| Summary: | The importance of landfast ice as a nursery and breeding ground for Arctic marine invertebrates was studied in Van Mijenfjorden (77°N, 15/16°W), southwestern Svalbard from March to May 2017. The collection of first-year ice cores with stations along a depth gradient allowed the investigation of both temporal and spatial differences in sympagic meiofauna community composition and abundance. Furthermore, water column samples were retrieved to examine the strength of sympagic-pelagic coupling. Overall, 13 taxa were identified from the ice and 15 taxa from the water column with low abundances of dominant ice fauna in the water samples. Total sympagic metazoan abundance peaked in late April with over 25,000 ind m-2, due to the reproduction of ice-associated nematodes. Throughout spring the presence of sexually mature nematodes and eggs supported the notion that sea ice in Van Mijenfjorden, especially at the main station (vMF/Mn – 50 m water depth), served as a breeding and reproductive ground to ice nematodes. Comparatively, through tight sympagic-benthic coupling, in late April, higher abundances of juvenile polychaetes (7,661 to 8,900 ind m-2), in the lowermost 10 cm of the ice, were recorded at the innermost stations (2 - 14 m water depth), compared to deeper stations (> 50 m water depth) (0 to 865 ind m-2). They were utilizing the sea ice as a nursery ground and refuge from predators. Higher growth rates and faster development of polychaete juvenile morphological features indicative of maturation were observed in situ, as well as in a growth experiment that mimicked natural ice algae concentrations. Conclusively, ice nematodes and juvenile polychaetes inhabited the sea ice in Van Mijenfjorden to exploit the highly concentrated ice algae located in the lowermost three centimeters of the ice for reproduction and growth. Molecular tools revealed that all sympagic nematodes, apart from one individual from Wahlenbergfjorden (Nordauslandet, Svalbard), belong to the genus Halomonhystera and the family Monhysteridae, which are known from sea ice habitats. Juvenile polychaetes belonged to the family Spionidae, which is known to reside in sea ice in other parts of the Arctic.
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