Titolo della tesi: A Mediterranean marine Priority Habitat restoration – the transplantation of Posidonia oceanica (L.) Delile 1813
P. oceanica is the most important and widespread endemic seagrass in the Mediterranean Sea, capable of forming extensive meadows from the sea surface up to 40 - 45 meters depth. Due to its physical, ecological, economic, and bio-indicator roles and for its high sensitivity to the smallest environmental variations, P. oceanica is protected both at the species and habitat level by several European directives and regulations, as well as local regulations and laws. Despite the legal framework protecting the species, P. oceanica shows regression phenomena in many parts of the Mediterranean Sea, mainly due to human-mediated disturbances that modify the environmental requirements needed for plant survival and growth.
To counter and reverse the habitat loss, the identification of local disturbances affecting the seagrass is a fundamental step for developing specific protection measures aimed at mitigating and regulating the pressures. Furthermore, mapping the meadows and comparing the actual status with previous historical maps, can provide a reliable method for determining the meadows’ temporal dynamic and inferring changes over time in the surface area colonized by the seagrass.
Due to the slow growth rates and the rare sexual reproductive events, natural recolonization by P. oceanica of previously occupied areas lost after major disturbances results to be difficult. For this reason, when the disturbing factors have been removed and the surrounding environmental conditions optimal for the survival and growth of the plant have been re-established, the restoration of the lost seagrass through active transplantation is an excellent solution to enhance or speed up the recovery processes of the plant.
This research aims at i) evaluating the temporal dynamic of a Posidonia oceanica (L.) Delile, 1813 meadow surrounding Giglio Island, ii) identifying the human-mediated disturbances affecting the seagrass, iii) assessing their effects on the meadow, and iv) investigating, from a broad spectrum of points of view, some aspects related to P. oceanica restoration. To this end, the research project has contributed to broadening knowledge in these areas thanks to the production of scientific articles, the contents of which are summarized below.
The first objective, aimed at mapping the upper limit of P. oceanica along the coasts of the island of Giglio (Grosseto, Tuscany) and at identifying the main human-mediated disturbances affecting the seagrass, was achieved through the application of the analysis of images derived from aerial orthophotos. Thirteen areas of the island characterized by the presence of seagrass were mapped from 1968 to 2013 approximately every 10 years, allowing to highlight phenomena of progression, regression, and stability over time. Subsequently, thanks to the orthophotos and the deep knowledge of the island, the main human-mediated disturbances affecting P. oceanica from 1968 until 2013 were identified: boat anchoring, coastal development, and harbor constructions. The Bayesian analysis showed that the shallowest part of the meadow, located near the coast, is mainly subjected to the detrimental effect of anthropogenic activities and is more affected than the deeper areas. Furthermore, the model showed how, among the disturbing factors present on the island, coastal and harbor constructions are the most impacting activities with a negative effect on P. oceanica, followed by anchoring, whose action is constant over the years, and by the presence of mines.
Areas with low magnitude disturbances and reduced in time show stable or increasing trends in seagrass coverage. This study highlights how the lasting presence over time of disturbing factors of moderate magnitude leads to enhance regression phenomena of some specific sectors of the P. oceanica meadow. On the contrary, in the absence or with minor disturbances and duration, P. oceanica can maintain stability or increase its coverage over time. Finally, this study highlights how despite the island's seabed presenting legally protected species and habitats such as P. oceanica, this condition does not limit the anthropogenic pressures, preventing or counteracting the loss of the seagrass.
The second objective, aimed at evaluating the effects on P. oceanica of multiple and synergic anthropogenic disturbances deriving from the impact of the Concordia cruise ship and subsequent removal activities, was achieved through the application of the seagrass structural descriptors, spatial metrics, and bionomic cartography. From 2012 to 2017, the physical and mechanical impact, sometimes in synergy, resulting from the presence of the wreck, supporting vessels, structures, and works necessary for the removal of the ship, led to the disappearance of a portion of P. oceanica meadow. During the study period, the acquisition during SCUBA diving of the seagrass structural descriptors, such as shoot density and seabed coverage, made it possible to highlight the regression of P. oceanica. Spatial metrics and bionomic cartography highlighted phenomena of disappearance, regression, and fragmentation of the meadow, showing over time a modification of the underwater landscape; this, once characterized by P. oceanica in good condition, following the accident is characterized by dead matte. Finally, the multivariate analysis permitted to highlight an impact gradient inversely proportional to the distance from the disturbing factors (wreck, supporting vessels, and structures for wreck removal): the moving away from the wreckage area, where the main disturbing factors were concentrated, led to a decrease in the impact on P. oceanica. This work has highlighted at a fine-scale how multiple severe disturbances that act synergically on P. oceanica, despite being concentrated in a short period and restricted geographical areas, are capable of leading to the total disappearance of the meadow.
