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Posidonia oceanica is an endemic Mediterranean seagrass that forms wide and dense meadows from the surface up to about 40 m depth. This species can develop a biogenic structure called matte, a typical terraced formation built up by itself, consisting of intertwined rhizomes, roots and sediment, which may allow shoots to reach the sea surface forming reefs (récif barrière), considered natural monuments. Posidonia oceanica reefs are particularly exposed to the ongoing increase in temperature and in storm frequency and intensity due to climate change, with negative repercussions on their conservation. Much more attention must be paid to the implementation of monitoring tools able to detect early signs of reef regression. In this study, the distribution of P. oceanica reefs located along southern coasts of Sicily (Italy, Mediterranean Sea) was investigated. A remote sensing approach was used to assess reef extension, morphological features (e.g., atolls), upper limit and occurrence of dead matte. In particular, very high-resolution drone images (~ 2 cm pixel size), combined with GPS field data, were used for 2D- and 3D-reconstruction of P. oceanica reefs. The 3D-model allowed to estimate bathymetrical distribution of P. oceanica reef enabling to achieve a more detailed and complete understanding of the P. oceanica reef architecture

Posidonia oceanica (L.) Delile grows on different substratum types, ranging from sand, which is easily penetrable by the roots, to rock, in which they are able to enter through crevices (1). In this study a survey was carried out on roots of plagiotropic rhizomes growing on sand (matte), and rock (dolonstone and calcarenite), to assess difference in morphological and anatomical features. In particular, for each substratum, nine rhizomes were randomly sampled by scuba diver at 10 meters of depth. In each rhizome histological and morphometric data (2, 3) were recorded on roots up to the second lateral order. Roots on rock were isolated from their substratum by chemical dissolution. Data analysis exhibited differences in roots anatomy and histology between substrata. The adventitious and second order lateral roots showed larger diameter on sandy sediment (from 3,111.88 ± 571.76 μm to 924.64 ± 146.11 μm) compared to ones growing on rocky substratum (from 2,695.14 ± 703.75 μm to 786.36 ± 168.18 μm). The outer cortex of roots was on average thicker on sand (229.41 ± 263.66 μm) than rock (123.49 ± 198.42 μm). Conversely, rhizodermis and mechanical hypodermis were thicker on rock (from 37.17 ± 5.97 μm to 233.19 ± 81.55 μm) than sand (from 30.66 ± 5.34 μm to 139.29 ± 74.04 μm). Similar trends in inner cortex and stele thickness were also found. In adventitious roots, the medulla was rich of well-lignified sclerenchymatic fibers, as they are responsible of the tensile strength, both on rock and sand, while variable results were obtained in first and second order lateral roots. Structures of adhesion have been detected (Fig. 1), very similar to ones discovered in seedlings (4), allowing to identify micro adhesive patterns. In particular, the root hairs genesis was reconstructed on rhizodermis, highlighting the attachment strategies to the substratum based on distal part of root hairs. The penetration strategy of adventitious roots (Fig. 2) was finally observed. Biochemical dissolution of carbonate cements in dolonstone or disintegration of particle components in calcarenite are used by the roots to penetrate into the rock for improving the anchoring capacity and the substrate exploration.

