NATURAL ENVIRONMENTAL CHANGES VERSUS ANTHROPOGENIC IMPACT IN THE NORTHERN ADRIATIC SEA DURING THE HOLOCENE TRANSGRESSION

The shallow northern Adriatic Sea formed at the end of the last glaciation by the rising sea level. Since historical times, it has been strongly influenced by human activities (fishing, coastal development, pollution, eutrophication), making it one of the most degraded marine ecosystems worldwide. Our study reconstructs environmental changes and anthropogenic impacts since the onset of the Holocene transgression using down-core changes in faunal composition as indicators for ecological shifts. Sediment cores from seven sites in the northern Adriatic Sea were taken with a self-developed piston corer. They are all about 1.5m long and cover vastly different time spans, ranging from millennia in areas with low- to centuries in areas with intermediate- to decades in areas with high sedimentation rates. The quantitative analysis of molluscs, ostracods and foraminifera from these cores, coupled with extensive age-dating of bivalve shells, reveals major changes of benthic communities during the Holocene transgression. They are related to 1) the rapidly rising sea level (until ca. 6000 y BP), 2) natural environmental changes under more or less stable sea level until a few hundred years ago, and 3) major changes over the past centuries, with distinct amplification in the 20th century that we attribute to strong human impact (most notably eutrophication, input of organic and inorganic pollutants, and destructive fishing techniques). The micro- and macrofauna show similar responses to natural environmental changes during the Holocene transgression and confirm eutrophication as the most important anthropogenic driver of community shifts. The macrofauna, however, more sensitively reflects the impact of hypoxic conditions and destructive fishing gear. Distinct epifauna assemblages, characterized by bioherms of filter-feeding organisms, developed only at certain periods and in certain regions of the study area with low sedimentation rates. A conspicuous oyster reef is visible in our sediment cores and on historical maps of the region, but due to the extensive use of destructive fishing gear it has meanwhile totally disappeared. Our results will help establish realistic baselines for restoration efforts for such richly structured epifauna.