Book excerpt: This study analyzes carbon-cycle conditions controlling the state of the Arctic ecosystem and their seasonal variations. Territory covered includes the Barents, White, Kara, Laptev, East-Siberian and Chukchi Seas, considering inter-correlations between sources of organic carbon, their fluxes, recycling and burial in bottom sediments. All biological communities (phythoplankton, macrophythobenthos, microphythobentos, bacterioplankton, zooplankton and zoobenthos) are taken into account regarding their participation in the carbon cycle.