Oceans Are Getting Darker, Shrinking the Livable Space for Marine Life, Study Finds

According to a new study, about one-fifth of the world’s oceans, spanning around 75 million square kilometers, has been getting darker over the past 20 years.

The study, published in the journal Global Change Biology, explored changes to the oceans’ photic zones. As the authors explained, these photic zones are where ecological interactions depend upon sunlight and moonlight. 

However, with the depth of photic zones decreasing in recent decades, these areas no longer receive enough light to support the ecological interactions, a phenomenon known as ocean darkening.

According to the study, 21% of the world’s oceans have become darker since 2003, with more than 9% of the oceans experiencing a photic zone depth reduction of more than 50 meters and 2.6% of oceans experiencing a reduction of more than 100 meters.

“If the photic zone is reducing by around 50 m 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. That could bring about fundamental changes in the entire marine ecosystem,” explained Tim Smyth, co-author of the study and Head of Science for Marine Biogeochemistry and Observations at the Plymouth Marine Laboratory.

These changes could be caused by a number of different factors, including nutrient, organic material and sediment loading from agricultural runoff, increased rainfall, algal blooms and increasing sea surface temperatures. The study also determined that some of the regions most impacted by ocean darkening include areas that are highly affected by climate change, including the Arctic, Antarctic and the top of the Gulf Stream.

Overall, the global trend is toward darkening oceans, although around 10% of oceans have become lighter, the researchers found.

Smyth and fellow author Thomas Davies, associate professor of marine conservation at the University of Plymouth, used data from the Ocean Color Web by NASA to track ocean darkening as well as solar and lunar irradiance models to investigate how the changes in light availability in the photic zones could potentially impact marine life. 

The scientists wrote that the impacts of ocean darkening on marine ecology are still largely unknown, but with 90% of all marine life dependent on these photic zones, the study authors warned that disruptions via ocean darkening were “likely to be severe” and far-reaching.

“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,” Davies said. “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.”