Kantaoui, Tunisia

The accumulation of the five Trace Metals (TMs) cadmium, copper, lead, nickel and zinc was measured in Posidonia oceanica leaves. Shoots were seasonally sampled at 8 - 10 m depth from four stations located in Port El Kantaoui area, Tunisia during four campaigns performed in 2012. Levels of the five TMs were analysed using inductively coupled plasma atomic emission spectrometry (ICP-AES) in three compartments of P. oceanica shoots: blades and sheaths of adult leaves, and intermediates leaves. Results showed a preferential accumulation of Cd, Pb, Ni and Zn in adult leaf blades. Therefore, we focus on the study of this compartment. TM levels of blades of adult leaves decreased in the following order: Zn > Ni > Cu > Pb > Cd, irrespective of the season. Levels of the five TMs significantly differed between seasons (p<0.01). Levels of Cd and Cu showed a seasonal pattern: Cd decreased from spring to winter while Cu increased during that same period of time. A significant correlation (p<0.01) was found between Cd-Cu and Cd-Pb. A significant correlation (p<0.05) was also noted between Cd-Ni in the adult leaves blades. A relationship was recorded between the foliar surface of the adult leaves blades and Zn accumulation. This survey allowed to highlight the annual variation of TM accumulation in adult leaves blades of P. oceanica, in relation with ecophysiology of this seagrass. Therefore, this study reinforces the usefulness and the relevance of this compartment of P. oceanica, easy to sample without destruction of whole shoot, as a bioindicator of Zn, Ni, Cd and Pb contamination.