Recently, scientists have turned to cutting-edge satellite technology named Day/Night Band (DNB) in order to investigate the intriguing phenomena of glow-in-the-dark milky seas, also known as mareel. This rare marine bioluminescence causes the nocturnal ocean surface to emit a steady, uniform, and expansive whitish glow.
The phenomenon of milky seas is a fairly uncommon occurrence, presenting itself only about two or three times annually across the globe. The phenomenon is primarily observed in the waters of the northwest Indian Ocean and off the coast of Indonesia and can sometimes stretch over an area exceeding 1,00,000 km2.
Understanding Bioluminescence and Milky Seas
Bioluminescence is light produced by a chemical reaction within a living organism. In milky seas, this bioluminescence arises from a saprophytic relationship between luminous bacteria and microalgae, happening on a macroscale. A strain of bacteria known as Vibrio harveyi, capable of producing light, colonizes the algae at the water’s surface.
When it comes to milky seas, these occurrences are not short-lived. Instead, they can persist for days to weeks, meandering within stagnant sea areas amid prevailing sea-surface currents. These phenoma seem to align with narrow ranges of sea-surface temperature and marine biomass, suggesting water mass isolation.
The Role of the Indian Ocean Dipole
The Indian Ocean Dipole (IOD), a significant climatic event, plays a role in the creation of milky seas. During the positive phase, the IOD leads to warm and wet conditions featuring warm pooling waters on the western side of the Indian Ocean. Conversely, cool and dry conditions with strong easterly winds prevail on the eastern side.
These easterly winds generate an upwelling of cool, nutrient-rich coastal waters, which drift offshore with the currents. This action triggers widespread algal blooms, and potentially creates conditions that are favourable for the genesis of milky seas.
Purpose of Bioluminescence
Luminous bacteria cause the particles they colonize to emit light. It is believed that the purpose of this bioluminescence could be to attract fish for consumption. These bacteria thrive within the gut of fishes, making a rich food supply. When the bacterial population becomes too large for the algae, the stomach of a fish makes a great secondary option.
Detection of Milky Seas
The awareness of milky seas primarily comes from reports by mariners concentrated in major shipping lanes. In 1995, satellite measurements provided the first overview of a milky sea off the Somalia coast.
Key instruments used for detection include the Operational Linescan System (OLS), carried by the US’ Defense Meteorological Satellite Program, and the Day/Night Band (DNB) instrument. These tools are capable of detecting very faint light sources, including the emissions from bioluminescent organisms. However, these instruments do have limitations, particularly in detecting more common bioluminescent events associated with disturbed water due to their typically small extent.
Limitations of Detection Instruments
Despite their usefulness, both OLS and the DNB are not devoid of limitations when it comes to detecting milky seas. The OLS, for instance, cannot detect the more common bioluminescence events associated with disturbed water. The DNB’s spectral response is also sensitive to mesospheric airglow emissions, which can interfere with the detection of milky seas. Atmospheric gravity waves can modulate the intensity of light and form patterns of brightness that mimic those expected from milky seas.