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Mysterious glowing oceans, known as milky seas, have been a nautical enigma for centuries. For over 400 years, mariners have documented this striking phenomenon where the ocean surface emits a widespread, ethereal bioluminescence, stretching to the horizon. This captivating ocean glow is now the subject of intense scientific scrutiny, with researchers developing new tools to predict and study these elusive displays.
The Enigmatic Glow of Milky Seas
Eyewitness accounts from seafarers vividly describe the surreal nature of milky seas. J. Brunskill, a ship officer, recounted in 1967 sailing through the Arabian Sea and witnessing a “phosphorescent glow” that illuminated the sea “from horizon to horizon,” making it appear lighter than the night sky after moonset.
Similarly, in 1976, Captain P. W. Price of the MV Westmorland reported navigating through a “large area of bioluminescence” in the same region. He described the sea as “glaring a brilliant and bright green,” so intensely luminous that it obscured whitecaps and waves, creating the illusion of a perfectly flat, glowing expanse.
These “milky seas,” a term coined by sailors, have presented a significant challenge to scientists due to their unpredictable and infrequent appearance in remote oceanic areas, far from regular human observation.
Advancing Milky Seas Research
Driven to unravel the mysteries of this bioluminescent spectacle, scientists are making strides in predicting the occurrence of milky seas. Justin Hudson, a doctoral candidate at Colorado State University, has compiled a comprehensive database of over 400 documented milky sea sightings. This includes historical reports like those from Brunskill and Price. This extensive database, detailed in a study published in the journal Earth and Space Science, is intended to facilitate dispatching research vessels to these events.
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Hudson, the study’s lead author, expressed hope that this database will encourage and enable broader scientific investigation into milky seas, potentially solving a maritime mystery that has persisted for centuries.
He emphasized that further research into milky seas is crucial to understanding their underlying causes and their ecological significance within marine environments.
“Milky seas” might indicate either a thriving, healthy marine ecosystem or, conversely, an imbalanced one; currently, their implications remain unclear. Predictive capabilities will allow researchers to address fundamental questions about their role in the global interconnected Earth system.
The Role of Bioluminescent Bacteria
Descriptions of milky seas often compare their luminescence to glow-in-the-dark stars. Sailors have reported the glow to be sufficiently bright to permit reading, a stark contrast to the typical darkness of the ocean at night. These events can endure for extended periods, sometimes months, and span vast areas, reaching up to 100,000 square kilometers (approximately 39,000 square miles). Larger occurrences are even visible from space, as indicated by satellite observations in the study.
While the precise mechanism behind this unusual ocean glow is still under investigation, the prevailing scientific theory points to high concentrations of bioluminescent bacteria, specifically Vibrio harveyi. This hypothesis is largely based on a serendipitous encounter in 1985 when researchers aboard a vessel collected and analyzed a water sample during a milky sea event.
Dr. Steven Miller, a co-author of the study and professor at Colorado State University, highlights the remaining unknowns. Despite the 1985 sample, “the circumstances for how they form and how they set about causing the entire ocean to glow like that is still highly unknown.” Miller, who has dedicated decades to studying milky seas, also led a 2021 study demonstrating satellite detection of larger milky sea events.
The extensive reach of “milky seas” allows them to be observed from space, illustrated in this enhanced color satellite image of an event. – Steven Miller
Analysis of the compiled milky sea records, encompassing both historical eyewitness accounts from sailors and satellite data, revealed recurring patterns. The researchers observed that milky seas predominantly occur in the Arabian Sea and Southeast Asian waters. Furthermore, they may be influenced by large-scale climate patterns, such as the Indian Ocean Dipole and the El Niño Southern Oscillation, according to Hudson.
Regions prone to milky seas often experience ocean upwelling. Upwelling brings cooler, nutrient-rich water from deeper ocean layers to the surface due to powerful winds. Hudson estimates that these regions may witness approximately one milky sea event per year.
Hudson notes, “They are locales that are ideally suited for significant biological activity. However, numerous places on Earth share these characteristics. The precise factors that make these particular regions conducive to milky seas remain a significant and unanswered question.”
Ecological Implications of Milky Seas
Distinct from more common bioluminescent displays caused by dinoflagellates, which emit brief blue flashes when disturbed, milky seas exhibit a sustained, steady glow. Dinoflagellate bioluminescence is believed to be a defense mechanism. In contrast, the milky sea bacteria are theorized to glow to attract fish. This attraction could facilitate bacterial proliferation within the fish’s digestive system, according to Miller, who also directs the Cooperative Institute for Research in the Atmosphere (CIRA) at Colorado State University.
Like many scientists captivated by bioluminescence, Miller hopes to personally witness a milky sea event. Dr. Edith Widder, an oceanographer and marine biologist, shares this aspiration. She was not involved in the current study but is a leading expert in the field.
In an email, Widder stated, “I have dedicated my career to observing and quantifying bioluminescence in the ocean. I have observed remarkable light displays, but I have yet to see a milky sea. It is a strong desire of mine.” She further commented, “By assembling this database, the authors significantly advance our ability to predict the timing and location of milky sea occurrences.”
Widder, CEO and senior scientist at the Ocean Research & Conservation Association, also raises questions about the ecological impact of these bioluminescent events. Specifically, she wonders how milky seas affect marine life, particularly creatures inhabiting the dark depths during daylight and ascending to feed under the cover of night.
“Light is a fundamental determinant of animal distribution and behavior in the ocean. What are the ramifications for the daily patterns of predator and prey when normally hidden animals are illuminated by widespread bioluminescence? What are the consequences for the carbon cycle? This natural phenomenon represents an opportunity to gain substantial insights into the dynamics of ocean life,” she elaborated.
Previous attempts to create milky sea databases have unfortunately been lost over time. This new database “re-establishes a valuable baseline for our comprehension and awareness of where and when milky seas are occurring globally,” Miller stated.
Miller emphasizes the dramatic nature of these bioluminescent ocean events, noting, “The bacterial population exhibits an extraordinary response, one that was previously beyond our expectations.”
Key unanswered questions persist, including the influence of climate change on the frequency of these glowing events and the subsequent ecosystem impacts, he mentioned.
“It is imperative to understand the mechanisms driving this process… because bacteria and phytoplankton form the base of the oceanic food web. Numerous higher-order species and fish depend on this food chain. Alterations to this food chain, driven by shifts in our planet’s systems, are critical aspects we need to investigate.”