Atlantification Alters the Reproduction of Jellyfish Aglantha Digitale in the European Arctic

Increasing heat content, salinity, and velocity of the Atlantic water masses passing northward through the Fram Strait accelerate the transition of the European Arctic toward more Atlantic state, a process referred to as Atlantification. A pronounced environmental shift leads to a poleward expansion of boreal species ranges and alters seasonal rhythms of local taxa, potentially affecting the whole food web structure. Here, we monitored a pelagic cnidarian, Aglantha digitale, commonly associated with boreal waters, throughout 12 consecutive summers (2003-2014) and along 2 branches of the West Spitsbergen Current, the main conveyor of Atlantic water to the Arctic. We documented a steady decrease in A. digitale abundance in the epipelagic waters of the European Arctic and a disproportionately larger northward advection of this jellyfish with the eastern branch of the West Spitsbergen Current compared to the western branch. Supported with modeling techniques, we found that year after year, A. digitale reproduced earlier in the southern region, thus leading to an earlier descent of the larger, more mature specimens, toward deeper waters, where they avoided our epipelagic sampling. Moreover, the prevalence of smaller jellyfish during years with record-level water temperature and salinity (2005-2007) in the southern region of the Fram Strait indicated that a 2(nd) reproductive cycle may have occurred. We also showed that the northern population of A. digitale, or at least part of it, originates in the south and is advected northward with the West Spitsbergen Current. In addition, our work highlights the need to include jellyfish in zooplankton monitoring.