The photic zone is the upper layer of the ocean, where sufficient sunlight and moonlight penetrates to support biological processes that range from photosynthesis and primary production to diel vertical migration, reproduction and various visually guided behaviours. Oceanic light is being increasingly attenuated by elevated levels of plankton and suspended or dissolved organic matter, which leads to a darkening of the photic zone. Writing in Global Change Biology in May 2025, Davies and Smyth quantify long-terms changes in underwater light availability over the past two decades. The authors examined global trends of MODIS Aqua’s diffuse attenuation coefficient of downwelling irradiance at 490 nm (Kd(490)), which is a measure of light attenuation in seawater from which the depth of the photic zone can be inferred. Between 2003 and 2022, the authors found that Kd(490) has increased across 21% of the global ocean, which indicates that the oceans have become darker during this time. By defining photic zone limits as the depth at which the minimum irradiance of 490-nm light elicits diel vertical migration in Calanus copepods, they report that photic zone depths have reduced by more than 50 m across 9% of the global ocean and by more than 10 m across 19%. Such dramatic changes across large spatial scales could have substantial ecological effects on the timing of phytoplankton blooms and other phenological events, and on diel and vertical light niches that are crucial for ocean biodiversity. We chose this paper for our Year in Review collection because it highlights an emerging global change issue with potentially severe consequences for marine food webs and global fisheries, as well as carbon and nutrient budgets.
Original reference: Glob. Change Biol. 31, e7022 (2025)
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