Ocean darkening involves a reduction in the photic zone-the sunlit layer of the sea that supports 90% of marine life. Using satellite observations and numerical modeling, researchers tracked changes in photic zone depth between 2003 and 2022, revealing that 21% of the global ocean has darkened. In some areas, photic zones shrank by more than 50 meters, affecting over 32 million square kilometers-an area comparable to Africa. About 2.6% saw declines exceeding 100 meters.
However, it's not a uniform trend. Roughly 10% of the ocean, or 37 million square kilometers, became lighter during the same period. Still, the overarching changes pose potential risks to marine ecosystems and their services, such as oxygen production and climate regulation.
While the research builds on more than a decade of studies into artificial light at night, scientists stress that ALAN is not a primary cause here. Instead, nearshore darkening is linked to sediment, organic matter, and nutrients from sources like agriculture and increased rainfall. In contrast, open ocean shifts are attributed to changes in algal blooms and sea surface temperatures that reduce water clarity.
"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. 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. 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," said Dr Thomas Davies, Associate Professor of Marine Conservation.
Professor Tim Smyth of Plymouth Marine Laboratory added: "The ocean is far more dynamic than it is often given credit for. For example, we know the light levels within the water column vary massively over any 24-hour period, and animals whose behaviour is directly influenced by light are far more sensitive to its processes and change. 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. That could bring about fundamental changes in the entire marine ecosystem."
To monitor these developments, the team used NASA's Ocean Colour Web data, analyzing ocean surface changes in 9km-wide pixels. An algorithm assessed light penetration, allowing them to map photic zone depth globally. Solar and lunar irradiance models helped gauge variations in daytime and nighttime lighting, with night changes proving smaller but still ecologically meaningful.
Notable darkening occurred in areas like the Arctic, Antarctic, and Gulf Stream-regions already affected by climate change. Coastal waters and enclosed seas such as the Baltic were similarly impacted due to land-based runoff fostering plankton growth that blocks light.
Research Report:Darkening of the Global Ocean
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