Хар тэнгисийн өнгө өөрчлөгдөж буй нь нүүрстөрөгчийн эргэлтэд нөлөөлж байна

Each spring, the Black Sea shifts from deep indigo to a milky, swirling turquoise. The color comes from coccolithophores, microscopic single-celled algae coated in tiny calcium carbonate plates that scatter sunlight in ways that dye entire ocean surfaces blue-green.

On June 22, 2026, NASA’s PACE satellite captured the sea mid-bloom, its waters lit up from above by the OCI (Ocean Color Instrument). A separate photograph, taken on May 27 by an astronaut aboard the International Space Station, showed the same effect flowing through the Bosphorus strait in Istanbul, with turquoise plumes tracing currents on both sides of the narrow waterway connecting the Black Sea to the Sea of Marmara.

What makes these images more than visually striking is a biological and chemical question that researchers have been tracking for decades: what controls when these blooms begin, how large they grow, and when they end? A study published in the journal Diversity by scientists from the Shirshov Institute of Oceanology found that in 2022 and 2023, coccolithophore blooms in the northeastern Black Sea lasted far longer than usual, persisting through July instead of fading by mid-June as they typically do.

The researchers identified a specific combination of wind conditions, water temperature structure, and nutrient imbalance that kept the blooms alive through summer and pushed the next phase of the seasonal cycle back by roughly two months.

Calm Winds and Unusual Chemistry Extend the Bloom Into Summer

The Black Sea normally follows a well-documented seasonal rhythm. In late spring, coccolithophores dominate the surface. By early summer, nitrogen levels drop, the water becomes less stable, and large diatoms, a different category of microscopic algae with silica shells, take over. Diatom blooms tend to darken water rather than brighten it, which is why the turquoise color fades as summer advances. In 2022 and 2023, that transition stalled.

According to the Diversity study, the primary driver was a prolonged absence of strong winds. In most years, wind-driven mixing disrupts the thin, stable layer near the surface known as the thermocline, a zone where water temperature drops sharply with depth. When the thermocline deepens or breaks apart, coccolithophores lose the calm, shallow, well-lit conditions they need to thrive.

In 2022, the thermocline held at depths of just three to eight meters through much of June and July, well shallower than the long-term average, giving coccolithophores an unusually long window of favorable conditions. In 2023, the period of minimal winds stretched from April through July, sustaining four months of carbonate pump activity.

Nutrient chemistry also played a role. Coccolithophores outcompete diatoms when nitrogen concentrations are low and phosphorus concentrations are relatively high, because coccolithophores need much less nitrogen to sustain growth. In July 2022, two bouts of heavy rainfall, with individual events dropping more than 30 millimeters in a single day, flushed additional phosphorus into the coastal waters, further extending conditions that favored Gephyrocapsa huxleyi, the dominant coccolithophore species in the Black Sea. Cell counts peaked at over nine million cells per liter in June of that year.

What Coccolithophore Blooms Mean for Carbon

The turquoise color is visible from orbit because coccolithophores become so densely packed during a bloom that the collective optical signature of their calcium carbonate shells changes the color of the sea surface. That density also matters for ocean chemistry. Coccolithophores form part of what oceanographers call the biological carbon pump, the collective set of processes by which marine organisms capture carbon dioxide from the atmosphere and transport it into the deeper ocean.

The specific mechanism coccolithophores drive is called the carbonate pump. When these organisms build their calcium carbonate plates, they use dissolved carbon from seawater, which in turn draws down atmospheric CO2. When they die, a portion of that bound carbon sinks to the seafloor, where it can remain stored for long periods rather than returning to the atmosphere. NASA’s Earth Observatory has documented this process in the Black Sea repeatedly, noting that coccolithophore blooms contribute to the ocean’s carbon cycle through this gravitational export of carbon-bearing material.

The catch is that coccolithophore calcification also releases CO2 as a byproduct, which increases the partial pressure of carbon dioxide in surface waters and slows the rate at which the ocean absorbs atmospheric CO2. This makes the carbonate pump different in character from the organic pump operated by diatoms, which capture carbon through photosynthesis without the same release effect.

When the carbonate pump dominates for an extended period, as it did in the northeastern Black Sea in 2022 and 2023, it affects how efficiently the ocean draws down atmospheric carbon overall.

The Black Sea as a Testing Ground for Carbon Pump Shifts

Researchers have long used the Black Sea as a model system for studying how the biological carbon pump responds to environmental change. Its relatively contained geography, predictable seasonal cycle, and long observational record make it well suited for tracking shifts that might be harder to isolate in open ocean settings.

The 25-year dataset analyzed in the Diversity study shows that coccolithophore blooms lasting through July were recorded only twice in that period: 2022 and 2023. The authors are careful not to classify this as an established trend, describing the events as anomalies while noting that the mechanisms driving them are sensitive to the kinds of meteorological changes associated with a warming climate.

Two competing scenarios emerge from that reasoning. If rising temperatures increase surface water stratification, the shallow, stable thermocline that coccolithophores prefer becomes more common, which would favor carbonate pump dominance and reduce the ocean’s ability to absorb CO2. If climate change instead intensifies storm activity and wind-driven mixing, deeper thermoclines would promote diatom growth and strengthen the organic pump.

The study’s authors note that distinguishing between these trajectories requires extended monitoring, because stochastic factors like individual storm events and rainfall episodes can mask longer-term patterns in records as short as a few years.

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