ANOTHER ONE BITES THE HOST: ASSESSING THE INFLUENCE OF ABIOTIC FACTORS ON SURGING PARASITE PREVALENCE DURING SEA LEVEL RISE, PO PLAIN AND NORTHERN ADRIATIC COAST, ITALY

Holocene deposits of the Adriatic coastal plain are rich with clams parasitized by trematodes (complex life cycle parasites), as indicated by diagnostic pits on the interior of their valves. Surges in parasite prevalence associated with past sea level rises have been documented by previous studies in transgressive coastal deposits (TST) and may indicate changes in ecosystem health with ongoing climate change. Similarly, trematode prevalence in the modern northern Adriatic, parallel past biotic trends. Indeed, in sediment-starved, TST-like settings (north of the Po Delta) trematode prevalence was found higher than in those settings along and south of the Po Delta and characterized by high sedimentation rates. Previous studies have ruled out taphonomy, host availability, diversity, turnover, and community structure as drivers of this pattern, yet proxy data for environmental variables are lacking. Here we analyze the spatial and temporal distribution of pooled δ¹⁸O and δ¹³C (V-PDB) values of C. gallina carbonate from modern death assemblages with known trematode prevalence values. We have analyzed 155 samples from five thanatocoenosis-sourced valves from three locations along the Adriatic coast. The step-wise positive shift in median δ¹⁸O values between Caorle (TST-analog; NE of Po delta) and Marina di Ravenna and Montemarciano (both high-sediment accumulation settings) potentially reflects the introduction of sediment and colder freshwater from the Po and Apennine rivers. Similarly, there is a significant positive shift in median δ¹³C values when comparing Caorle and Marina di Ravenna with Montemarciano. δ¹³C variance values in the locations on and south of the delta are negatively skewed and have significantly higher variance values than Caorle, potentially indicating higher input of organic matter due to higher river discharges. Ongoing work will produce data from 30 C. gallina valves retrieved in 11 thanatocoenoses sampled along the Northern and Central Adriatic coasts and ~ 30 sub-fossil valves from transgressive and regressive Holocene coastal deposits of the Adriatic. We will complement these analyses with trace element concentration data and clumped isotope paleothermometry to determine the influence of environmental variables on trematode surges in coastal settings during sea level rise.