In algal cultivation, nighttime biomass loss due to respiration and cell mortality can considerably reduce the amount of biomass produced during daylight. The adverse effect can be counteracted by mixotrophic cultivation, where an organic carbon (OC) source is used to supply the energy required for cell maintenance and division during darkness. The potential for mixotrophic cultivation to mitigate night biomass loss has yet to be tested under outdoor, large-scale conditions that use raw industrial waste streams, particularly during low-light seasons. We investigated night biomass loss in cultivation of the strain Monoraphidium minutum KAC90 in outdoor 1 m³ raceway ponds during the Nordic autumn. Flue gas condensate (nitrogen source) and cheese whey (phosphorus and OC source) were used for the mixotrophic treatment, while potassium monophosphate (phosphorus source) was used for the photoautotrophic control. Results indicate that under high OC availability, the mixotrophic treatment had a night biomass gain of 33% +/- 16%, whereas it experienced a night biomass loss of 10% +/- 9% under low OC. In contrast, the photoautotrophic control showed a night biomass loss of 5% +/- 15%. In the mixotrophic treatment, algal biomass had a higher carbohydrate content, but lower levels of lipids and proteins than the photoautotrophic cultures. The cultivation of algae using cheese whey may increase biomass accumulation in darkness, enhancing the overall production of algal biomass rich in carbohydrates.