EFFECTS OF THE 20TH CENTURY HYPOXIC EVENTS ON MOLLUSCAN COMMUNITIES AND THEIR PRESERVATION POTENTIAL IN SEDIMENT CORES (ADRIATIC SEA)

Estimating the timing of local extinctions and turnovers in the composition of macrobenthic communities driven by anthropogenic eutrophication is challenging because the early 20th century surveys are sparse and the corresponding intervals in sediment cores are mixed by bioturbation. Here, we account for mixing with extensive shell dating and assess response of the bivalve Corbula gibba to changes during the 20th century in sediment cores from two sites with high accumulation (~2 cm/y) in NW Adriatic (Po prodelta) and one site with moderate accumulation (<0.2 cm/y) in NE Adriatic (Isonzo prodelta). This approach allows us to assess preservation potential of hypoxic events and their association with molluscan turnover in the subfossil record. We find that, first, pervasively-mixed muds typical of natural highstand conditions deposited in the early 20^th century are replaced by muds with relicts of flood layers and high content of total organic carbon (TOC) deposited in the late 20^th century at Po prodelta. Second, the shift from the early to the late 20th century is characterized by a decrease in stratigraphic disorder and by an increase in temporal resolution from decadal to yearly resolution in both regions. This shift is not associated with higher sedimentation rate or with higher frequency of floods. However, incomplete mixing generated by burrowers after 2000 AD penetrates to depths > 100 cm and disturbs previously weakly-bioturbated layers. Third, the increase in production of C. gibba reconstructed on the basis of age unmixing occurred in both regions during the late 20th century. This increase is associated with a large-scale change in molluscan community composition, with a major decline in abundance of previously dominant infaunal and epifaunal suspension-feeders. These trends in bioturbation and molluscan community composition coincide with (1) an increase in TOC and (2) an increase in frequency of seasonal hypoxia documented independently on the basis of oceanographic observations. The cumulative effects of hypoxia driven by the 20th century eutrophication probably triggered the dominance of opportunists and reduced bioturbation. However, post-depositional mixing by burrowers in the aftermath of hypoxic events effectively removes the signature of hypoxic events in habitats with moderate sedimentation rates.