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Posidonia oceanica meadows are declining at alarming rates due to climate change and human activities. Although P. oceanica is considered the most important and well-studied seagrass species of the Mediterranean Sea, to date there has been a limited effort to combine all the spatial information available and provide a complete distribution of meadows across the basin. The aim of this work is to provide a fine-scale assessment of (i) the current and historical known distribution of P. oceanica, (ii) the total area of meadows and (iii) the magnitude of regressive phenomena in the last decades. The outcomes showed the current spatial distribution of P. oceanica, covering a known area of 1,224,707 ha, and highlighted the lack of relevant data in part of the basin (21,471 linear km of coastline). The estimated regression of meadows amounted to 34% in the last 50 years, showing that this generalised phenomenon had to be mainly ascribed to cumulative effects of multiple local stressors. Our results highlighted the importance of enforcing surveys to assess the status and prioritize areas where cost-effective schemes for threats reduction, capable of reversing present patterns of change and ensuring P. oceanica persistence at Mediterranean scale, could be implemented.

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Bioconstructions such as coralligenous outcrops and maërl beds are typical Mediterranean underwater seascapes. Fine-scale knowledge on the distribution of these sensitive habitats is crucial for their effective management and conservation. In the present study, a thorough review of existing spatial datasets showing the distribution of coralligenous and maërl habitats across the Mediterranean Sea was undertaken, highlighting current gaps in knowledge. Predictive modelling was then carried out, based on environmental predictors, to produce the first continuous maps of these two habitats across the entire basin. These predicted occurrence maps for coralligenous outcrops and maërl beds provide critical information about where the two habitats are most likely to occur. The collated occurrence data and derived distribution model outputs can help addressing the challenge of developing basin-wide spatial plans and to guide cost-effective future surveys and monitoring efforts towards areas that are presently poorly-sampled

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Abstract. The ecosystem associated to the Mediterranean seagrass Posidonia oceanica shows a clear distinction in two subcompartments regarding turnover time: aboveground and belowground. Aboveground parts (leaves) are highly dynamic, and most of the leaf material is decomposed or exported in less than one year, representing a net loss of nutrients. In contrast, belowground biomass (roots and rhizomes) has a turnover time of the order of centuries, with a consequent accumulation of organic matter in the sediment. The accumulation rates for the single elements rank in the order C > N > P. This ecosystem may be considered as a sink for biogenic elements.

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