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Highlights: [•] Posidonia oceanica upper limit is strongly controlled by surf-related effects. [•] We compared the position of the meadow upper limit with the beach morphodynamics. [•] We provided evidence that the natural landwards extension of meadows can be predicted. [•] We elaborated a model to define the reference condition of P. oceanica upper limit. [ABSTRACT FROM AUTHOR]

The recognized ecological importance of Posidonia oceanica, the most important seagrass of the Mediterranean Sea, makes it crucial to assess the state of health of its meadows, discriminating natural from anthropogenic impacts. In this paper, the hydrodynamic conditions at the upper limit of P. oceanica meadows along the Ligurian coast (NW Mediterranean Sea) were investigated. A relationship between the distance of the upper limit of the meadow from the shoreline and the morphodynamic domain of the beach ( i.e. distinctive types of beach produced by the topography, wave climate and sediment composition) was found. A zonation of the state of the shallow portions of the meadows down the submerged beach profile was identified. Zone a, from the shoreline to the breaking limit, is naturally critical for the development of the meadow. Zone b, from the breaking limit to the closure depth, is subjected to natural and human impacts. Zone c, below the closure depth, is little influenced by coastal dynamics. This study quantifies for the first time how much the status of the shallow portions of P. oceanica meadows is dependent on coastal dynamics, which is important for their proper management. [ABSTRACT FROM AUTHOR]

The article discusses the patterns of a wide-scale substitution within meadows of the seagrass Posidonia oceanica in North West Mediterranean Sea. It notes that the endemic seagrass meadows in the region are presently facing invasion by alien algal species, particularly where they are already showing signs of regression due to other stresses. Those invaders were found to be the stronger colonizers, in terms of habitat occupancy within meadows, as compared to natives.

Abstract: A system of five adjacent Posidonia oceanica meadows facing the waterfront of Genoa city (Ligurian Sea, NW Mediterranean) was investigated over different spatial scales (meters–kilometers) using three environmental indices: conservation index (CI), substitution index (SI) and phase-shift index (PSI). CI revealed differences mostly at large spatial scale, distinguishing the poor condition of the meadows closest to Genoa centre and harbour from the comparatively healthy condition of the farthest meadows. SI showed differences mostly at small spatial scale (i. E. , within meadows), suggesting the influence of local factors in the re-colonisation of regressed meadows by the seagrass Cymodocea nodosa and/or the invasive alga Caulerpa racemosa. Mapping of PSI showed that the meadows closest to Genoa centre and harbour have undergone a nearly total phase shift and have no real potential for recovery: attempts to re-establish P. Oceanica there might be a waste of time and money. On the contrary, the meadows farthest from Genoa centre and harbour showed a comparatively low level of phase shift and could still fully recover given specific management actions. [Copyright &y& Elsevier]

Abstract: Side Scan Sonar (SSS) is a common method for mapping seagrass meadows that allows to define meadow extent and typology. Should sonogram interpretation be efficient, repeated mapping would allow evaluation of spatial meadow dynamics through time. Here, maps of Posidonia oceanica (L.) Delile meadows from western Liguria (NW Mediterranean Sea) produced in 1990, 1991 and 2006 through SSS surveys, were processed with GIS (Geographical Information System) and compared by means of diachronic analysis to establish concordances and discordances. While any difference between maps of 1990 and 2006 might provide information about change in seagrass cover with time, the availability of maps taken at only one year of distance (1991 vs 1990) offered the unique opportunity to evaluate the efficiency of SSS mapping, as the slow dynamics of P. oceanica makes significant change over such a short time scale unrealistic in absence of any large-scale environmental catastrophe. Results showed obvious differences in P. oceanica meadow distribution among the three periods in most of the areas. The largest differences were found between the maps of 1990 and 1991. This discrepancy was probably due to erroneous interpretation of small, fragmented shallow-water meadows on sonograms. Errors in sonogram interpretation combine with inefficient positioning in old cartographies to blur the evaluation of change over large time scales (i.e. between 1990 and 2006). Extensive sea-truthing based on a rigorous design is mandatory for efficient acoustic mapping of seagrass meadows. [Copyright &y& Elsevier]

Abstract: Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, forms large meadows from the sea surface down to 40 m. The depth of the lower limit of the meadows marks the boundary between the infralittoral and the circalittoral zone, and is said to be normally set by light attenuation underwater, while the role of water movement has been little explored. In this paper, the position (i.e. distance from the shoreline and depth) of P. oceanica meadow lower limits along the whole Ligurian coastline (about 350 km) was related to the annual storm wave base. This depth represents the limit of interaction between waves and seafloor and corresponds to L 0/2, where L 0 is the annual offshore wave length. In all meadows, the lower limit has never been found deeper than the annual storm wave base, and its depth (Zc ) showed related to L0 according to the equation Zc  = 0.32∙L 0 + 5.62. In the coastal tracts affected by the least intense waves, the reduction of water movement with depth represents the most important constraint to the seaward development of the meadow, whereas light availability plays a major role in meadows affected by the most intense waves. The present study represents the first attempt at understanding the role of hydrodynamic factors in setting the depth limit of seagrass meadows. If corroborated by future research at other sites, this will have important implications for both basic and applied science, as it would imply rethinking about the relative importance of water movement and light in seagrass depth distribution, and could allow for a better estimate of the extent of meadow regression in anthropized areas. [Copyright &y& Elsevier]

Abstract: The results of 15 years of monitoring of Posidonia oceanica in the “Cinque Terre” Marine Protected Area (NW Mediterranean) are presented. Seasonal data on meadow characteristics (cover and shoot density), plant phenology (leaf number, leaf length and width, leaf brown portion, undamaged leaves), lepidochronology, leaf epiphyte cover and herbivore pressure collected from three stations at 5, 10 and 17m depth were compared. Time-series analyses showed both medium-term (5

Abstract: Habitat fragmentation in meadows of Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, was investigated at a region-wide spatial scale using a synthetic ecological index, the Patchiness Index (PI). We tested the hypothesis that human impacts are the major factor responsible for habitat fragmentation in P. oceanica meadows contrasting fragmentation of meadows located in “anthropized” areas with that of meadows located in areas with low anthropization and considered as virtually “natural”. We also related fragmentation of meadow with the morphodynamic state of the submerged beach (i.e. distinctive types of beach produced by the topography, the wave climate and the sediment composition) in order to investigate the influence of one natural component on the seagrass meadow seascape. Results demonstrated that fragmentation in the P. oceanica meadows is strongly influenced by the human component, being lower in natural meadows than in anthropized ones, and that it is little influenced by the morphodynamic state of the coast. The use of landscape approaches to discriminate natural disturbance from human impacts that affect seagrass meadows is thus recommended for the proper management of coastal zones. [Copyright &y& Elsevier]