The dense and well-preserved meadow that characterized the underwater landscape before the shipwreck was replaced by a dead matte substrate without a canopy, surrounded by fragmented limits with few and isolated shoots of P. oceanica. Within this degraded environment, it was possible to achieve the third objective, in which the consequences of the P. oceanica habitat loss were assessed through SCUBA diving investigations and photographic surveys. The disappearance and regression of the seagrass led to the weakening of the habitat integrity, facilitating the spread of the invasive species Caulerpa cylindracea, followed by a settlement and colonization of the seabed, once colonized by P. oceanica, by the algal species. The results showed that the absence of the seagrass from the seabed led to a greater presence of the algal species, with coverage values up to 70% during the summer months, while the presence of the seagrass (evaluated in areas without impact with P. oceanica in good condition) limited, due to competition, that of the invasive species to a maximum coverage value of 10%. The study emphasizes the importance of habitat integrity, demonstrating how the absence of structuring species such as P. oceanica from the seabed makes the environment more vulnerable and susceptible to the settlement and colonization of alien species such as C. cylindracea.
The fourth objective, aimed at evaluating the feasibility of the restoration of the meadow lost due to the Concordia accident and the subsequent removal works and aimed at developing a specific protocol for the restoration of P. oceanica, was achieved through a pilot study based on experimental transplantation lasted almost 5 years, from 2016 to 2021. This transplantation was developed following the removal of the anthropogenic disturbing factors that led to the disappearance of the seagrass from the seabed and after that, the surrounding environmental conditions have returned to optimal values for the seagrass’ survival and growth and similar to those characterizing non-impacted areas. To carry out the transplantation, “recycled” vegetal material derived from both the action of boat anchoring and the effect of storms was used as the only source. The vegetal material, represented by P. oceanica cuttings detached from clods of matte of various sizes, was attached to the dead matte substrate with specifically developed degradable metal stakes. The structural (survival rate of transplanted cuttings and variation in the number of leaf bundles) and functional (leaf elongation) descriptors of the seagrass showed over time a high survival rate of transplanted cuttings and a considerable increase in the number of bundles. In addition, cuttings transplanted to a depth similar to that at which they were collected showed a greater growth of leaf bundles than cuttings transplanted to depths greater or less than that at which they were collected. Furthermore, the leaf elongation of the transplanted cuttings showed significantly higher values than the growth of natural plants used as a procedural control, demonstrating how, under environmental conditions suitable for survival and growth, P. oceanica grows as quickly as possible to colonize the seabed. This study, therefore, demonstrated how the area is suitable for the survival and growth of P. oceanica cuttings, validating the feasibility hypothesis of a large-scale transplant aimed at restoring the meadow lost following the impact of the Concordia shipwreck. The work also demonstrates how the recycled material derived from boat anchoring and the erosive action of storms is a valid and non-destructive alternative to the use of donor meadows. The study highlights for the first time that transplanting cuttings to a depth similar to that of the collection leads to better results than the procedures practiced up to now in P. oceanica restoration. Finally, the results related to the variation in the diameter of the metal stakes showed a corrosion rate such as to estimate the time required for the metal to be corroded and no human artifacts left on the seabed at 20-25 years. The protocol developed in this study, applicable also for other rhizomatous phanerogams outside the Mediterranean Sea, is an excellent tool for carrying out transplants of P. oceanica, using an efficient, non-destructive, and degradable transplanting method capable of leaving, in a limited period, the seabed characterized by the seagrass only.
Confirmed the feasibility of a large-scale restoration and developed a specific transplant protocol, the fifth objective, aimed at enhancing and accelerating the recovery of the meadow impacted in the past by the Concordia accident, was achieved through large-scale transplantation. Starting from 2019 and up to 2023, 2048 m2 of dead matte were selected, within the surface of P. oceanica which was lost following the shipwreck and subsequent works, to be re-vegetated. In the first two years from 2019 to 2020, 1149 m2 of the surface were covered with P. oceanica cuttings while the remaining surface will be accomplished in the following years. The use of high-spatial-resolution photogrammetric techniques and SCUBA diving made it possible to verify respectively on a wide and fine-scale the success of the transplant. The results acquired both on the entire surface of 1149 m2 through high spatial resolution photomosaics and in detail within monitoring squares, showed that both the survival of the transplanted cuttings and the variation of the shoot density were similar to those observed in the previous experimental work. The restoration described in this work is configured as one of the most extensive so far carried out in the Mediterranean Sea, with the highest shoot density values ever used and results that suggest a successful transplanting action. Furthermore, the study demonstrates how large-scale transplantation activates and speeds up the recovery process of previously damaged meadows, becoming an excellent tool for seagrass management and impact compensation.
Considering that seagrass transplantation is becoming more and more frequent and taking into consideration that the spatial scales at which such activities are carried out are very extensive, it is necessary to develop new remote monitoring methods capable of assessing the survival of transplanted cuttings, their growth, and verify the effective success of the transplant. For this reason, the sixth objective, aimed at creating a new monitoring method to remotely follow the survival and growth of transplanted cuttings, was achieved through the use of photogrammetric methods with high spatial resolution and the use of a real-time kinematic positioning system (GPS RTK). The results showed how the accurate georeferencing of photomosaics at high spatial resolution (at centimeter scale) is able to identify every single cutting present on the seabed, evaluate its vitality and follow its development. The method proposed in this study is configured as a valid and excellent tool for evaluating, on a large surface, the coverage rate of the seabed by transplanted seagrasses, to follow over time the vitality and development of each cutting and evaluate the topographical change of the seabed. Furthermore, considering the ease of use, the low cost, and the repeatability of the method, this can be exported for further applications, other areas than the Mediterranean Sea, and further seagrasses than P. oceanica.