Fauna Associated with Sea Ice, Sediment, and Water Column in the Ermolinskaya Inlet (Kandalaksha Bay, the White Sea)

The abundance and diversity of meiofauna, with a focus on nematode communities, were studied across three habitats (ice, water, and sediment) during three seasons (winter, spring, and summer) in the Ermolinskaya Inlet of the White Sea between 2020 and 2021. During the study period, seasonal ice cover with a thickness of 2.5 to 51 cm was observed. From December 2020 to April 2021, the average abundance of meiofauna increased in sea ice (0.4 ± 0.01–1.4 ± 0.3 ind/10 cm2), bottom sediments (76.4 ± 20.3–1250 ± 394.2 ind/10 cm2), and the water column (1989.4 ± 464.3–3330.1 ± 679.1 ind/m3). The Ermolinskaya Inlet is a shallow water body (<5 m deep), where the ice cover lies very close to the seafloor or even contacts it throughout the ice-covered season. This proximity could have facilitated the colonization of sea ice by organisms from both the water column (rotifers, copepods and their nauplii) and sediment (nematodes, acoel flatworms, bryozoans, bivalves, and gastropods). A case study of ice-associated nematodes revealed a seasonal shift in dominant species. In winter, adults of Daptonema gelida and Hieminema obliquorum predominated, whereas in spring, juveniles of Cryonema crassum. The study highlights the key role of local hydrodynamic regimes in enabling nematode dispersal, especially during the dynamic periods of ice formation and melting.