Posidonia oceanica is considered a key species due to its different roles as primary producer, substrate for many species, shoreline erosion protector and long-term carbon store (1).The importance of P. oceanicahas stimulated several studies aimed at quantifying its status. In particular growth performance of rhizomes has become among the most used descriptors for monitoring changes of P. oceanicameadows induced by human or naturalexogenous factors (2). However, ability to detect any change of growth in space or in time is often confounded by natural age-induced variations, which involves serious interpretation problems (3). A general approach adopted to overcome this problem is to build growth charts as reference tool for comparison purposes. Charts describing patterns of biometric features conditioned to age are increasingly used as comparison tools, even if almost exclusively in Auxology(4). Their use can be extended to other disciplines, including ecological studies, although very large data sets are required for obtaining reliable estimates and curves should be flexible enough to account for non-linear growth pattern over age (5). In this work reference growth charts involving different P. oceanicagrowth performance measures (speed of growth and primary production of rhizomes) will be presented. Curves have been built using proper statistical frameworks (GLMM, Segmented and Quantile Regressions), based on more than 13000 annualgrowth data recorded by lepidochronology (6) on about 1600 shoots collected at 4–32 m depth range along Sicilian coasts.Growth patterns exhibited distinct trends as regards the relationships with depth: neither speed of growth nor primary production of rhizomes depended on depth until 14 m, while at deeper stands significant linear decrease by 3.5–2.0% for 1 m increase in depth was observed, due to light and sedimentation reduction. The considerable size of the dataset allowed to estimate the accurate shapes of the percentile curves (from 5thto 95th), revealing non monotonic relationships of growth with respect to shoot age with an initial increase followed by an overall decrease of 40% during the following years of the explored lifespan. The accompanying model-based classification procedures presented, will allow to obtain comparable results also when age of shoots is largely different (up to 20 years) (7). The growth charts may represent a noteworthy tool for researchers involved in studying of different aspects of seagrass monitoring. It is hoped that the proposed framework will facilitate assessment of growth performance status and comparative analysis of growth data from different populations around the Mediterranean Sea

Over the past three decades an increasing amount of dating records dealing with Posidonia oceanica growth performance was incorporated into a variety of studies, from which a dualistic nature of the factors influencing seagrass growth arose. A large amount of literature focused on the role of exogenous factors in explaining rhizome growth variations, while only few studies invoked the importance of endogenous factors in driving growth. A particular attention was paid on the confounding role of shoot age, as endogenous factor, when the effect of exogenous variables on growth performance is analyzed. Shoot age confounding implies serious interpretation problems, since it is difficult to distinguish between the effectson P. oceanica growth due to spatio-temporal exogenous variations or simply to the unbalanced age structure of samples. A practical tool to overcome the problems linked to age confounding is to build references growth charts just used as yardstick for comparison purposes. References growth charts represent the distribution of a given biometric measurement changing with age, and typically are used in Auxology for establishing whether a generic individual at a given age lies within the ‘normal’ range. Their use can be extended to other disciplines, although very large data set are needed. In this study, P. oceanica references growth charts have been built using statistical methods, including GLMM, Segmented Regression and Nonparametric Quantile Regression, working on 4·104 lepidochronological measures collected along Sicilian coasts from 1 to 32 m depth. We detected two different trends of growth along depth, separated by a change point estimated at 15 m depth. Above and below this depth two distinct references growth charts have been built. Different curves of growth performance vs. age have been estimated at the 10th, 25th, 50th, 75th, 90th percentile, showing nonlinear patterns with the highest values of rhizome elongation and primary production at shoot age of about 4 years, followed by a monotonic decrease with aging. These results highlight the need to control for shoot age and recommend references growth charts as new tools to assess growth performance of P. oceanica samples coming from different depths and areas, accounting for their demographic structure.

This work benefited partly from the financial support of the European program ΓNTERREG IIIA Corsica, Sardinia. Tuscany, The conservation of the coastal marine environment requires the possession of information that enables the global quality of the environment to be evaluated reliably and relatively quickly. The use of biological indicators is often an appropriate method. Seagrasses in general, and Posidonia oceanica meadows in particular, are considered to be appropriate for biomonitoring because of their wide distribution, reasonable size, sedentary habit, easy collection and abundance and sensitivity to modifications of littoral zone. Reasoned management, on the scale of the whole Mediterranean basin, requires standardized methods of study, to be applied by both researchers and administrators, enabling comparable results to be obtained. This paper synthesises the existing methods applied to monitor P. oceanica meadows, identifies the most suitable techniques and suggests future research directions. From the results of a questionnaire, distributed to all the identified laboratories working on this topic, a list of the most commonly used descriptors was drawn up, together with the related research techniques (e.g. standardization, interest and limits, valuation of the results). It seems that the techniques used to study meadows are rather similar, but rarely identical, even though the various teams often refer to previously published works. This paper shows the interest of a practical guide that describes, in a standardized way, the most useful techniques enabling P. oceanica meadows to be used as an environmental descriptor. Indeed, it constitutes the first stage in the process., peer-reviewed

