Abstract <p>An investigation was conducted in the Kara Sea to study the effect of small doses of γ-radiation from an external source of ⁴⁰K (Eγ = 1.46 MeV) on carbon assimilation by natural populations of microplankton. The results of measuring the rate of carbon assimilation by plankton communities under the influence of γ-radiation from external sources with activities of 269 and 376 Bq showed a multidirectional effect of its influence on carbon assimilation in the light and dark conditions. In the surface layer, the rate of carbon assimilation in light samples with a source activity of 269 Bq increased on average by 1.2 times. It was shown that under a γ-radiation source activity of 376 Bq the mean rate of carbon assimilation in the light condition increases on average by 1.4 times. Comparison of the rate of carbon assimilation in darkness under γ-radiation of sources with an activity of 269 and 376 Bq showed its decrease on average by 1.6 times relative to the background, regardless of the dose rate.</p>