This presentation aims to summarize the most relevant results of a doctoral project that developed new concepts and scientific tools for the modelling of coupled physical and biogeochemical processes in ice-covered regions. The main idea behind the coupling is the definition of a dynamical time-varying biologically active layer (BAL) in sea ice. The BAL concept, assumptions and features will be introduced. The advantage of this new formulation – in comparison with the standard formulation of a prescribed ice thickness where the biology is modelled – will be shown. The generality of the BAL concept and the potentialities to apply the BAL theory to other models will be described. The BAL development allows a direct coupling with a new biological model implemented in sea ice (BFM-SI), derived directly from an already existing and widely used model (BFM). The comprehensiveness of the theory and the modularity of the structure were the model’s fundamental features which enabled the reproduction of a complex and realistic biological system in sea ice. Model results at two different ice sites – one in the coastal Baltic and one in the coastal Greenland – will show that algae growth is primarily controlled by light availability, followed by nutrient replenishment and competition. Several experiments on different forms of light adaptation and acclimation will show that rapid changes in light conditions may lead to different community structure. The ability of organisms to down-regulate their internal chl:C ratio depending on light conditions becomes crucial in ice-covered regions where it will be shown that it may result in very different contributions in terms of biomass. The direct coupling between the sea ice and the oceanic ecosystems also allows a new modelling approach for the study of the sinking vs seeding effect of the sympagic and pelagic communities. Some final speculations will be made on the evolution of the polar ecosystems in climate change scenarios: model results suggest that shorter ice seasons and thinner sea ice may surprisingly support higher production rates and diversity of the sea-ice biological community in a warming ice-covered ocean.