The Use of Lepidochronology to Assess the Impact of Terrigenous Discharges on the Primary Leaf Production of the Mediterranean Seagrass Posidonia oceanica

The impact of terrigenous discharges on the leaf production of the Mediterranean seagrass Posidonia oceanica has been investigated. The shoot leaf production has been estimated by using the lepidochronological technique (a reconstruction method based on the cyclic annual variation of the sheath thickness), whose sensitivity as a tool to detect this kind of man-induced impact has also been tested. For this purpose, a number of 20 shoots were collected at each of the four sites investigated, represented by two impacted and two control sites, located along the northern coast of the Ischia Island (Gulf of Naples, Italy). The annual leaf production per shoot of the year in which the impact occurred and that of the two previous years has been estimated. A decrease of the leaf production was observed in P. oceanica shoots from the impacted sites after the terrigenous discharges, while such a trend was not detected at the control sites. The decrease of the primary leaf production (around 20%) was mainly attributable to the decreased mean number of leaves produced per year, whereas mean leaf density and length remained constant. The results obtained in this study confirm that P. oceanica is a sensitive indicator of the environmental quality of coastal waters and that lepidochronology is a useful tool to provide evidence of the impact induced by terrigenous discharges on coastal zones colonized by this seagrass.