Fauna of shallow water Arctic seep related to dynamic water masses

Most shallow-water methane seeps studied in the Svalbard-Barents Sea region fall within relatively warm and saline Atlantic water masses which have been hypothesized to impact epimegabenthic species composition and select against chemosymbiotrophic fauna. Seeps on the shelf off Svalbard’s southernmost fjord Hornsund are subject to colder Arctic water masses, which theoretically could favor chemosymbiotrophic fauna, but were nonetheless devoid of chemosymbiotrophic siboglinids. It is likely that the region is still exposed to Atlantic water masses; our CTD data captured high salinity values which suggest transformed Atlantic water. Furthermore, water masses are highly dynamic in this region; Atlantic water intrusions might be common, favoring fauna directly dependent on phytodetrital material over chemosymbiotrophic species. Current community composition could also represent a recent transition since Atlantic water presence has increased over the past few decades. Otherwise, epimegabenthic community composition of this shallow-water location resembled that of many other Arctic seeps, consisting of background benthic taxa. Four nudibranch morphotaxa were seen, including the kelp or rocky substrate associated species Edmundstella pedata, which has never been observed north of mainland Norway, nor on soft sediment before. This highlights the extent to which non-invasive, image-based methods can reveal about benthic fauna, including significantly expanding distribution ranges and providing novel insight into habitat associations.