Basin-wide infaunalisation of benthic soft-bottom communities driven by anthropogenic habitat degradation in the northern Adriatic Sea

Shallow coastal seas play an important role in the economy of many countries by sustaining fisheries, tourism, aquaculture and other economic activities. Their exploitation has largescale ecosystem effects that are easily overlooked, as they often built up over decades or centuries, and historical ecological reference data are rarely available. Here, we assess these effects by comparing live and surface death assemblages (recording historical community states) of soft-bottom molluscs across a range of habitats in the northern Adriatic Sea, using the degree of mismatch between the 2 assemblages as a proxy for ecological change. We found a consistent live-dead mismatch at all stations. Although the degree of mismatch varied between stations with low and high time-averaging (i.e. the range of post-mortem shell ages), the community change followed the same trend over the entire Adriatic basin regardless of the type of sedimentary environment, with a loss of epifaunal species and the reduction of grazers, carnivores, and herbivores. In turn, the abundance of infaunal and opportunistic species feeding on plankton or detritus strongly increased in the living communities. Directionality and magnitude of these changes cannot be explained by time-averaging or by differences in species durability, but reflect a true ecological shift in response to multiple, long-lasting anthropogenic pressures, mainly bottom trawling, eutrophication and hypoxia. The original heterogeneous assemblages characteristic of different sedimentary habitats are thus replaced by a more infaunal, functionally impoverished and less diverse benthic community representing a new ecological baseline shaped by human impact.