Spatial and Temporal Patterns of Macrozoobenthic Communities Around Offshore Gas Structures in the Adriatic Sea

Spatial and temporal variability of macrozoobenthic communities were investigated around three offshore gas structures with different architecture (a subsea well-site, a four-leg platform, and a one-leg platform) in the NW Adriatic Sea. Four post-installation surveys (two per year over two years) were conducted by sampling sediments at increasing distances from each structure (approximately 0, 30, 60, 120 and 1000 m from the structure edge). A total of 233, 271 and 260 taxa were recorded around Structures A, B and C, respectively, with polychaetes representing the dominant taxonomic group at all sites. Across all structures, community composition showed significant variability both among surveys and along the distance gradient. Near-structure stations (0–60 m) most frequently accounted for spatial dissimilarities, whereas communities at 120 and 1000 m were generally more similar. Early surveys around the well-site and the four-leg platform were characterised by low diversity and high dominance of the opportunistic polychaete Ditrupa arietina, suggesting a short-term disturbance related to installation. Post-installation trajectories differed among structures: community descriptors stabilized faster around the subsea well-site, while changes near the platforms extended for at least two years. At the one-leg and four-leg platforms, the progressive development of a bivalve mound (specimens of Mytilus galloprovincialis and/or Neopycnodonte cochlear fell from the submerged parts of the platforms) coincided with increased abundance, species richness and occurrence of hard-bottom associated taxa at 0 m stations. Overall, the results indicate that offshore gas structures can locally influence macrozoobenthic assemblages by modifying habitat heterogeneity and promoting site-specific community responses. Although based on post-installation observations only, this study provides site-specific evidence useful for future decommissioning planning in soft-bottom shelf areas.