The Phytoplankton and the plants play a major role in the Arctic Marine Food Chains and Ecosystems. The plants underwater use Photosynthesis to make their own food. The plants in the Arctic Marine Food Chains are the producers (they make their own food). And the other creatures are the consumers (they eat the plants and other animals).
So, the plants underwater's niche would be to produce and use Photosynthesis, otherwise the whole food chain would fall apart.
If the plants all didn't make their own food or survive then, the small creatures wouldn't have anything to eat and if they didn't have anything to eat they wouldn't exist anymore and if they didn't exist anymore the bigger creatures wouldn't have anything to eat and viseversa and so forth.