Holocene ecosystem shifts and human-induced loss of Arca and Ostrea shell beds in the north-eastern Adriatic Sea

Highlights: • North-eastern Adriatic mollusc communities changed significantly over the past 10,000 yrs. • Main drivers of change are the rising sea level, grain size shifts and, more recently, human impact. • Early infauna-dominated communities recede in favour of epifaunal assemblages by ~ 3000 yrs BP. • Epifaunal dominance culminates in the establishment of extensive Arca and Ostrea shell beds. • Multiple anthropogenic impacts in the 20th century reset the community to infaunal dominance. - Abstract: The molluscan assemblages in a sediment core from the north-eastern Adriatic show significant compositional changes over the past 10,000 yrs related to (1) natural deepening driven by the post-glacial sea-level rise, (2) increasing abundance of skeletal sand and gravel, and (3) anthropogenic impacts. The transgressive phase (10,000–6000 BP) is characterized by strongly time-averaged communities dominated by infaunal bivalves. During the early highstand (6000–4000 BP), the abundance of epifaunal filter feeders and grazers increases, and gastropods become more important. Epifaunal dominance culminates during the late highstand (4000–2000 BP) with the development of extensive shell beds formed by large-sized Arca noae and Ostrea sp. bivalves. This community persists until the early 20th century, when it falls victim to multiple anthropogenic impacts, mainly bottom trawling, and is substituted by an infauna-dominated community indicative of instability, disturbance and organic enrichment. The re-establishment of this unique shell-bed ecosystem can be a goal for restoration efforts.