Effects of urban effluents on spatial structure, morphology and total phenols of Posidonia oceanica: Comparison with a reference site

In the Mediterranean, the discharge of urban effluents in coastal areas adversely affects Posidonia oceanica, since it induces nutrient enrichment and a decrease of water clarity, but data from the southern Mediterranean are very sparse. In this paper we examine the differences in P. oceanica parameters from a disturbed site (Bou Ismaïl) and a reference site (Kouali), at meadow level (shoot density, meadow cover), at individual level (leaf biometry, A coefficient, epiphytic index) and at biochemical level (total phenols). The differences were examined at the upper limit (− 5 m) and the lower limit (− 20 m), during an annual cycle. Results indicated a significantly lower mean meadow cover at the upper limits of the disturbed site compared to the reference site. The leaf length and leaf area were also lower at the disturbed site, and this difference was recorded all year round at the upper limit. The epiphytic index and the A coefficient of adult leaves were higher only in July and October at the upper limits of the disturbed site. In contrast, total phenol concentration of the leaves did not show any response to disturbances related to urban effluents. However, the biosynthesis of phenolic substances at the two sites was significantly higher in the intermediate leaves, in winter, when growth slows down. The unexpected higher value of total phenol concentration recorded in the intermediate leaves and sheaths in spring at the reference site is discussed.