In the Mediterranean Sea, organic and trace metal pollution has increased over the last 20 years [59,60], and this has had a perceptible effect on marine environments [61]; however, the seagrass meadows investigated in this study can be described as healthy in terms of density and biomass. The nutrient levels in the surrounding waters were low, and the trace metal levels in dead sheaths of P. oceanica were, in most cases, comparable to meadows in unpolluted areas. Thus, the meadows investigated in this study appear to be relatively unaffected by the increase of industrial, agricultural, and urban activities, and the surface waters exhibit low anthropogenic (organic and trace metals) discharges. Seagrasses in northern Corsica are currently safe from human pollution, although there are some sites showing impacts of urban centers (i.e., Bastia) or mining waste (i.e., Nonza).
Our study shows that P. oceanica can be used as a bioindicator of trace metals in the Mediterranean Sea because of its widespread distribution and because it allows retro dating and smoothing of seasonal variations (lepidochronology). In particular, this plant seems to be useful as a biomonitor for Cu, which is essential for plant growth and metabolism, because variations in its distribution in sheaths reflect the original leaf concentrations. The sheaths were less useful for recording changes in the concentrations of Cr, Ni, Zn, Cd, and Pb. Unfortunately, the As and Se levels could not be tested because they had only been measured in sheaths and not in leaves. Further studies on these latter eight metals should allow determination of whether they can be examined using lepidochronology techniques.
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