El Niño hits the base of the ocean food chain. What is happening to plankton?

For most of us, El Niño is associated with the heatwaves that have been affecting Europe since the beginning of summer. However, NASA satellite observations also reveal significant changes beneath the ocean surface: along the equator in the Pacific, the amount of phytoplankton is declining significantly. The consequences of this phenomenon for marine life could be serious.

El Niño is already visible from space

The El Niño phenomenon began to dominate in June 2026 and, according to experts from NOAA’s Climate Prediction Center, will intensify through the end of the year. In its latest statement issued on 31 July, the World Meteorological Organization (WMO) even warned of an exceptionally strong episode forecast for the period from August to October, which could cause record temperature anomalies.

Let us recall that El Niño is the warm phase of the El Niño-Southern Oscillation (ENSO), which influences weather around the world and can cause flooding in desert regions, delayed monsoons and an increased frequency of tropical cyclones. Its arrival is signalled by a rise in sea surface temperatures in the equatorial Pacific and higher-than-usual water levels. These changes can be observed through satellite monitoring, which revealed another phenomenon: a clear reduction in the presence of chlorophyll-a in the equatorial belt of the Pacific Ocean.

Why is the Pacific losing its “fertilisation”?

Chlorophyll-a is a green pigment found in most phytoplankton. Its concentration in water is measured using the OCI (Ocean Color Instrument), installed on a specialised NASA satellite. A comparison of images from June 2025 and 2026 shows that the green belt along the equator has significantly diminished.

Experts from the Ocean Ecology Laboratory at NASA’s Goddard Space Flight Center explain that during El Niño, the easterly equatorial trade winds weaken and the warm surface layer of the ocean extends deeper. This inhibits the process of upwelling of cold, nutrient-rich water, which normally drives phytoplankton growth.

So far, the changes mainly affect the central Pacific, but scientists fear that further development of El Niño could cause them to spread.

Less phytoplankton, consequences for the entire food web

Why are observations of phytoplankton so important? Microscopic plant-like organisms are crucial to life in aquatic ecosystems. Diatoms, dinoflagellates, green algae and cyanobacteria provide food for zooplankton, which in turn forms the basis of the diet of many species of fish and whales. Fish are eaten by larger fish and marine mammals, while also constituting an important part of the human diet.

Disrupting the biological balance at the lowest level of the ocean food chain can cause serious disturbances in marine life. Fortunately, as NASA researchers explain, the effects will be temporary, because after El Niño subsides and the cool phase of La Niña arrives, chlorophyll-a concentrations usually increase, phytoplankton blooms occur and fish populations in the Pacific Ocean grow.

El Niño and ocean warming are not the same thing

The Copernicus Climate Change Service reported that the average sea surface temperature in non-polar regions was the highest on record last month, reaching 20.96°C and exceeding the previous record from July 2023. The development of El Niño had a significant influence on this record, but it was not the only cause.

As a natural climate phenomenon, El Niño does not generate new heat, but changes its distribution around the world. The ocean, however, is warming mainly because of human activity, more specifically the increasing concentration of greenhouse gases in the atmosphere. Estimates by a team of scientists led by the Institute of Atmospheric Physics of the Chinese Academy of Sciences indicate that in 2025, the world’s oceans absorbed an additional 23 zettajoules of energy. This is equivalent to the energy generated by approximately 12 Hiroshima-scale atomic bombs exploding every second of every day.

Meanwhile, most scientists agree that a warmer ocean may increase the likelihood of very strong El Niño episodes, drawing attention to a key problem: the global need to reduce the use of fossil fuels and halt deforestation around the world.

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main photo: Glen Turner/Pexels

Sources:

https://phys.org/news/2026-08-el-nio-marine-life-pacific.html

https://oceanographicmagazine.com/news/ocean-hits-highest-ever-recorded-temperature-in-july

https://wmo.int/media/news/strong-el-nino-expected-intensify

Pan, Y., Cheng, L., Abraham, J. et al. Ocean Heat Content Sets Another Record in 2025. Adv. Atmos. Sci. (2026). https://doi.org/10.1007/s00376-026-5876-0