A model developed for Zostera marina was adapted and used to select suitable areas for Posidonia oceanica transplantation in the Gulf of Palermo, where recent rehabilitation programmes have reduced human pressure. This model consists of three steps: (1) habitat selection, by calculation of the Preliminary Transplant Suitability Index (PTSI); (2) field assessments and test-transplanting, to evaluate the site suitability and to estimate the effects of tearing on transplant units (about 50%); (3) identification of suitable restoration sites, by calculation of the Transplant Suitability Index (TSI). A new parameter was added to the literature model: the number of grids detached, which is linked to factors (hydrodynamic regime, anchoring, fishing) that have a potentially great effect on the final outcome of the transplant. Only one site (TSI = 16) in the Gulf of Palermo was indicated as potentially suitable for restoration with P. oceanica. In this site, a transplant of 40 m2 was implemented. From 2008 to 2014, transplant effectiveness was evaluated in terms of establishment, detachment and mortality of cuttings and shoot density. The long-term monitoring (6 years) allowed us to detect changes in the structural conditions of the transplanted meadow and to identify the possible turning point in P. oceanica recovery (2 years after transplanting). Moreover, 6 years after transplantation the P. oceanica meadow has exceeded the transplant shoot density of about 16%, with a mean and a maximum value of 11.6 and 17 shoots per cutting, respectively.

Growth performance of rhizomes has become among the most used descriptors for monitoring Posidonia oceanica seagrass dynamics and population status. However, ability to detect any change of growth in space or in time is often confounded by natural age-induced decline. To overcome this problem, we have produced reference growth charts, which in other areas are universally recognized as a very powerful tool for comparing growth of living beings during their ontogeny. Reference growth charts involving different P. oceanica growth performance measures (speed of growth and primary production of rhizomes) have been built using proper statistical frameworks (GLMM, Segmented and Quantile Regressions), based on more than 13 × 10 3 annual growth data recorded by lepidochronology on about 1600 shoots collected at 4–32 m depth range. Growth patterns exhibited distinct trends as regards the relationships with depth: neither speed of growth nor primary production of rhizomes depended on depth until 14 m, while at deeper stands significant linear decrease by 3.5–2.0% for 1 m increase in depth was observed. According to these results, the depth of 14 m was used as breakpoint for building two distinct sets of reference growth charts. The considerable size of the dataset allowed to estimate the accurate shapes of the percentile curves (5th, 10th, 25th, 50th, 75th, 90th, and 95th), revealing non monotonic relationships of growth performance with respect to shoot age with an initial increase followed by an overall decrease of about 40% during the following years of the explored lifespan. The accompanying model-based classification procedures described in this paper, allow to obtain comparable results also when age of shoots is largely different (up to 20 years). While this approach is flexible enough to produce satisfactory results using any growth measure (speed of growth and primary production of rhizomes, annual or cumulative), it maintains low complexity with trivial computations performed straightforwardly. The growth charts may represent a noteworthy tool for researchers involved in studying of different aspects of seagrass monitoring. It is hoped that the proposed framework will facilitate assessment of growth performance status and comparative analysis of growth data from different populations around the Mediterranean Sea.

Posidonia oceanica is the most common, widespread and important monocotyledon seagrass in the Mediterranean Basin, and hosts a large biodiversity of species, including microorganisms with key roles in the marine environment. In this study, we ascertain the presence of a fungal endophyte in the roots of P.oceanica growing on different substrata (rock, sand and matte) in two Sicilian marine meadows. Staining techniques on root fragments and sections, in combination with microscope observations, were used to visualise the fungal presence and determine the percentage of fungal colonisation (FC) in this tissue. In root fragments, statistical analysis of the FC showed a higher mean in roots anchored on rock than on matte and sand. In root sections, an inter- and intracellular septate mycelium, producing intracellular microsclerotia, was detected from the rhizodermis to the vascular cylinder. Using isolation techniques, we obtained, from both sampling sites, sterile, slow-growing fungal colonies, dark in colour, with septate mycelium, belonging to the dark septate endophytes (DSEs). DNA sequencing of the internal transcribed spacer (ITS) region identified these colonies as Lulwoana sp. To our knowledge, this is the first report of Lulwoana sp. as DSE in roots of P.oceanica. Moreover, the highest fungal colonisation, detected in P.oceanica roots growing on rock, suggests that the presence of the DSE may help the host in several ways, particularly in capturing mineral nutrients through lytic activity.

