Methane-related carbonate cementation of marine sediments and related macroalgal coralligenous assemblages in the Northern Adriatic Sea

This chapter presents a multidisciplinary study of the major characteristics of a set of submarine rock outcrops in the Northern Adriatic Sea: the rock occurrence, embedded down to about 1 m from the seafloor in the bottom sediments; numerous gas accumulations in the proximity of the outcrops; small-scale mud volcanoes; and microbial mats around the seepage site. From the biological point of view, a total of 112 macroalgal taxa were recorded. The different number of taxa recorded at the San Pietro and Bardelli sampling sites have been related to the different distance from the coast and to water depth. The data highlights that the seabed is almost flat, locally showing the occurrence of rock outcrops, with a range of sizes, shapes, and spatial orientations. On the side-scan survey (SSS) data, they appear as high backscatter features. The seabed is mainly composed of sands. Some species that characterize these outcrops are acknowledged as important bioconstructors distinctive of the Mediterranean area. The seafloor sedimentary deposits show a gradual increase of the fine fraction from the coast toward the offshore; then a progressive increase in the coarse fraction in the central part of the study area (residual sands of the TST) is recognized. The fragile biological equilibrium of these structures, the slow-growing calcareous organisms considered as nonrenewable resources, and the dredging and bottom trawling pressure that strongly threatens the physical characteristics of the habitat, and negatively affect the associated biota, all motivate the need to protect the Trezze and their surrounding seabed.