HISTORICAL ECOLOGY OF THE NORTHERN ADRIATIC SEA — A REGIONAL APPROACH USING MOLLUSCAN DEATH ASSEMBLAGES FROM SEDIMENT CORES

The northern Adriatic Sea is among the most degraded marine ecosystems worldwide and is particularly suited to study ecosystem modification under human pressure. Multiple cores of 1.5 m length and diameters of 90 and 160 mm were taken at seven sampling stations throughout the northern Adriatic Sea, covering different sediment types, nutrient conditions and degrees of habitat exploitation. We study the down-core changes in the ecosystem composition on the basis of several higher taxa in these cores, most notably molluscs, foraminifers and ostracods. For the mollusc analyses, the cores were sliced into smaller subsamples and analysed for species composition, abundance, taxonomic similarity, evidence for ecological interactions (i.e., frequencies of drilling predation) and taphonomic condition of shells. In total, 120 bivalve and 207 gastropod species were recorded from almost 128,000 valves and shells. Almost 2000 shells of four bivalve species were dated with radiocarbon-calibrated amino-acid racemisation. They indicate vastly different time spans covered by the cores: about 6-8 ky in the NE Adriatic at the Brijuni islands and off Piran, but also at the station off the Venice lagoon, essentially covering the whole Holocene transgression, not more than 1000 years at Panzano Bay (near the Isonzo river mouth), and less than 250 years in the Western Adriatic near the river Po. Surface mixed-layer assemblages tend to show right-skewed postmortem age-frequency distributions whereas subsurface assemblages show unimodal or uniform shapes of distributions in most cores. In accord with the amino-acid racemization results, sedimentation rates derived from ²¹⁰Pb dating differ by one-order-of-magnitude between stations (1.9-2.5 cm/yr at the Po delta, Italy versus 0.15-0.34 cm/yr at Brijuni and Piran stations). Molluscan assemblages showed significant interregional differences that are strongly correlated with sedimentation rates, grain size, and the presence and abundance of organic and inorganic pollutants and heavy metals. Down-core changes in molluscan communities are conspicuous in all cores. These changes partly reflect environmental change during the Holocene transgression while in the uppermost core sections we interpret them to indicate anthropogenic impacts.