Is the ocean getting darker?
New research found 21% of the global ocean had experienced a reduction in the depth of its lit zones, which are home to 90% of all marine life, during the past 20 years
image:
A world map showing changes in global photic zones between 2003 and 2022. Reds indicate regions where the oceans are getting darker, while blues indicate regions where oceans are getting lighter and white indicates regions where there was no statistically significant change over the period.
view moreCredit: University of Plymouth
More than one-fifth of the global ocean – an area spanning more than 75million sq km – has been the subject of ocean darkening over the past two decades, according to new research.
Ocean darkening occurs when changes in the optical properties of the ocean reduce the depth of its photic zones, home to 90% of all marine life and places where sunlight and moonlight drive ecological interactions.
For the new study, published in Global Change Biology, researchers used a combination of satellite data and numerical modelling to analyse annual changes in the depth of photic zones all over the planet.
They found that between 2003 and 2022, 21% of the global ocean – including large expanses of both coastal regions and the open ocean – had become darker.
In addition to this, more than 9% of the ocean – an area of more than 32million sq km, similar in size to the continent of Africa – had seen photic zone depths reducing by more than 50metres, while 2.6% saw the photic zone reduced by more than 100m.
However, the picture is not solely of a darkening ocean with around 10% of the ocean – more than 37million sq km – becoming lighter over the past 20 years.
While the precise implications of the changes are not wholly clear, the researchers say it could affect huge numbers of the planet’s marine species and the ecosystem services provided by the ocean as a whole.
The study was conducted by researchers from the University of Plymouth and Plymouth Marine Laboratory, who have spent more than a decade examining the impact of artificial light at night (ALAN) on the world’s coasts and oceans.
They say that is not directly connected to ocean darkening, however, with the changes likely being as a result of a combination of nutrient, organic material and sediment loading near the coasts, caused by factors such as agricultural runoff and increased rainfall.
In the open ocean, they believe it will be down to factors such as changes in algal bloom dynamics and shifts in sea surface temperatures, which have reduced light penetration into surface waters.
Dr Thomas Davies, Associate Professor of Marine Conservation at the University of Plymouth, said: “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.”
Professor Tim Smyth, Head of Science for Marine Biogeochemistry and Observations at the 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.”
Assessing changes in the ocean’s photic zones
To assess changes in the photic zone, the researchers used data from NASA’s Ocean Colour Web, which breaks the global ocean down into a series of 9km pixels.
This satellite derived data enabled them to observe changes on the ocean surface for each of these pixels, while an algorithm developed to measure light in sea water was used to define the depth of the photic zone in each location.
They also used solar and lunar irradiance models to examine particular changes that might impact marine species during daylight and moonlight conditions, demonstrating that changes in photic zone depth at night were small compared to daytime, but remained ecologically important.
A shifting global picture of ocean change
The most prominent changes in photic zone depth in the open ocean were observed at the top of the Gulf Stream, and around both the Arctic and Antarctic, areas of the planet experiencing the most pronounced shifts as a result of climate change.
Darkening is also widespread in coastal regions and enclosed seas – such as the Baltic Sea – where rainfall on land brings sediment and nutrients into the sea, stimulating plankton growth and reducing light availability.
Journal
Global Change Biology
Method of Research
Imaging analysis
Subject of Research
Not applicable
Article Title
Darkening of the Global Ocean
Article Publication Date
27-May-2025
Ocean Shot Award propels discovery through innovation
image:
Bigelow Laboratory Senior Reserach Scientist John Burns, who is leading the team that just received a significant award to advance new technology for ocean discovery, fixes a device on the deep-sea submersible during the previous expedition to test and refine the technology.
view moreCredit: Credit: Brennan Phillips, University of Rhode Island
A multidisciplinary team led by scientists at Bigelow Laboratory for Ocean Sciences have received funding that will expand the possibilities for discovering and describing exotic, even gelatinous, animals in the deep ocean with the next generation of ocean exploration technology.
