Scientists Warn of What's Happening in Oceans: 'Real Cause for Concern'

Ocean darkening is a phenomenon that occurs when, as a result of changes occurring in the highest level of the ocean, the depth of the so-called photic zones, where 90 percent of all marine organisms live, decreases. According to the latest study, over two decades, the darkening phenomenon occurred in an area of ​​over 75 million km2. The results were published in "Global Change Biology."

Why are the oceans getting darker?

Scientists from the University of Plymouth and Plymouth Marine Laboratory used satellite data and numerical modelling data, including NASA’s Ocean Colour Web, which divides the global ocean into a series of 9km pixels. This allowed them to observe changes in the ocean surface for each of these pixels. An algorithm developed to measure light in seawater was then used to determine the depth of the photic zones at each location. In addition, the researchers used models of solar and lunar irradiance to see how changes in irradiance affected marine animals.

They found that as much as 21 percent of the global ocean surface became darker between 2003 and 2022, including large areas of coastal regions and the open ocean. Furthermore, over 9 percent of the ocean (an area of ​​32 million km ² , similar in size to Africa) the photic zone depth decreased by more than 50 m, and over 2.6 percent of the ocean it decreased by more than 100 m.

The question is, what is causing this phenomenon? According to scientists, there may be several causes. As for the darkening of waters off the coast, it is likely the result of the accumulation of nutrients, organic material, and sediments due to agricultural runoff and increased rainfall. In turn, the darkening in the open ocean may be caused by changes in the dynamics of algal blooms and changes in water temperatures.

Ocean darkening poses serious threat to marine life

While the exact impacts of these changes are not yet known, scientists say they could impact a huge number of marine species and ocean-based ecosystem services.

"Our results provide evidence that such changes are causing widespread darkening, which reduces the amount of ocean available to animals that rely on the Sun and Moon to survive and reproduce. We also rely on the ocean and its photic zones for the air we breathe, the fish we eat, our ability to combat climate change , and the overall health and well-being of the planet. Given all this, our findings are a real cause for concern," said Thomas Davies, one of the study's authors.

The phenomenon of ocean darkening could in practice have a huge impact on the entire marine ecosystem. Tim Smyth of Plymouth Marine Laboratory, the second author of the study, admitted that "if the photic zone shrinks by about 50 m over large areas of the ocean, animals that need light will be forced closer to the surface, where they will have to compete for food and other resources they need."

In which specific regions has ocean darkening been observed? The most pronounced changes in the depth of the photic zone in the open ocean have been recorded at the top of the Gulf Stream and around the Arctic and Antarctic, where the effects of climate change are greatest. In coastal regions and enclosed seas, darkening has been particularly evident in the Baltic Sea, the North Sea and Celtic Sea, the east coasts of England and Scotland, the coastlines of Wales, and the northern Irish Sea.

Fortunately, there is some good news. It turns out that over the past 20 years, about 10 percent of the ocean (an area of ​​over 37 million square kilometers) has become brighter. This includes much of the English Channel and regions stretching from northern Scotland to Orkney and Shetland.