El Niño is currently warming part of the equatorial Pacific. This change affects more than just water temperature: it can also reduce phytoplankton, the microscopic organisms that feed much of marine life.
Phytoplankton consists of algae and other tiny organisms capable of using sunlight. It is the primary food source for many small marine animals. These are then eaten by fish, which are themselves consumed by larger predators.
The green color represents the concentration of phytoplankton in the Pacific, which has fallen sharply over one year.
NASA illustration
It all begins with the winds that usually blow from east to west near the equator. They push some of the warm surface water toward the western Pacific. Colder water then rises from the depths in the eastern part of the basin. It carries nutrients that phytoplankton need to grow.
During El Niño, these winds weaken. Warm water spreads farther east and forms a thicker layer near the surface. This layer slows the upwelling of cold, nutrient-rich water. Phytoplankton then has fewer resources available to multiply.
NASA has tracked this phenomenon for several decades using satellites. They do not see every organism. Instead, they measure the color of the ocean and the amount of chlorophyll present near the surface. This pigment, which is used for photosynthesis, serves as an indicator of phytoplankton abundance.
During previous El Niño events, satellites recorded a clear drop in chlorophyll in the equatorial Pacific. This decline can then affect the entire food chain. Less phytoplankton means less food for zooplankton, potentially followed by fewer resources for some fish, birds, and marine mammals.
The 2026 El Niño event was officially declared in June. Satellite observations showed that it was continuing to strengthen in the central and eastern Pacific. The question now is how far this warming will alter the upwelling of deep water, and then the amount of phytoplankton available over the coming months.