During normal conditions, strong trade winds blow from east to west, pushing surface waters away from coastlines. This displacement allows cold, nutrient-rich deep water to rise to the surface, a process known as upwelling. These nutrients, such as nitrates and phosphates, serve as essential fuel for phytoplankton, which form the base of the marine food web.
During an El Niño event, these trade winds weaken or even reverse. This suppresses upwelling, meaning the nutrient-rich deep water remains trapped below a layer of warm, nutrient-poor surface water. Without the constant influx of minerals, phytoplankton populations crash, leading to a significant reduction in available food for zooplankton, fish, and larger predators.
The long-term implications for marine biodiversity are severe. Frequent or intense El Niño cycles can lead to ecosystem shifts where productive, specialist species are replaced by less productive generalists. Persistent nutrient depletion can cause collapses in commercial fisheries and disrupt migratory patterns of large marine mammals and seabirds. Over time, this instability can reduce the overall resilience of the ocean ecosystem, making it more vulnerable to further environmental stressors like climate change and ocean acidification.
