Assessment of benthic macrofaunal communities: setting a baseline for european flat oyster (Ostrea edulis) reef restoration in the northern Adriatic Sea

In the past, oyster reefs of European flat oyster (Ostrea edulis) were common type of habitat in the coastal areas of the north Adriatic Sea. They served as biodiversity hotspots, shelter and hunting grounds for other animals, but also as coastal protection and climate change indicators. Now, after years of overfishing that decreased oyster population and destroyed their natural habitat as well as pathogens that caused mass mortality, the ability of natural restoration is unfairly limited. In this study, which is a part of PNRR MER project, is a large-scale assessment aiming to set a pre-intervention baseline for planned active restoration of native oyster beds in the north Adriatic Sea. The purpose of the baseline is to assess the habitat characteristics during 2024 and 2025, so as to quantify any subsequent changes in the system as a consequence of the restoration actions. In each of three sites, 3 stations for sediment collection were set. For each station, 3 replicate sediment samples were collected using a Van Veen grab. Laboratory analysis included: infauna species identification and abundance assessment, and sediment properties analysis (grain-size composition, loss on ignition, total organic carbon, and total nitrogen). Community diversity and composition were examined using diversity metrics and multivariate ordination. Across the two sampling campaigns, 159 taxa and 7,393 individuals were recorded. The three sites differed markedly in their assemblage composition and sediment characteristics. Tegnùe was associated primarily with sandy sediments, whereas Lignano and the LNG Terminal contained larger proportions of fine sediment. Principal Coordinates Analysis showed consistent spatial separation of the sampled communities in both years. Total organic carbon was strongly associated with community composition in 2024 and 2025, while total nitrogen showed a weaker and less consistent relationship. Although sampling effort differed slightly between years, the results reveal substantial spatial variability among the restoration sites, as well as some temporal variation within them. Future monitoring should therefore evaluate each site relative to its initial conditions and consider background environmental variability when assessing changes potentially associated with oyster-reef restoration.