Microbial eukaryotes or single-celled protists are organisms that play a fundamental role in energy flow and the cycling of elements in the marine ecosystem. Protist communities in the Arctic are highly seasonal. Despite the abundance and importance of pico- and nano- sized eukaryotes, most protist research so far has focused on larger organisms that can be studied under a microscope. However, recent advances in molecular techniques have made it possible to identify pico- and nano-sized protists and quantify their biodiversity. Furthermore, molecular genetic tools make it possible to connect taxa to specific biogeographic conditions and link distinct protist assemblages to environmental factors.In this project we will examine seasonal changes within the pelagic community of microbial eukaryotes in the seasonally ice-covered fjord Billefjorden (BAB station), using this as a model system for a high-Arctic fjord. The resulting data will be compared to time-series data from the "atlantified" Isfjorden-Adventfjorden (ISA) time series - a model system for an ecosystem influenced by climate change. The ongoing global warming leads to loss of sea-ice habitats, making it particularly urgent to obtain more knowledge on the current status and potential climate impacts on the marine ecosystems in seasonally ice-covered arctic fjords.