From atmospheric light to bioluminescence: Determining transition depths using gliders in the Mediterranean Sea

Light in the ocean is an important driver of organism behavior and primary productivity. Bioluminescence, or light produced by living organisms, contributes to marine lightscapes. While this trait is ubiquitous throughout marine species, observations of bioluminescence remain relatively sparse compared to other oceanographic variables. Here we present glider based observations of bioluminescence and atmospheric light in the north west Mediterranean Sea as a part of the BIOLUminescence Marine, Observations spatio-temporelles in situ par Planeur Sous-marin (BIOLUMOPS) campaign. We consider the relationship between these variables down to a depth of 600 m using data from two deployments in 2025. We calculate the depth at which the light field is dominated by bioluminescence in this region, termed the bioluminescence transition depth. Our findings suggest a strong diel signal in transition depths from the surface at midnight to depth ranges between 246 and 465 m at midday. This depth range approximately follows a range of photosynthetically active radiation (PAR) isolumes from 2.1 × 10 −5 to 1.8 × 10 −4 μ mol photon m −2 s −1 , consistent across both the winter and summertime deployments. These findings present a new perspective on lightscapes in the Mediterranean Sea, with implications on light-mediated behavior and the effectiveness of communication between organisms.