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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 δ13C 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 δ13C 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 δ13C 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. [ABSTRACT FROM AUTHOR]

The effects of diving activity in different Mediterranean subtidal habitats are scarcely known. This study evaluates diver behaviour (for example time spent in each habitat), use (contacts made with the substrate) and immediate effects of diver contact on benthic species in a marine protected area (MPA) in Sicily. Over a two-year period, intentions of 105 divers were observed within seven subtidal habitats: algae on horizontal substrate, algae on vertical substrate, Posidonia oceanica, encrusted walls, caves, sand and pebbles. Divers selected a habitat in proportion to its availability along the scuba trail. On average, each diver made 2.52 contacts every seven minutes, and no differences were detected among the levels of diver scuba certification. The highest rates of total and unintentional contacts were recorded on caves and encrusted walls, and the slow growing species Eunicella singularis and Astroides calycularis were the most frequently injured by divers. Most of the contacts were concentrated in the first minutes of the dives. The identification of diving effects in different habitats will enable management strategies to specifically control this impact at a habitat scale, for example restricting the start of the dive to low vulnerability habitats would reduce damage to benthic organisms, allowing sustainable use of MPAs. [ABSTRACT FROM PUBLISHER]

Abstract Marine coastal ecosystems are facing structural and functional changes due to the increasing human footprint worldwide, and the assessment of their long-term changes becomes particularly challenging. Measures of change can be done by comparing the observed ecosystem status to a purposely defined reference condition. In this paper, a geospatial modelling approach based on 2D mapping and morphodynamic data was used to predict the natural position of the upper limit (i.e., the landward continuous front) of Posidonia oceanica seagrass meadows settled on soft bottom. This predictive model, formerly developed at the regional spatial scale, was here applied for the first time at the Mediterranean spatial scale in eight coastal areas of Spain, France, Italy, and Greece showing different coastal morphologies and hydrodynamic characteristics, and affected by a number of natural and/or human local disturbances. The model was effective in measuring the regression (i.e., seaward withdrawal) of the meadow upper limit. In all the meadows investigated the upper limit was regressed, laying deeper than the reference condition, with the proportion of regression ranging from 17.7% to 98.9%. The highest values of regression were found in Spain and in France, and were consistent with the highest levels of fragmentation detected with map analysis and of coastal pressures. This geospatial modelling approach represents an effective tool to define the reference conditions when proper pristine areas or historical data are not available, thus allowing the assessment of long-time changes experienced by seagrass ecosystems due to human impacts. Highlights • Reference conditions are needed to measure long-term change in marine ecosystems. • We modelled the upper (landward) limit of seagrass meadows at Mediterranean scale. • All meadow upper limits appeared regressed and fragmented due to local disturbances. • Modelling represents an effective tool for evaluating seagrass meadow conditions. [ABSTRACT FROM AUTHOR]