In this paper we present the seabed maps of the shallow-water areas of Lampedusa and Linosa, belonging to the Pelagie Islands Marine Protected Area. Two surveys were carried out ("Lampedusa2015" and "Linosa2016") to collect bathymetric and acoustic backscatter data through the use of a Reson SeaBat 7125 high-resolution multibeam system. Ground-truth data, in the form of grab samples and diver video-observations, were also collected during both surveys. Sediment samples were analyzed for grain size, while video images were analyzed and described revealing the acoustic seabed and other bio-physical characteristics. A map of seabed classification, including sediment types and seagrass distribution, was produced using the tool Remote Sensing Object Based Image Analysis (RSOBIA) by integrating information derived from backscatter data and bathy-morphological features, validated by ground-truth data. This allows to create a first seabed maps (i.e. benthoscape classification), of Lampedusa and Linosa, at scale 1:20 000 and 1: 32 000, respectively, that will be checked and implemented through further surveys. The results point out a very rich and largely variable marine ecosystem on the seabed surrounding the two islands, with

Abstract. Coastal ecosystems produce and store carbonate particles, which play a significant role in the carbonate dynamics of coastal areas and may contribute to the sediment budget of adjacent beaches. In the nearshore seabed of temperate zones (e.g. Mediterranean Sea and South Australia), marine biogenic carbonates are mainly produced inside seagrass meadows. This study quantifies the contribution of biogenic sediments, mainly produced in Posidonia oceanica seagrass meadows and secondarily in photophilic algal communities, to the sediment budget of a Mediterranean beachdune system (San Giovanni beach, western Sardinia, western Mediterranean Sea). A set of geophysical, petrographic and sedimentological data was used to estimate the sediment volume and composition of the beach-dune system as a whole. The San Giovanni beach-dune system contains 3 797 000 ± 404 000 t of sediment, 83 % (3 137 000 ± 404 000 t) of which is located in the coastal wedge, 16 % (619 000 ± 88 000 t) in the dune fields and 1 % (41 000 ± 15 000 t) in the subaerial beach. The sediments are composed of mixed modern bioclastic and relict bioclastic and non-bioclastic grains from various sources. The system receives a large input of modern bioclastic grains, mainly composed of rhodophytes, molluscs and bryozoans, which derive from sediment production of present-day carbonate factories, particularly P. oceanica seagrass meadows. Radiocarbon dating of modern bioclastic grains indicated that they were produced during the last 4.37 kyr. This value was used to estimate the longterm deposition rates of modern bioclastic sediments in the various beach compartments. The total deposition rate of modern bioclastic grains is 46 000±5000 t century −1 , mainly deposited in the coastal wedge (39 000 ± 4 000 t century −1 ) and dunes (7000 ± 1000 t century −1 ), and 46 000 t represents ∼ 1.2 % of the total beach-dune sediment mass. Carbonate production from coastal ecosystems was estimated to be 132 000/307 000 t century −1 , 28 % (15 % / 34 %) of which is transported to the beach-dune system, thus significantly contributing to the beach sediment budget.The contribution to the beach sediment budget represents a further ecosystem service, which our data can help quantify, provided by P. oceanica. The value of this sedimentsupply service is in addition to the other important ecological services provided by seagrass meadows. The dependence of the beach sediment budget on carbonate production associated with coastal ecosystems has several implications for the adaptation of mixed and carbonate beaches to the loss of seagrass meadows due to local impacts and the changes expected to occur over the next few decades in coastal ecosystems following sea level rise.

Linosa Island represents the emergent tip of a mostly submarine, much wider volcanic edifice, with at least 96%of its areal extent lying below sea level. Marine geological surveys carried out in 2016 and 2017 allowed to reconstruct the submarine portions of Linosa and to characterize the main volcanic features, providing new, unexpected insights on the evolution of this little-explored volcanic complex. In particular, the submarine setting of the NW offshore is represented by a ~10-km long volcanic belt punctuated by a number of small eruptive cones, appearing more recent with respect to the assumed Mid/Late-Quaternary age of volcanism on the island. This evidence suggests that the growth of the volcanic edifice has likely been more complex than that claimed on the base of subaerial volcanism only, and supports a north-westward migration of the activity over time. The submarine southern flank of the volcanic edifice is also characterized by eccentric eruptive cones, but mostly without evidences of recent activity. The main processes responsible for the growth and evolution of Linosa volcano and their possible relative chronology are discussed in the framework of what previously known on the base of the limited subaerial portions, with implications on the potential hazard of the volcanic edifice (considered as not-active in recent times). Similarity with the Pantelleria volcano, located in the NW Sicily Channel, are also evidenced, especially for what regards the distribution and morphometric characteristics of eruptive cones occurring in the submarine portions of both islands.

Striped seagrass meadows are formed by narrow ribbons which are elevated over the seabed and separated by channels. Limited information on the genesis and development of this morphological pattern, including the adaptive responses of associated biota, is preventing holistic insight into the functioning of such protected ecosystems. This paper assessed the structural dynamics of a Posidonia oceanica striped meadow and the distribution and 3D orientation of the associated bivalve Pinna nobilis . Our analysis of the interaction between bedforms, bottom currents, and the distribution of P. nobilis revealed that the striped seascape is the result of a self-organisation process driven by feedback interactions among seagrass growth, sediment deposition, and hydrodynamics. The results suggest that the ribbon wall is the most suitable sub-habitat for this species, because it supports the highest density of P. nobilis, compared to the meadow top and bottom. Here, specimens can take advantage of the resuspension induced by hydrodynamics and open their shells towards the current, thus enhancing food intake. Therefore, our results show that self-organisation in striped seagrass meadow affects the distributional pattern of P. nobilis, providing new insights into the autoecology of this species beyond the conservation implications for its habitat.