Micro- and macrofaunal responses to major environmental changes in Holocene highstand sediments from the Northern Adriatic Sea

Benthic communities in the Northern Adriatic Sea experienced major environmental and ecological changes during the late Holocene, particularly in the late 20^th century due to anthropogenic induced stressors such as hypoxic events. These events lead to mass mortalities and changes in benthic communities. Here, we assess stratigraphic changes in bulk sediment geochemistry and sedimentological attributes to quantify the magnitude and timing of environmental changes and to correlate them to ecological changes. We focus on the reconstruction of the micro- to macrobenthic community composition (foraminifera, ostracods, bivalves and gastropods) prior and after major anthropogenic impacts. We investigate the differences in responses of these taxonomic groups to environmental changes and account for the impact of time-averaging.

The 3-m-long gravity core was collected at 31 m water depth, off the Po prodelta in the western part of the northern Adriatic Sea. The upper 60 cm of the core represent a condensed record determined by sediment bypassing and winnowing during the early and late sea level highstand. In total, 50 shells of the common bivalve Corbula gibba were dated by ¹⁴C-calibrated amino acid racemization (AAR) from the upper 30 cm and plant remains were dated by ¹⁴C from deeper parts. These analyses show that median shell ages of Corbula decline downcore, from ~50 years in the top 2.5 cm to 1,400 years in the 5-7.5 cm increment, 2,900 years in the 10-12.5 cm increment, and 4,500 years in the 17.5-20 cm increment. Median age in the 28-33 cm increment is again 3,600 years, indicating effects of mixing. The youngest shell corresponds to 24 years BP in the top 2.5 cm and the oldest shell to to 7800 years BP at the base at 30 cm.  The 60 cm-long highstand record can be divided in 4 major intervals:

(1) Early-highstand sediments cover the development of a baseline community. Total abundances of micro-and macrobenthic species increase upwards (2) In the late-highstand sediments (around 12.5-15 cm), micro-and macrobenthic absolute species abundance are highest. Increase in eutrophication and heavy metal pollution is indicated by rising N levels and Pb content in bulk sediments. (3) At 5 cm depth, a major anthropogenic environmental shift indicated by strong pollution (Pb and Hg) and eutrophication (TOC) coincides with a strong decline in micro-and macrobenthic abundance and diversity (4) The surface-mixed layer yields a slight increase in micro-to macrobenthic abundances, next to a slight decrease of heavy metal pollution and eutrophication.

¹⁴C-calibrated AAR shell ages indicate a relatively limited, centennial time averaging (measured by interquartile age ranges) of Corbula in the uppermost increment but then show a millennial-scale time-averaging below the uppermost surface-mixed layer. This can be linked to a decrease in bioturbation in the 20^th century and to a slight increase in sedimentation rate. Although the record is affected by time-averaging, the micro-and macrobenthic community abundances show a distinct pattern that can be related to environmental changes from geochemical sediment proxies. Benthic foraminifers, ostracods and mollusks abundance show similar responses to sedimentological and geochemical tracers in these condensed sediments.