El Niño reduces Pacific phytoplankton
NASA satellite data shows a major drop in chlorophyll across the Central Pacific in June 2025, indicating a decline in phytoplankton due to the ongoing El
A strengthening El Niño is causing significant disruption to the marine food web in the equatorial Pacific. NASA's Earth Observatory reports satellite measurements from June 2025 show a substantial drop in surface chlorophyll concentrations across the Central Pacific compared to neutral conditions.
Chlorophyll is the pigment that enables phytoplankton to generate energy from sunlight. Its reduction signals a decline in these microscopic plants, which form the foundational layer of the ocean's food web.
Disruption of nutrient supply
El Niño conditions weaken the easterly equatorial trade winds. Matthew Kehrli and Graham Trolley, oceanographers at NASA's Goddard Space Flight Center, explain this causes the warm surface layer of the ocean to extend deeper, suppressing the upwelling of cold, nutrient-rich water from the depths.
Hyung-Gyu Lim, an associate professor of oceanography at Chonnam National University in South Korea, confirmed the link. "This may trigger less nutrient supply. It is not surprising for me," Lim told Mongabay. He noted that a developing El Niño can slow the upwelling process.
The NASA Earth Observatory states that as El Niño progresses, chlorophyll concentrations are likely to decrease further over a broader region.
Ecological and fishery impacts
The cascading effect of fewer nutrients means less food for zooplankton. This shortage then impacts fish, seabirds, and marine mammals higher up the food chain. The consequences for commercial fisheries are direct and severe.
Peru's anchovy fisheries, for example, historically see significant catch declines during El Niño events. This is driven by the combination of warmer surface waters, reduced upwelling, and lower phytoplankton levels. This year, the country has repeatedly suspended its vital anchovy fishery to protect vulnerable stocks.
The ecological stress is visible on shore. Starving pelicans have begun invading urban ports in search of food.
NOAA's Climate Prediction Center estimates a greater than 90 percent chance the current El Niño will persist into early 2027.
Recovery and observation challenges
Despite severe immediate disruptions, oceans often experience a "chlorophyll rebound" once an El Niño subsides. Research led by Hyung-Gyu Lim points to key drivers for this recovery.
His studies suggest higher iron concentrations, supplied by ocean currents and dust blown from dry land in parts of Central and South America, help fuel the resurgence of chlorophyll. "Oceanic drivers facilitate a post-El Niño chlorophyll rebound fueled by subsurface iron supply," Lim said. He added that atmospheric dust-iron deposition can significantly modulate marine productivity.
Lim cautions that surface satellite maps only show part of the ecological picture. To truly capture the dynamics, he argues advanced observation systems must integrate both surface and subsurface data from the oceans.
