Photo: Matt Paul Catalano//Unsplash
New research says 21 percent of our oceans have become darker in the 20-year period from 2003 to 2022. The study was conducted by researchers at the University of Plymouth and its results were published in Global Change Biology this week, “indicating global scale shifts in ocean ecology” and bringing a phenomenon to light known as “ocean darkening.” Meanwhile, a smaller portion of the ocean — about 10 percent — became lighter over the same time period.
The phenomenon of ocean darkening occurs when it becomes difficult for light to penetrate ocean water. This obviously happens regardless at certain depths, but the study reveals that the upper band of water where the ocean receives sunlight and moonlight is shrinking. Since more than 90 percent of marine species live in that space closest to the surface, this leaves less space for those species to thrive.
This darkening affected 21 percent of the global ocean between 2003 and 2022. This meant photic zones (the uppermost layer of a body of water where sunlight penetrates, allowing for photosynthesis) became 50 meters shallower over approximately nine percent of the ocean, and about 2.6 percent of the ocean saw that same zone shrink by 100 meters.
“Marine organisms use light for a whole variety of purposes. They use it for hunting, for mating, for timing reproductive events. They use it for basically every single part of their biology,” explained Dr Thomas Davies, a marine conservationist at the University of Plymouth. “With ocean darkening, they have to move up the water column, and there is less space, they’re all being squished up towards the surface.”
Researchers propose a variety of factors causing this trend, including algal blooms, changes in sea surface temperatures, and even artificial light.
“Darkening has typically been associated with coastal regions. Our results indicate that increasing attenuation of light is not restricted to the coastal zones but also widespread in the major oceans, particularly in polar regions, the North East Atlantic and North West Pacific Oceans,” the study says. “This suggests that light attenuation across the oceans is not driven exclusively by localized nutrient loading, run-off and upwelling in coastal regions. While these factors will clearly influence light attenuation in neritic coastal waters, darkening in the open oceans may be driven by warming of the surface oceans and climate-driven changes in the ocean circulation patterns.”
