Effect of Holocene environmental change on the size structure of mollusc indicator species of the northern Adriatic Sea

Anthropogenic disturbances such as pollution, eutrophication and increased frequency of hypoxia led to profound changes in the abundances of marine species and overall community structure. They also led to changes in the population size structure of some species, a phenomenon still little understood. In this project, we use data from sediment cores to document long-term changes in population size structure of two common mollusc species of soft bottom communities of the northern Adriatic Sea (NAS): the bivalve Varicorbula gibba (Olivi, 1792) and the gastropod Turritellinella tricarinata (Brocchi, 1814) (= Turritella communis Risso, 1826). They are similar in their habitat preferences, but V. gibba is an opportunistic species with a higher tolerance of natural and anthropogenic disturbances. Ecological monitoring and analyses of sediment cores have shown that T. tricarinata was replaced by V. gibba as the dominant species of soft bottom communities in the NAS during the 20^th century, probably due to increased eutrophication and a higher frequency of hypoxia. It has also been shown that V. gibba did increase in average size.

We test whether size changes have also occurred in T. tricarinata using material from sediment cores from the area off the Po delta in the western-, and Panzano and Koper Bays in the eastern NAS. The cores capture the last 150 to 7,000 years depending on the location. Measurements of shell length and width are used to analyze population size structure. Changes in sediment composition and geochemical proxies of pollution and eutrophication are studied as possible drivers of size changes and are placed in a chronological framework based on radiocarbon-dated shells.

Our preliminary results indicate that although abundance of T. tricarinata decreased throughout the region in the second half of the 20th century, shell length increased in some stations during that time. This pattern contrasts with the results of previous studies, which suggested that larger specimens of molluscs are more vulnerable to hypoxia. Our results may reflect complex interactions between nutrient enrichment and oxygen availability in controlling population dynamics of this species. However, more data are necessary to document long-term trends in population size structure driven by natural environmental change and to establish a baseline for size variability in T. tricarinata and V. gibba in the NAS prior to increased anthropogenic impact. Our study will provide a better understanding of how population size structure of marine benthic species changes in response to anthropogenic disturbances.