The team represents six different institutions and will receive $2.2 million, over three years, from the Ocean Shot Research Grant Program, an initiative to encourage bold research in ocean discovery and technology through the Sasakawa Peace Foundation’s Ocean Policy Research Institute, supported by the Nippon Foundation. With the funding, they’ll continue to advance an innovative approach using cutting-edge imaging technology and underwater robotics to produce high-resolution 3D images, preserve tissue, and generate reference genomic data for new marine species. The effort will transform how scientists understand the ocean’s vast — and understudied — midwater region.
“This funding call is really visionary for taking a leap for the sake of innovation and discovery,” said Bigelow Laboratory Senior Research Scientist John Burns, the project’s principal investigator. “This opportunity will allow us to discover new species and examine existing ones in new ways in a part of the ocean that’s been largely inaccessible to scientists.”
Burns will be attending the One Ocean Science Congress, a special event of the UN Ocean Conference, in Nice, France, this June to meet with the foundation and help kick off this exciting project.
In addition to Bigelow Laboratory, the team includes experts from Harvard University, the University of Rhode Island, Baruch College, the Monterey Bay Aquarium Research Institute (MBARI), and the Japan Agency for Marine-Earth Science and Technology.
The researchers represent diverse expertise in underwater imaging, robotics, taxonomy, and genetic sequencing. Together, they’ve created new tools for observing and describing even the most fragile marine animals in their natural environment, which has the potential to speed up the process for classifying new species and provides previously impossible levels of detail on an animal’s genetics, behavior, and structure.
Last year, the team described their multi-prong approach in the journal Science Advances. That study, part of the “Designing the Future” project funded by Schmidt Ocean Institute, included the first applications of the method to four animals collected during an expedition off the coast of San Diego in 2021.
“Testing this remarkable suite of technologies on board our research vessel was an exciting peek into a future where we will be able to identify marine species and understand their behavior in their natural habitat,” said Schmidt Ocean Institute Executive Director Jyotika Virmani. “This will revolutionize the way in which scientists study and understand the ocean’s inhabitants.”
With this new, substantial support, the team will transform the imaging and robotics technology, and the genetic methods, described in that original study.
On the robotics side, the team intends to reimagine their novel, origami-inspired robotic encapsulation device by outfitting it with a biopsy tool, modeled on the biology of the mantis shrimp, that can non-destructively sample gelatinous organisms floating in the water. The ultimate hope is to enable “catch and release sampling,” where they can enclose an animal in an underwater chamber and use various sensors and devices to collect tissue samples for genetic analysis before releasing the animal unharmed.
Likewise, the team is building out software to process the complex data produced by their cutting-edge imaging systems. The goal is to develop new algorithms for processing multidimensional imaging data and AI tools to automatically identify the animals they image. They will also deploy a new shadowgraph system that essentially takes pictures with shadows to capture the internal structure of the animals, which will help with taxonomic classification of new species.
The researchers are working to secure opportunities to take their new methods into the field on several expeditions to test them and collect additional samples from new areas of the ocean.
“We’re planning to go to regions of the South Atlantic and Southern oceans that people haven’t looked at closely, especially in the midwater environment, for these kinds of animals,” Burns said. “We’re almost guaranteed to find lots of new species because these areas are so understudied.”
During those research cruises, they plan to collaborate with another large, interdisciplinary team, led by researchers in Australia, Japan, and the United States. That group was also recently funded by Ocean Shot to develop new taxonomic methods for studying and classifying unknown midwater species.
“Their team is focused on novel studies of morphology and behavior of animals on the ship, while we’re trying to create these digital recreations underwater,” Burns said. “By tackling these questions from different angles, we’ll be stronger together and be able to really understand the biological diversity of this environment.”
That kind of creative, cross-border collaboration reflects the goals of the Ocean Shot Research Grant Program, which provides funding for nonprofit institutions who are pushing the frontier of discovery in the ocean.
“Ocean Shot’s approach is to get the best people together and see what we can innovate, learn, and discover in places that haven’t been well studied,” Burns said. “It’s very exciting to be a part of.”
