Impact of Holocene environmental change on northern Adriatic populations of Echinocyamus pusillus

The consequences of prolonged human stressors on populations of marine benthic species are generally difficult to assess due to the paucity of long-term monitoring data. The youngest fossil record, preserved in sediment cores and grab samples, provides a rich historical archive that allows tracking ecological responses to natural environmental changes and reconstructing the state of populations prior to the onset of anthropogenic impacts. However, most studies using these paleoecological records focus on microfossil- or mollusc assemblages, and only limited knowledge is available on the responses of other key components of marine ecosystems, such as echinoids. This study presents a novel record of well-preserved tests of the infaunal clypeasteroid Echinocyamus pusillus from Holocene marine sediment cores, which provide new insights into the long-term population dynamics and ecological responses of the widely distributed echinoid. We investigated trends in test size and abundance of E. pusillus and relate them to sedimentological conditions during the last ~11,000 years in the shallow northern Adriatic Sea, where coastal marine habitats have been under prolonged anthropogenic pressure. We combined these data with radiocarbon dating of multiple specimens to extract ecological signals from time averaged assemblages. The sediment cores and grab samples collected from four stations, which cover different benthic habitats, document environmental changes during the post-glacial transgression and the Holocene-Anthropocene transition, as evidenced by shifts in molluscan assemblages and geochemical proxy records. The data suggest significant fluctuations in E. pusillus abundance, but remarkably stable test sizes over the past millennia and across sampling stations. Moreover, the age-dating results indicate a paucity of dead tests originating from the late 19^th and 20^th centuries in the surface mixed layer. This points to a strong decline in test production and thus population densities over the past ~200 years. This pattern can be linked to pronounced environmental changes during the onset of the Anthropocene, including siltation, eutrophication, and increased frequency of hypoxic events. Rapid decline of populations during that time combined with temporal mixing of fossil assemblages may thus explain the overall lack of body size shifts observed in sediment cores. Our results provide a fundamental baseline for evaluating temporal and spatial changes in body size and abundance of E. pusillus in the northern Adriatic Sea. They can be compared with recent monitoring and fossil data from other areas and serve to disentangle natural population variability from responses to recent intensification of human impacts.