Environmental controls of autotrophic biofilm biomass and community composition in subarctic lakes and streams in Greenland

Photosynthetic biofilms are key components of Arctic freshwater ecosystems, supporting primary production and forming the base of aquatic food webs. While several environmental factors regulating biofilms are known, their relative importance and connection to catchment characteristics across different Arctic ecosystems remain unclear. This study assessed epilithic biofilm biomass and autotrophic community composition in lakes and streams near Narsaq, South Greenland. Lake biofilms were dominated by cyanobacteria, with autotrophic biomass positively associated with catchment greenness and water conductivity. In streams, biofilms primarily comprised diatoms and green algae, with autotrophic biomass linked to phosphate, pH, and temperature. Total biofilm biomass in lakes was also related to catchment greenness and conductivity, while no consistent environmental drivers were found for stream biomass. These findings underscore how environmental controls on biofilm structure differ between lentic and lotic systems. As climate warming intensifies tundra greening and alters nutrient regimes, autotrophic biofilm biomass is likely to increase, potentially affecting food web dynamics and carbon cycling in Arctic freshwater ecosystems. Our findings advance the understanding of Arctic freshwater biofilm dynamics and their sensitivity to climate‐driven changes.