A composite image of the kinds of gelatinous, fragile deep-sea animals that the new approach will enable scientists to more easily and comprehensively study.
Credit
Schmidt Ocean Institute
The ROV SuBastian aboard the Schmidt Ocean Institute research vessel, Falkor, is outfitted with the dodecahedron, an origami-inspired chamber in the bottom right, which will be used to encapsulate and sample animals.
Credit
Brennan Phillips, University of Rhode Island
Sensing color cues to monitor coral health in the Red Sea
King Abdullah University of Science & Technology (KAUST)
Coral reefs form a vital part of the marine ecosystem, playing host to diverse species and supporting multiple industries, including fisheries, tourism, and recreation. However, these fragile ecosystems are under increasing threat from climate change, with warming oceans increasing stress on the coral animals and their symbiotic algal partners.
A new remote sensing tool developed by KAUST researchers has created an effective and efficient method of monitoring and predicting both the scope and severity of coral bleaching in the Red Sea[1]. The tool – developed by KAUST in partnership with SHAMS, General Organization for the Conservation of Coral Reefs and Turtles in the Red Sea – could aid conservation management and policymaking by enabling targeted, integrated management strategies to prioritize specific areas for intervention. It is applicable in the Red Sea as well as across the world.
The algae living within corals share nutrients and resources, giving corals their distinctive color. When corals are under stress and competing for limited resources, they ‘kick out’ their algal partners that help with nutrition. This results in bleaching, where corals lose their pigmentation and gradually turn white. This process weakens the coral animal and leaves it more vulnerable: prolonged bleaching events can kill corals and decimate reefs.
“Monitoring the health of coral reefs amid climate change is crucial, and satellite remote sensing provides a cost-effective strategy that is more efficient than traditional field sampling, which can be time-consuming and resource-intensive,” says Elamurugu Rajadurai Pandian at KAUST, who worked on the project during his Ph.D., under the supervision of KAUST’s, Ibrahim Hoteit.
The new tool utilizes the extensive datasets collected by satellite imaging every five days. While previous studies have used satellite imaging to monitor coral bleaching events, the team took this technique a step further by including an analysis of the severity of bleaching. This means that areas can be rapidly graded according to how intense the bleaching is likely to become.
“Healthy and bleached corals reflect light differently, and satellites can detect these variations,” says Rajadurai Pandian. The researchers took advantage of these differences in color and brightness in thousands of satellite images to accurately identify bleached corals.
Firstly, they analyzed how much light was reflected from the ocean floor by both healthy and bleached corals. Then, they used a mathematical technique called the least-squares approach. This helped identify patterns in the data and accurately segregated bleached corals from healthy ones, making the overall detection rate more precise and reliable.
“The detection accuracy depends on the atmospheric correction of satellite imagery, which is complicated by the Red Sea’s proximity to deserts and frequent dust storms,” says Rajadurai Pandian. “We used an advanced algorithm that removed erroneous reflectance values caused by aerosols and particulates. This significantly improved the accuracy of satellite ocean color data retrievals in our model, particularly over the complex, reef-filled shallow waters of the Red Sea.”
Their model also has improved spatial resolution compared to previous models, providing detailed analyses of coral health every ten meters. By monitoring bleaching severity, ranging from low to high, scientists can gain deeper insights into coral resilience and recovery potential.
“By serving as an early warning system for coral bleaching, our method will enable faster responses and better conservation strategies,” concludes Hoteit. “Such high-resolution monitoring will support sustainable fisheries and tourism management while also contributing to climate change research by tracking environmental changes in marine ecosystems.”
Reference
- Gokul, E.A., Raitsos, D.E., Brewin, R.J.W., Carvalho, S., Asfahani, K. & Hoteit, I. Remotely sensing coral bleaching in the Red Sea. Remote Sensing in Ecology and Conservation, early online 26 February (2025).| article
Journal
Remote Sensing in Ecology and Conservation
