The natural recolonisation process of the seagrass Posidonia oceanica (L.) Delile after the introduction of the Italo-algerian methane pipeline in the SW Mediterranean Sea.

This work investigates the recolonisation process and patterns of the seagrass Posidonia oceanica after a man-induced impact. The installation of a methane pipeline connecting Italy and Tunisia destroyed a large seagrass bed in the South Mediterranean Sea (SW Sicily, Italy). Pipes were buried and backfilled using terrestrial calcareous rubble. As a results of dumping activities, the rubble formed a mound-like seascape changing dramatically the sea bed features of the area. This research was articulated in two parts, the former involving an ecological approach to the problem where attention was focused on whether rubble mounds represent a suitable substrata for P. oceanica recruitment. Attention has been drawn on the role of environmental factors on such a process, and in particular how the feedback between plant recruitment, sediment dynamics and water flow is created in this new artificial environment. The results indicated how P. oceanica recruits via vegetative fragments which break from the meadow during storms. However, vegetative recruitment only occurs in sheltered (valley) areas. Once recruited, fragments are able to become rooted and draw on sediment nutrients providing the resources needed to grow. In valleys, once patches are well established, they create a positive feedback between plant canopy, water flow attenuation and sediment deposition. Sheltered (valley) and exposed (crests) locations on the mounds show different physical and geological characteristics. Differences in the sediment dynamics between exposed (crests) and sheltered (valleys) locations on the mound field entail differences in the amount of nutrient available to the plants for growth and photosynthesis and hence plant morphology. The latter part of this thesis was focalised on the physiological response of the plant to this new environment. Nutrient limitation and plant phenology have been considered to draw a complete picture not only on how a hard substratum might affect the recruitment of P. oceanica but also to assess how this species might have adapted to this new, artificial environment. Indeed, in the presence of the canopy in valleys favours particle settlement, so contributing to the formation of a sediment layer which provides the necessary nutrients for plant growth. However, the rubble topography might play an important role in seagrass resource allocation to the different biomass compartments. Although N and P content found in the valleys cannot be considered as limiting for seagrass growth plants might still experience some nutrient limitation. Thus, seagrass morphological features, biomass and production are dependent on local factors, such as substratum type and resource availability. The plasticity of P. oceanica modules allow the plant to adapt to a new environment. Plants living in valleys increase resource allocation to the roots to best exploit porewater nutrients. As plant on valleys need a better anchorage on the rubble they tend to allocate more resources to the below-ground organs, while above ground biomass is reduced.This study is the first to report on the recovery of P. oceanica on an artificial substratum following a human impact. After a series of unsuccessful project on Posidonia oceanica restoration, this work indicates that P. oceanica is able to recover by means of vegetative growth after a large human-induced disturbance. Moreover, this thesis intends to evaluate the possibility of employ artificial substrata to favour the recovery of lost seagrass beds in the Mediterranean Sea. This would allow the restoration of damaged areas as well as minimise the effect of future marine operations that involves impacts to natural seagrass communities