Climate change is not only causing irreversible damage to the green cover, they are equally impacting marine life. A new study seems to suggest that more than one-fifth of the global ocean has considerably darkened in the last two decades. This alarming state of the oceans has been brought to light by the study, Darkening of the Global Ocean, conducted by researchers from University of Plymouth.
The study by Dr Thomas Davies, Professor Tim Smyth, and team, suggest that ocean darkening at this scale is not only the latest ecological crisis, but one that comes with grave implications for marine life and overall planetary health.
What is ocean darkening?
Darkening of the ocean is essentially shrinking of the photic zones or those layers of water where sunlight can pass and induce the process of photosynthesis which is key to all biological processes. The photic layers can go down to about 200 meters and they also act as a base for nearly 90 per cent of the world’s marine life. This layer is responsible for increasing the productivity of the ocean which also involves regulating climate and even supporting global fisheries or related activities.
As part of the study, Davies and Smyth used satellite data along with sophisticated modelling techniques to analyse changes in how the oceans have absorbed light in the last two decades. The duo tracked the changes using the diffuse attenuation coefficient (Kd 490), a measure of how rapidly light fades as it passes through seawater.
The finding of the study is critical as it indicates that between 2003 and 2022, nearly 21 per cent of the global ocean experienced darkening. While nine per cent (equivalent to the area of the continent of Africa) saw a decline in photic zone depth greater than 50 metres, 2.6 per cent saw reduction of over 100 metres.
Mixed global picture
Even though the study states massive loss of light, around the same two decade-span, a considerable part of the global ocean has actually become lighter. This shows that the predictability of ocean darkening is not even across all waterways. The North Sea, the eastern UK coastline, and the Arctic have lost more light than any other regions, while there have even been small areas (e.g. regions of the English Channel) that have had more light. These patterns are the result of varying environmental conditions in different regions which includes varied rainfall, land use, and ocean currents.
However, the most prominent darkening was observed in the open ocean, particularly in climate-sensitive zones like the Arctic, Antarctic, and Gulf Stream region. Coastal areas like the Baltic Sea have also lost photic depth because of erosion of sediments and nutrients from lands.
Why is this happening?
In coastal zones, darkening is usually due to higher runoff of agricultural nutrients, organic matter, and sediments into the ocean with rain which directly promotes algal blooms that block light. In the open ocean, the likely causes are changes in plankton dynamics, increase in sea surface temperature and changes in ocean circulation.
“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,” Davies explained.
“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”, he added.
Marine life in the shadows
The contraction of photic zones may lead to fundamental shifts in marine ecosystems. Those species that use sunlight and moonlight cues to feed, move, hide, and reproduce will be competing for shallower zones. This might upset marine food webs that are already working hard against an ecosystem with minimal fishing activity.
Smyth, who is the head of Science for Marine Biogeochemistry and Observations at the Plymouth Marine Laboratory, highlighted the ecological danger. “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 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,” Smyth said.
The indicator species used in this experiment was Calanus copepods, a highly photosensitive zooplankton. These animals are at the center of the marine food chain and use very faint light cues from the Sun and Moon for vertical migrations during the day and various other behaviours.
Underwater habitat crisis
The study infers ocean darkening as maybe one of the biggest global habitat loss cases ever in recent memory. Animals depending on light are being constrained to narrow vertical spaces, increasing predation and greater competition for resources. In due course, this may kill biodiversity, disrupt oceanic carbon cycling, oxygen production, and ocean buffering against climate change.
(This article has been curated by Prachi Mishra, who is an intern with The Indian Express)
A map focused on the UK and North Atlantic Ocean showing changes identified through this project. While the red markings show areas darkening, blue shows areas lightening. (Image: University of Plymouth) 
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