1. What Does a Blue Tide Look Like, and Why Isn't It Plankton?
Blue tides usually occur from late summer to autumn. Chiba Prefecture, which sits on the eastern shore of Tokyo Bay, describes the event as the sea surface changing to a "milky blue or milky white" [3]. A prefectural research institute in Hokkaido, the northern island of Japan, gives the color as milky white to bluish white [4]. A research paper on Tokyo Bay explains the name in plain terms: ao means blue and shio means tide [1].
A red tide is different. In a red tide, huge numbers of plankton color the water. In a blue tide, the color is not made by living things. It comes from chemical reactions in the water [4]. So the next question is what the milky stuff actually is.
2. What Are the White Particles in a Blue Tide Made Of?
They are made of sulfur. Seawater naturally holds a lot of sulfate, a form of sulfur that is dissolved and invisible. Where there is no oxygen, bacteria called sulfate-reducing bacteria use sulfate in a process similar to breathing and give off hydrogen sulfide, the gas that smells like rotten eggs [4][3].
That hydrogen sulfide can sit in the bottom water for a while. Then, if the water is pushed up toward the surface, the hydrogen sulfide meets oxygen and is oxidized into very small particles of sulfur, called colloidal sulfur [1][4]. The particles float instead of sinking. They scatter light strongly, and the surface looks milky white or bluish white [4]. Some of the hydrogen sulfide escapes into the air as well, which is why a blue tide can come with a rotten-egg smell [4].
One detail is still missing from the sources used here. They say the particles scatter light, and one Chiba report says the water scatters blue light and looks bluish white [2], but none of them explains in detail why the tint leans blue. The safe statement is the first half: tiny sulfur particles scatter light and make the water look milky.
3. Why Does the Bottom of a Bay Run Out of Oxygen in Summer?
Tokyo Bay is a good example. Rivers and cities send a lot of nutrients into it, and plankton grow in the bay itself. When that plankton and other organic matter die and sink, bacteria on the bottom break it down. Breaking it down uses up oxygen [2].
In summer, the warm, lighter water sits on top of the cooler water below, like layers that do not want to mix. This is called stratification. Air can supply oxygen to the surface layer, but the layering stops it from reaching the bottom, so the oxygen down there keeps falling [2][3]. A reading below 2 milligrams of dissolved oxygen per liter is usually treated as "hypoxic," meaning very low in oxygen [4].
The bay's water quality has improved in recent years, but the oxygen-poor patches have not gone away. Chiba Prefecture counted 1, 4, 6, 4 and 2 blue tide events in the fiscal years 2020 to 2024 (Japan's fiscal year starts in April), an average of 3.4 a year [3]. Low oxygen at the bottom is only half the story, though. The surface does not turn white until that water comes up.
4. What Pushes Oxygen-Free Bottom Water Up to the Surface?
Wind and water currents can [4]. A Chiba diagram explains one common case: when a northeast wind blows, surface water is carried away from the shore, and bottom water with little oxygen rises to take its place [3]. A research paper on the northeast shore of Tokyo Bay also links repeated northeast wind to this kind of upwelling, which can kill many shellfish and other animals [1]. Falling air temperature is listed as another trigger [2].
But the wind direction is not a simple rule. In October 2023, scientists measured the water off Urayasu, in Tokyo Bay, on the day before a blue tide was spotted. Near the surface the oxygen was still above 7 milligrams per liter. Below about 6 meters (20 feet), it was close to zero, and a measure called redox potential was negative from about 7 meters down, a sign of very little oxygen. No blue tide had formed yet. It was seen the next day, October 6, near the mouth of the Sakai River [2]. Weather records for those days show a northwest wind, not a northeast one [2].
So a calm, normal-looking sea can have a layer of oxygen-free water only a few meters beneath it. Color at the surface alone does not show what is below.
5. How Did Scientists Tell a Blue Tide from a Red Tide in Northern Japan?
Until 2025, blue tides had mostly been recorded in warm, closed bays such as Tokyo Bay and Osaka Bay. Hokkaido had seen them only in enclosed brackish lakes, not on the open sea coast [4].
That changed in September 2025, when a fishing cooperative and the town office of Samani, on Hokkaido's Pacific coast, reported that the water in Tojima fishing port was black and smelled of sulfur. Staff measured dissolved oxygen at the inner end of the harbor and found only 0.02 to 0.36 milligrams per liter at the surface, which is clearly hypoxic. On October 2, part of the inner harbor turned milky white [4].
At first the change was suspected to be a red tide, so water samples were analyzed in detail at the prefecture's fisheries research station. The research organization reported it as the first blue tide recorded on Hokkaido's Pacific coast [4]. It shows that the same sequence (water with almost no oxygen rises, then sulfur particles form) can happen outside the places where people expected it. It also shows why an observer cannot name a milky sea from color alone.
6. How Can You Follow Blue Tides Safely?
Stay away from the water. Hydrogen sulfide is harmful to living things at high concentrations, and blue tides sometimes kill fish and shellfish [4]. If you see strangely colored water or smell rotten eggs at the shore, watch from a safe distance and tell an adult.
You can learn more safely by looking at records. Chiba Prefecture publishes its blue tide dates. You can look up weather records for the same days and see whether the wind blew from the northeast, as the usual diagram suggests, or from some other direction, as in October 2023 [3][2].