The world’s northern oceans used to have permanent ice cover, but that ice has thinned and melted to such as extent that the region can now be navigated.
Not just by boats, but also sea creatures from giant whales to single-cell plankton.
Plankton consists of microscopic plants called phytoplankton and tiny animals called zooplankton. Together they form the main food for many marine animals.
The arrival of a Pacific Ocean species of phytoplankton is of particular concern.
As yet the impact of this new species is unknown, but it will compete with existing types which may affect the availability of food for other sea life, including commercial fish.
