Tracking Pelagia noctiluca scyphomedusae by combining modeling and stable isotope approaches

Isabella D'Ambra^1,2*Simona Saviano³Maria Assunta Ambrosio³Vincenzo Botte¹Daniele Iudicone¹Maria Grazia Mazzocchi¹Louise Merquiol^1,4,5Daniela Cianelli³

• ¹Department of Integrative Marine Ecology, Anton Dohrn Zoological Station Naples, Naples, Campania, Italy
• ²National Biodiversity Future Center (NBFC), Palermo, Italy
• ³Department of Research Infrastructures for Marine Research, Stazione Zoologica Anton Dohrn, Naples, Campania, Italy
• ⁴Aix Marseille University, Université de Toulon, CNRS, IRD, MIO, Marseille, France
• ⁵UMR TELEMMe, MMSH, Aix-Marseille University, CNRS, Aix-en-Provence, France

True jellyfish (Cnidaria, Scyphozoa) often appear in large aggregations along the coasts, where they interfere with human activities (tourism, fisheries, power plants). Therefore, defining their distribution and predicting their outbreaks is crucial for effective coastal management. In this study, we tested the combination of modeling based on the Lagrangian approach and stable isotope (SI) analysis to define the trajectories of the scyphomedusa Pelagia noctiluca in the Gulf of Naples (GoN, western Mediterranean Sea) during 4 outbreaks recorded in March, June, July, and November 2019. SIs were determined in scyphomedusae and their potential planktonic prey collected at the Long Term Research site MareChiara (LTER-MC) during the outbreaks and during the previous three weeks, to account for the turnover rate of medusae. Numerical simulations were performed using a particle tracking model forced by a Regional Ocean Modeling System (ROMS) developed for the GoN. Lagrangian simulations were performed releasing particles 20 days before the outbreaks to align with SI determinations. SI ratios of scyphomedusae indicated offshore foraging, with Lagrangian simulations confirming offshore-to-coastal transport via south Tyrrhenian surface dynamic and southern winds regime. During the outbreak in November, carbon and nitrogen SIs of medusae (-18.7‰ and 1.9‰, respectively) reflected the SIs of plankton typically found in offshore waters. The model corroborated this finding, suggesting a rapid transport of medusae by surface currents driven by intense southerly winds (gusts up to 18.7 m/s). During the other three outbreaks, SI values of medusae (δ 13 C ranging between -20.1 and -18.5‰, δ 15 N between 4.6 and 5.9‰) were intermediate between prey found offshore and those in the coastal area. Simulations indicated that surface circulation patterns promoted the permanence of medusae within the coastal area, particularly in summer. Our results suggest that SI ratios of scyphomedusae are intimately dependent on their movements across diverse isoscapes. Therefore, predictive models integrating SI analysis and ocean circulation data could improve early warning systems for jellyfish outbreaks, aiding coastal management.