Calanus species complexity in the transitional fjord system Isfjorden: from cold Arctic to warm Atlantic climate.

The West Spitsbergen Current carries warm and salty Atlantic water (AW) northwards, and with high input of AW, more Calanus finmarchicus is transported to the Arctic. Isfjorden is strongly impacted by Atlantic inflow and is ice-free year-round, except the innermost part, Billefjorden, with a shallow sill, so the dense AW can't enter, and the sea temperature remains cold. C. finmarchicus may not manage to establish an own local population in the high-Arctic, since its success depends on the timing of the phytoplankton spring bloom and the length of the growth season to successfully reproduce. It is predicted that with higher sea water temperatures, reduction in sea ice and a longer growth season for phytoplankton will create a more favourable environment for C. finmarchicus in the Arctic, suggesting it to be similarly or even more successful than Calanus glacialis in future. C. glacialis, however, has also shown to thrive better in warmer water, likely due to higher growth rates and a more favourable match of the timing of spring bloom and reproductive females being present. However, individual copepod body sizes are likely to be reduced with warmer climate meaning that individual copepods will be less lipid-rich, which will lead to less lipids per prey captured for their predators. This can be negative for predators that pick one by one prey item, like the little auk (Alle alle) which the last decade has been declining in numbers in Isfjorden.