INCREASE IN SIZE OF THE BIVALVE CORBULA GIBBA DRIVEN BY CHANGES IN GROWTH RATE IN RESPONSE TO THE 20TH CENTURY EUTROPHICATION IN THE ADRIATIC SEA

The size of the bivalve Corbula gibba increased by a factor of two during the 20^th century in the Gulf of Trieste and at Po prodelta in the northern Adriatic Sea. However, it is unclear whether present-day large-sized populations of this species have analogues either in the older Holocene record of the northern Adriatic Sea and/or in earlier pre-Holocene record of the Mediterranean Sea and Paratethys and what mechanisms are responsible for this trend. Here, we assess temporal size variation in Holocene death assemblages of C. gibba in the NW and NE Adriatic, in combination with age dating based on radiocarbon-calibrated amino acid racemisation and taphonomic clock based on conchiolin preservation. We find that, first, C. gibba populations inhabiting locations with high sediment accumulation rates (death assemblages time-averaged to decades) prior to the late 20th century were not dominated by large-sized individuals (> 10 mm in length). Second, C. gibba increased in size also at locations with slow sediment accumulation rate in the NE Adriatic (off Istria), but the proportion of large-sized individuals in surface death assemblages (time-averaged to millennia) is small. However, the size increase is stronger when shells without conchiolin preservation (that are probably older than 100-200 years) are omitted from analyses, indicating that C. gibba grew to sizes > 10 mm effectively in the whole northern Adriatic Sea. Death assemblages off Istria, affected by mixing of the late 20th century large-sized shells with much older, small-sized shells, are thus inert to late 20th century changes in size structure of C. gibba populations, in contrast to death assemblages at sites with high sedimentation rate. We conclude that the temporal shift from right-skewed distributions to bimodal distributions (with high abundance of shells > 10 mm) is a unique feature of the late 20th century and correlates with an increase in the frequency of hypoxia and eutrophication and with delayed recovery of hypoxia-sensitive species in the wake of mass mortality events. Analyses of growth increments indicate that the size increase is primarily underlain by an increase in growth rate.