Late Holocene changes in the composition of foraminiferal,ostracod and molluscan communities in condensed sediments(northern Adriatic Sea)

Human-induced changes in sedimentation rates, pollution, and eutrophication significantly transformed the benthic communities in the northern Adriatic Sea during the late Holocene, particularly in recent centuries. Fossil assemblages from sediment cores record these changes but are often affected by stratigraphic condensation and mixing. Here, we show that molluscan, foraminiferal, and ostracod assemblages preserved in a condensed sediment core collected off the Po delta at 31 m water depth, still archive information about the composition of benthic communities prior to anthropogenic changes. All three groups exhibit a similar trend in total abundance (density): a gradual increase peaking in a 10 cm-thick shelly lag (≈ 2 – 6 kyr BP) with millennial time-averaging, followed by a significant decline in the uppermost 5 cm of highstand silts (representing the past 2 kyr BP) with centennial time-averaging. The molluscan assemblage in the shelly lag integrates across several baseline community states. The assemblage mainly comprises shallow-subtidal filter feeders and soft-bottom infauna, with the bivalve Varicorbula gibba and the gastropod Turritellinella tricarinata dominating and increasing in proportional abundance in the highstand silts. The ostracod and benthic foraminiferal assemblages in the shelly lag are dominated by opportunistic species such as Cytheridea neapolitana, Ammonia beccarii, and Haynesina germanica. In contrast, the foraminifera Nonionella sp. and the ostracod Loxoconcha sp. increase in proportional abundance in the highstand silts, characterised by an increase in filter-feeders among the molluscs, infaunalisation and a decrease in epiphytic species. Although an increase in net sediment accumulation primarily causes the decline in fossil density in the uppermost part of the core, upward changes in the relative abundance of species and functional groups reveal a difference between the baseline and impacted community states. Therefore, the time-averaged fossil assemblage in the shelly lag provides a valuable long-term record of an ecosystem in the region before human impact.