ISTANBUL

More than 20% of the world’s oceans have darkened in the last two decades, posing a threat to marine life and ecosystems, according to research published Tuesday.

Researchers from the University of Plymouth and Plymouth Marine Laboratory used satellite data and modelling to study changes in ocean light zones, building on more than a decade of research into the effects of artificial light at night on marine environments.

The study in the journal Global Change Biology found that 75 million square kilometers (29 million square miles) of ocean, equivalent in size to Europe, Africa, China, and North America combined, have been affected by darkening.

The phenomenon is disrupting the upper layers of the ocean, which support 90% of all marine life. While sunlight can penetrate ocean waters to depths of up to 1 kilometer, it is typically limited to the top 200 meters, where most marine organisms live.

“Darkening in the open oceans may be driven by warming of the surface oceans and climate-driven changes in the ocean circulation patterns,” according to the study.

Researchers identified several contributing factors, including increased coastal runoff, sediment accumulation, changing rainfall patterns, algal blooms and rising sea surface temperatures.

The study noted that reduced light in the oceans would push marine species into narrower surface waters, leading to increased competition for resources and higher risks of predation.

“The implications for marine food webs, global fisheries, carbon and nutrient budgets could be severe,” it added.​​​​​​​