The oceans are losing their light – and with it, possibly their life. A new study reveals more than 20% of the Earth’s oceans, an area larger than Asia, has darkened over the last two decades. These findings raise concerns about how this spreading shadow could reshape marine ecosystems that depend on sunlight and moonlight for survival.
Almost 90% of marine life lives in what is known as the photic zone; the surface part of the ocean where sufficient light penetrates to stimulate biological processes like photosynthesis and animal behavior. With an average depth of 200 meters (656 ft), the photic zone is vital for fish stocks and the global nutrient cycle. “Ocean darkening” occurs when sufficient light cannot penetrate these photic zones of the ocean.
“There has been research showing how the surface of the ocean has changed color over the last 20 years, potentially as a result of changes in plankton communities,” says Dr Thomas Davies, Associate Professor of Marine Conservation.
“But our results provide evidence that such changes cause widespread darkening that reduces the amount of ocean available for animals that rely on the sun and the moon for their survival and reproduction.”
To examine the changes in the photic zone, researchers at the University of Plymouth utilized NASA’s Ocean Color Web, which tracks the amount of light that penetrates seawater by analyzing its color. Using data from 2003 to 2022, the team found nearly 10% of the ocean’s photic zone has been squeezed by 50 m (164 ft). More dramatically, photic zones in some oceans have shrunk by 100 m (328 ft).
This means that marine organisms, whose behaviors are influenced by photons, will have insufficient light to grow, move, hunt, and reproduce, forcing them to migrate vertically, closer to the ocean surface. When light-dependent species are forced upward into a smaller belt near surface waters, the competition for resources and food intensifies and animals are exposed to an elevated risk of predation.
Such conditions may have severe implications for marine food webs. They could trigger fundamental changes in the entire ecosystem, and even disrupt the ocean’s role in regulating climate.
As for why our oceans are darkening, the causes likely differ by region. Along coastlines, increased rainfall and agricultural runoff wash nutrients and sediments into the sea, spurring plankton growth. Additionally, dissolved organic matter and decaying vegetation absorb some of the light available. Meanwhile, in the open ocean, rising sea surface temperatures and algal blooms are possibly impacting how light travels through the upper layers.
“We also rely on the ocean and its photic zones for the air we breathe, the fish we eat, our ability to fight climate change, and for the general health and wellbeing of the planet,” says Davies. “Taking all of that into account, our findings represent genuine cause for concern.”
The study has been published in Global Change Biology.
Source: University of Plymouth