A changing marine lightscape: Two decades of satellite data to map the global benthic mesophotic zone and explore trends of change

Underwater light plays a key role in the distribution and survival of many marine species; however, the spatial and temporal variations of the light regime at mesophotic depths (from 30 m depth down to where net photosynthetic rates are no longer positive) are mostly unexplored. Benthic mesophotic habitats are especially at risk since rely upon the presence of specific, limited light conditions. Here we present the first assessment of the extent of the global benthic mesophotic zone based on two decades of satellite data, showing how its current distribution varies depending on the region and climate zones. We also explored temporal trends in the seasonal extension of the benthic mesophotic zone and in the value of Photosynthetically Active Radiation (PAR) at seabed during the period 2003–2021. Our results revealed a global-scale spatial reorganization of the benthic mesophotic zone and variation in the associated light intensity over the past two decades that may have major implications for mesophotic benthic habitats in the future. • The global assessment of benthic mesophotic zone extent from satellite data shows its distribution varies regionally and across climate zones. • Seasonal analysis reveals that major changes in the extent of the benthic mesophotic zone occurred from 2003 to 2021. • Significant trends in seabed PAR values suggest changes in mesophotic light conditions over the last 20 years. • The spatial reorganization and changing light conditions in the mesophotic zone may have global impacts on benthic habitats.