All seagrass species known from the Mediterranean basin have been recorded along the Sicilian coast, where studies have been carried out at a very local scale and information is fragmented or confined to the grey literature. The objective of this article is to summarise and evaluate current knowledge on seagrass species on the Sicilian coasts, providing an overview of species distribution, genetic diversity, biology and ecology, based on the literature and unpublished data. Most literature studies have been carried out on Posidonia oceanica meadows because of their wide distribution, complexity and ecological importance. In this study, the analyses carried out on P. oceanica structural and functional features show that the Sicilian meadows are in good condition with respect to the Mediterranean average, probably because of relatively low anthropogenic pressure and favourable ecological conditions. The available data on this species summarised in this article represent an important starting point from which to build effective plans for understanding levels of environmental threats and for supporting conservation strategies for these important ecosystems. Conversely, the limited information available on other seagrasses only allows the description of some structural and functional features, and does not permit to drive overall conclusions on their general health status.

1. Explosive volcanic submarine activity is expected to affect seagrass communities due to sudden and dramatic changes in the physical and chemical features of sea water and sediments, with possibly large ecosystem effects. However, seagrass response to the harsh environmental conditions that arise due to explosive volcanism is as yet unexplored as it is not easy to predict when and where an eruption will occur. Here, we investigate the uptake of hydrothermal carbon within the seagrass Posidonia oceanica by the analysis of d13C and growth rates in tissue laid down before and after an exceptional and massive hydrothermal gas release in the Aeolian Islands (Italy, Mediterranean Sea). 2. Hydrothermal submarine activity was recorded by P. oceanica, which showed a large and persistent 13C-depletion in both scales and rhizomes in the site close to the eruption. Both increased CO2 availability and reduced carbon demand, as a consequence of stressful environmental conditions (e.g. light limitation due to turbidity, high temperature), combined to give much lower d13C signatures. Our results suggest that the explosive volcanism caused physiological stress in the seagrass, leading to a reduction in productivity, whereas slower, more diffuse release of hydrothermal CO2 is known to enhance seagrass productivity. 3. Synthesis. We analysed the effect of a sudden and large hydrothermal event on d13C interannual variations and growth of the seagrass P. oceanica. Our results confirm that shallow submarine hydrothermal vents can be used as natural laboratories for exploring biological responses to acute and often extreme environmental conditions. P. oceanica can record geological events by capturing CO2 derived from vent systems, with implications for habitat management to mitigate against raising CO2 levels and ocean acidification. This leads the way to further studies to evaluate the effects of hydrothermal carbon on shallow ecosystems.

The Stagnone of Marsala is an oligotrophic lagoon situated on the western coast of Sicily. The area has been under study since the mid-nineteenth century. Research has focused on its physiographic, chemical-physical, biological, ecological features and, particularly, on submerged vegetation. Its main biotic component, a meadow of Posidonia oceanica, is distributed among peculiar reef formations inside and outside the lagoon, which are extremely interesting from an ecological and environmental point of view. The seagrass grows near to its thermal and salinity tolerance level in the inner lagoon, where it is partially replaced by Cymodocea nodosa and Caulerpa prolifera meadows. Algal assemblages are characterised by both attached and/or unattached seaweeds that accumulate in areas of different extension, where they remain lying freely and slowly rolling on the seabed. Over the past few years, as a result of the global climate change, a noticeable reduction in the lagoon biodiversity has occurred due to decreased water exchange, and an increase in both confinement and temperature. Therefore, a consistent, suitable and well organised monitoring plan should be developed for the protection and the recovery of the Stagnone of Marsala lagoon.