Terrestrial Runoff Divides the Central Arctic Ocean and Shapes the Biologically Relevant Marine Lightscape

The Arctic Ocean is undergoing drastic changes in its sea ice cover, but is also optically complex. Observations from summer 2022 across the western Eurasian Basin, show increased colored dissolved organic matter (CDOM) absorption from terrestrial‐derived runoff within the Transpolar Drift (TPD) in the Amundsen Basin, extending down to a depth of 100 m. This is in stark contrast to the low‐CDOM waters of the Atlantic‐influenced Nansen Basin. The higher CDOM absorption increases the diffuse attenuation coefficient by around 35% relative to Atlantic‐influenced waters, and changes the spectral quality of light at depth. An irradiance model demonstrates that the decreased light availability can delay or inhibit under‐ice phytoplankton blooms, affect zooplankton behavior, and cause enhanced solar heating in the upper 10 m. These findings show that simplified representations of light attenuation in present Earth System Models can hinder accurate assessments of light limitations on Arctic marine ecosystems.