Analysis of coastal dune vegetation in relation to Posidonia oceanica seagrass

Abstract

Coastal systems are situated at the interface between marine and terrestrial environments and are particularly sensitive to the changes in the surroundings. They are strongly linked to marine environments and an intense exchange of energy and nutrient occurs through the accumulation of wrack derived from marine seagrasses and macroalgae. In the Mediterranean, consistent deposits of seagrasses wrack accumulate along sandy shores. These deposits are mainly constituted of material from the endemic seagrass Posidonia oceanica. Beach-cast wrack plays several roles and influences both the morphology and the ecological process of coastal environments. Although coastal vegetation is fundamental for dune formation processes and for the ecosystem maintenance, few authors have investigated the effect of beach-cast wrack on plant species. This decomposing material could be a relevant subsidy in sandy shores, which are typically nutrient poor. We hypothesize that the accumulation of P. oceanica beach-cast wrack on coastal dunes has an important positive effect during the germination and early stages of plant growth, and consequently, it may contribute to a significant increment of the total plant diversity. This project aims at analyzing the effects of P. oceanica beach-cast wrack on coastal dune vegetation, at species and community levels. In particular, we focused on the effects of wrack on: 1) seed germination of the most typical coastal plant species 2) seedling development 3) the structure of coastal dune plant communities The effect of P. oceanica wrack on seed germination was investigated through germination tests. Germplasm of most typical dune plant species (Cakile maritima, Elymus farctus and Crucianella maritima) was collected along the coasts of Lazio, Sardinia and Balearic Islands. The effect of light, temperatures, NaCl and KNO3 was tested. In this PhD thesis only the results for Crucianella maritima are shown. The seed germination of this species was characterized by photo-inhibition, absence of primary dormancy and a secondary dormancy imposed by salt concentrations, with no requirement for nutrient-rich soils. IV Plant growth experiments were carried out in a greenhouse, comparing plants growing on substrates with and without P. ocenica wrack. The presence of wrack enhanced the plant growth, fertilized the substrate, kept it humid and favoured the nutrient uptake. The analysis of coastal dune vegetation at community level was studied through the comparison of phytosociological relevés (obtained from a literature review) from beaches with high or with low volumes of beach-cast wrack. High levels of wrack incremented species cover and influenced species composition. Our results revealed that P. oceanica beach-cast influences the vegetation at both species and community level. Moreover, our findings could provide useful information for coastal management plans.