According to a study published in May 2025 and conducted by the marine scientists, more than 20 per cent of the ocean, i.e., over 75 million square kilometres, across the globe has darkened over a period of past 20 years. That is, there has been a significant reduction in the photic zones of the oceans compared to what they were earlier.
This research was carried out by the marine biologist, Thomas Davies, who works at the University of Plymouth, along with the marine biogeochemist, Tim Smyth, who works at the University of Exeter. They asserted that though scientists are aware of ocean darkening and its consequences, the real extent of ocean darkening still needs to be measured.
Understanding Ocean Darkening
Ocean darkening implies that the photic zones of the ocean are contracting. This prevents the sunlight from entering deeper into the ocean, which, in turn, will impact the ecosystems found in the ocean that rely on sunlit waters as well as the process of photosynthesis that is carried out inside the ocean.
What Photic Zone is
The photic zone includes the sunlit waters of the ocean where 90 per cent of the marine life lives and thrives. In other words, this zone encompasses the waters in the upper surface of the ocean that receives the sunlight. The marine life inhabiting this zone functions with the sunlight and moonlight, thereby contributing to the ecological balance on Earth.
The shallower parts of this zone mainly include primary producers like kelps, seagrasses, and phytoplankton that carry out the process of photosynthesis by using Sun’s energy. Other marine creatures like fish, crustaceans, corals, and marine mammals also occupy this zone, as they survive by feeding on primary producers.
Reduction in Photic Zones According to the scientists, the extent of the photic zone is up to 200 metres beneath the surface of the ocean. In this region of the ocean waters, infinite marine ecosystems are present.
However, there are some distressing changes in the photic zone as per the research. The research states that there is a drastic reduction in the critical photic zones of the oceans around the world. This ocean darkening covers an area greater than that of South America. Due to the shrinkage in the vital marine light across all the oceans of the world, the marine life inhabiting the oceans has been endangered.
Findings of the Research
According to Davies, earlier research showed that the colour of the ocean surface has been changed over the past two decades, probably due to altered plankton communities. It became evident in the research conducted by Davies and Smyth that such changes were responsible for ocean darkening on a large scale. As a result, the marine animals that survive and reproduce with the help of sunlight and moonlight entering the ocean surface have limited surface of ocean. Now, they have to compete for resources available to them.
When Davies and Smyth combined numerical modelling and satellite data from the Ocean Colour Web of the National Aeronautics and Space Administration (NASA), for the period between 2003 and 2022, a certain pattern of the depth of photic zone was revealed. This pattern shows that a large number of marine creatures have limited area as their habitats. These habitats are continually getting smaller and smaller day by day.
The analysis of this data also showed the depth of the ocean waters where the sunlight reached. As per the research, dramatic reductions were seen in the depths of the photic zones. It means in around 9 per cent of total waters of the world’s oceans, the depth of the photic zone shrank over 50 metres. To our utmost surprise, the depth of photic zone in 2.6 per cent of total ocean waters has reduced by more than 100 metres.
According to Smyth, there is huge variation in the levels of light within the ocean waters over a period of 24 hours. The processes and changes in the light affect those marine animals the most that rely on the light. This shows how dynamic our ocean is.
Reasons behind the Darkening of Ocean
When sediment, plankton, organic matter, and other dissolved substances come in the path of the sunlight falling on the ocean waters, it leads to ocean darkening. These substances dim down the sunlight moving underwater by being scattered all over the surface of ocean waters and absorbing the light. As per the researchers, heavy downpour and agricultural runoff are the main causes of darkening of waters near coastal regions. Consequently, a large amount of sediment and nutrients get deposited into waters near coasts.
In the regions with open ocean waters, the main causes of darkening are surface temperatures of the sea, alteration in ocean currents, and movement of algae blooms here and there. These regions include parts of the Antarctic Ocean, the Arctic Ocean, and parts affected by the Gulf Stream. These regions, being subject to climate change, become more vulnerable to darkening of their waters.
Consequences of Ocean Darkening
The patterns of natural light given out by Sun or reflected by Moon are useful to the marine life in many ways. Phytoplankton, the primary producer, can perform photosynthesis only in the presence of sunlight. It forms the basis of ocean food webs. Other marine animals need sunlight or moonlight for their significant behaviours, like spawning, hunting, and migration.
One of the noticeable migrations is diel vertical migration. It is the largest migration event that takes place inside the ocean every day. At the nightfall, billions of marine creatures and their many species migrate upwards to the surface of the ocean waters to feed on abundant plankton and also to hide from predators in the dark. However, as the sun rises, they descend down deep into the dark ocean waters to hide from predators. This migration is significant for maintaining the ecology of the world’s ocean. With limited photic zone, these tiny organisms have to adjust in cramped spaces and fight for the limited food available. This might also threaten their habitat.
Further, the spawning event of coral reefs and other marine creatures could be disturbed due to the low light in the ocean waters. It is a vital process in which these marine creatures breed, guided by the moonlight. But in the low moonlight or in its absence, these creatures could not reproduce properly, and there is a decline in their population.
Marine creatures that depend on light conditions within the ocean waters have to assemble near the surface of the ocean waters in the condition of reduced photic zones. As a result, they compete intensely for food and survival, which may transform marine ecosystems considerably.
Apart from impacting individual marine species, the photic zones play a vital role in ocean productivity and carbon cycling across the globe, which maintain the climate on Earth. Shrinking photic zone may affect the productivity of the primary producers, which may be insufficient for the fish. With low fish produce, humans and other animals that feed on fish may be affected.
Further, reduced photic zone may impact ocean chemistry, which may alter levels of oxygen and nutrient cycles. This may damage the marine ecosystems, endanger biodiversity, and disrupt global ecosystems.
Contrary to this, the recent study also revealed that some parts of global ocean waters have been brightened. That is, it was noted that around 10 per cent of the world’s oceans, viz., the region spanning 37 million square km, has become lighter.
Conclusion
To conclude, the researchers asserted that ocean monitoring should be a regular activity. Besides, people across the globe should be made aware of this phenomenon so that appropriate and timely measures can be taken. By tackling climate change, regulating sediment and nutrient runoff, and improving monitoring of the ocean, as well as gaining global cooperation, we can deal with ocean darkening, thereby conserving marine ecosystems.
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