The Arctic Ocean (AO) is particularly affected by climate change, as illustrated by its vanishing sea-ice and it is warming two times faster than the average global ocean. Those changes have great consequences for both the ecosystem and biogeochemical cycles and biogeochemical models are the predilection tool to project their future changes. However, biogeochemical models, such those used in the IPCC assessment, show the greatest uncertainties in the AO due to underconstrained or missing specific processes. Here, we will make use of a biogeochemistry model (FESOM2-REcoM) to investigate changes in the future ecosystems and carbon sequestration pathways. But first, based on observations, we show how we implement an important and enigmatic missing phytoplankton group, phaeocystis. Phaeocystis is thought to become prominent in the polar regions due to global warming, by outcompeting diatoms in some regions.