Ocean darkness affects 75 million square kilometers

Over the past two decades, more than one-fifth of the Earth’s oceans have become darker, creating what researchers say is one of the largest habitat losses on Earth.

A new study shows that between 2003 and 2022, 75 million square kilometers of oceans (about twice the size of North America) have reduced light permeability, fundamentally changing marine ecosystems, which account for 90% of the marine ecosystem.

This phenomenon is called ocean blackening when the optical properties of water decrease the sun and moonlight can penetrate the surface. These photon regions are the most productive habitats in the ocean, support global fish reserves and drive key biogeochemical cycles that regulate the Earth’s climate.

Huge habitat compression

The researchers used satellite data from NASA’s MODIS Aqua sensors to track changes in light attenuation at 9.8 million ocean sites around the world. Their discovery depicts a wide range of ecological destruction that goes far beyond the coastal waters entering the open ocean.

The scale of habitat loss is alarming. The depth of the optical belt has been reduced by 50 meters in 32 million square kilometers (roughly equivalent to real estate in Africa). In some areas, light-dependent marine organisms lose more than 100 meters of vertical habitat space.

“There are studies showing how ocean surfaces have changed over the past 20 years due to changes in plankton communities,” explains Dr. Thomas Davies, associate professor of marine conservation at the University of Plymouth. “But our results provide evidence that such changes can lead to widespread blacking, reducing the amount of ocean available to animals that rely on the sun and the moon to survive and reproduce.”

Beyond the coastal waters

Although ocean blackening was previously mainly associated with coastal pollution and runoff, this study revealed phenomena that affected the vast ocean. The biggest changes occur in the polar regions, the Northeast Atlantic and the Northwest Pacific waters – the region undergoes a significant climate-driven shift.

What makes this discovery particularly concerned is how it affects the largest daily migration of the ocean. Zooplankton and other marine life take a large vertical journey every day, rising to surface water at night and feeding deeper water during the day to avoid predators. As the available light shrinks, these migrations are compressed into increasingly narrow water bands.

“The dynamics of the ocean are far beyond people’s dynamics,” said Tim Smyth, director of marine biogeochemical science at Plymouth Marine Laboratory. “If in a large ocean, the optical zone is being reduced by about 50m, animals that need light will be forced to get close to the ground, where they have to fight for food and other resources they need.”

Moonlight is also important

The study breaks new groundwork by studying how ocean blackening affects nighttime ecosystems. Many marine species synchronize critical life events, from spawning to migration, with the lunar cycle. Even though the moonlight provides much lower lighting than sunlight, the study found that darkness affects the optical zone of the 50 million square kilometers of ocean at night.

This discovery challenges conventional approaches to defining marine habitats, often focusing only on sun-driven photosynthesis. The researchers used the photosensitive of Calanus copepods – small crustaceans, which migrate vertically in response to sun and moonlight – to establish a more biologically relevant threshold for light-dependent marine life.

Climate connection

The study shows that ocean darkness comes from multiple interconnected causes. Near the coastline, agricultural runoff and increased rainfall cleans nutrients and deposits into the ocean, stimulating the flowering of plankton that absorbs and scatters light. In an open ocean, rising sea surface temperatures and cyclic patterns appear to be changing the distribution and abundance of absorbed photoorganisms.

These changes create a feedback loop using the climate system. Optical regions play a crucial role in the carbon cycle, where marine organisms capture atmospheric carbon dioxide through photosynthesis and transport it to deeper waters. As these areas shrink, the ocean’s ability to regulate atmospheric greenhouse gases may be compromised.

Interestingly, not all ocean areas are turning black. Around 37 million square kilometers of oceans have become lighter throughout the study period, highlighting the complex and regional changing nature of these changes.

Ecosystem cascade effect

These implications are spread throughout the marine food network. As light-dependent populations of organisms gradually shrink surface water, competition for resources will intensify and predation pressure will increase. This compression may trigger a cascade effect, thus reshaping the entire marine ecosystem.

Schedule is not more critical. Many marine species are already facing the pressure of warm water, acidification and overfishing. The darkening of the ocean adds another layer of pressure that can push vulnerable people toward adaptability.

How will marine ecosystems cope with the unprecedented loss of this luminous habitat? The researchers stress that while their 20-year dataset provides compelling evidence of change, understanding the entire ecological consequences will require continuous monitoring and research on how different species cope with changing light conditions.

Davis highlighted the broader implications: “We also rely on the oceans and their optical regions to breathe air, the fish we eat, the ability to change climate and the general health and well-being of the planet. With all this in mind, our findings represent a real cause of concern.”