Ocean Darkening Affects 75 Million Square Kilometers

More than one-fifth of Earth’s oceans have become significantly darker over the past two decades, creating what researchers describe as one of the largest habitat losses on the planet.

A new study reveals that 75 million square kilometers of ocean – an area roughly twice the size of North America – experienced reduced light penetration between 2003 and 2022, fundamentally altering the marine ecosystems that 90% of sea life depends upon.

The phenomenon, known as ocean darkening, occurs when changes in water’s optical properties reduce how deeply sunlight and moonlight can penetrate the surface. These photic zones serve as the ocean’s most productive habitats, supporting global fish stocks and driving critical biogeochemical cycles that regulate Earth’s climate.

Massive Habitat Compression

Using satellite data from NASA’s MODIS Aqua sensor, researchers tracked changes in light attenuation across 9.8 million ocean locations worldwide. Their findings paint a picture of widespread ecological disruption extending far beyond coastal waters into the open ocean.

The scale of habitat loss is staggering. Across 32 million square kilometers – roughly equivalent to Africa’s landmass – the photic zone has shrunk by more than 50 meters in depth. In some regions, light-dependent marine life has lost over 100 meters of vertical habitat space.

“There has been research showing how the surface of the ocean has changed colour over the last 20 years, potentially as a result of changes in plankton communities,” explained Dr. Thomas Davies, Associate Professor of Marine Conservation at the University of Plymouth. “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.”

Beyond Coastal Waters

While ocean darkening has previously been associated primarily with coastal pollution and runoff, this study reveals the phenomenon affecting vast expanses of open ocean. The most dramatic changes occurred in polar regions, the northeast Atlantic, and northwest Pacific waters – areas experiencing pronounced climate-driven shifts.

What makes this discovery particularly concerning is how it affects the ocean’s largest daily migration. Zooplankton and other marine organisms undertake massive vertical journeys each day, rising to surface waters at night to feed and descending to deeper waters during daylight to avoid predators. As available light shrinks, these migrations become compressed into increasingly narrow bands of water.

“The ocean is far more dynamic than it is often given credit for,” noted Professor Tim Smyth, Head of Science for Marine Biogeochemistry and Observations at Plymouth Marine Laboratory. “If the photic zone is reducing by around 50m in large swathes of the ocean, animals that need light will be forced closer to the surface where they will have to compete for food and the other resources they need.”

Moonlight Matters Too

The research breaks new ground by examining how ocean darkening affects nighttime ecosystems. Many marine species synchronize critical life events – from spawning to migration – with lunar cycles. Even though moonlight provides far less illumination than sunlight, the study found that darkening affects nighttime photic zones across 50 million square kilometers of ocean.

This finding challenges conventional approaches to defining marine habitats, which typically focus only on sunlight-driven photosynthesis. The researchers used the light sensitivity of Calanus copepods – tiny crustaceans that migrate vertically in response to both sun and moonlight – to establish more biologically relevant thresholds for light-dependent marine life.

Climate Connection

The study suggests ocean darkening stems from multiple interconnected causes. Near coastlines, agricultural runoff and increased rainfall wash nutrients and sediments into the sea, stimulating plankton blooms that absorb and scatter light. In the open ocean, rising sea surface temperatures and shifting circulation patterns appear to be altering the distribution and abundance of light-absorbing organisms.

These changes create a feedback loop with climate systems. Photic zones play crucial roles in carbon cycling, with marine organisms capturing atmospheric CO2 through photosynthesis and transporting it to deeper waters. As these zones shrink, the ocean’s capacity to regulate atmospheric greenhouse gases may be compromised.

Interestingly, not all ocean regions are darkening. About 37 million square kilometers of ocean have actually become lighter over the study period, highlighting the complex and regionally variable nature of these changes.

Ecosystem Cascade Effects

The implications extend throughout marine food webs. As light-dependent organisms crowd into shrinking surface waters, competition for resources intensifies while predation pressure increases. This compression could trigger cascading effects that reshape entire ocean ecosystems.

The timing couldn’t be more critical. Many marine species already face mounting pressures from warming waters, acidification, and overfishing. Ocean darkening adds another layer of stress that could push vulnerable populations beyond their ability to adapt.

How will marine ecosystems respond to this unprecedented loss of illuminated habitat? The researchers emphasize that while their 20-year dataset provides compelling evidence of change, understanding the full ecological consequences will require sustained monitoring and research into how different species respond to altered light conditions.

Davies stressed the broader implications: “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. Taking all of that into account, our findings represent genuine cause for concern.”