El Niño is a recurring climate pattern characterized by the anomalous warming of sea surface temperatures in the tropical central and eastern Pacific Ocean. This phenomenon, the warm phase of the El Niño-Southern Oscillation (ENSO), significantly alters global weather patterns and oceanic conditions. The changes in the ocean’s physical structure fundamentally disrupt the marine food web, particularly in the highly productive Eastern Pacific. This warming suppresses the natural mechanisms that support the base of the ocean’s productivity.
The Physical Mechanism of Ocean Disruption
Under normal conditions, strong easterly trade winds push warm surface water west, causing colder, denser water from the deep ocean to rise near South America, a process called coastal upwelling. This mechanism keeps the thermocline—the boundary layer—close to the surface, allowing nutrient-rich bottom water to fertilize the surface layer.
During an El Niño event, the trade winds slow down or reverse direction, allowing warm surface water to flow eastward toward South America. This influx of lower-density warm water dramatically deepens the thermocline in the Eastern Pacific, pushing the nutrient-rich cold water layer far below the surface.
The deepening thermocline effectively cuts off the supply of deep-ocean nutrients to the sunlit surface waters. The warmer surface layer becomes highly stratified, resisting mixing with the colder, deeper water. This suppression of upwelling starves the surface ecosystem, leading directly to the collapse of primary productivity across the Eastern Pacific.
Collapse of Primary Producers
The immediate biological consequence of this physical disruption is the severe reduction in primary producers, specifically phytoplankton. These microscopic algae form the base of the marine food chain and rely entirely on nutrients delivered by the upwelled deep water. When the deepening thermocline prevents these nutrient-rich waters from reaching the euphotic zone, phytoplankton populations rapidly decline.
This reduction in primary productivity is visible in satellite data as a marked drop in sea surface chlorophyll-a concentrations. The lack of essential nutrients can cause a near-total collapse of certain phytoplankton groups, such as diatoms. This biological desert at the base of the food web sets the stage for a dramatic cascading effect through all higher trophic levels.
Cascading Effects on Higher Trophic Levels
The collapse of the phytoplankton population initiates a severe bottom-up trophic cascade. Zooplankton, the immediate consumers, experience a rapid decline in their food source, leading to a drop in biomass. This shortage directly impacts forage fish, such as anchovies and sardines, which are obligate plankton feeders.
Forage fish populations respond to the scarcity of food and warmer water by migrating to cooler, more productive areas or moving into deeper depths. Those that remain face starvation, reduced growth rates, and reproductive failure, leading to significant population declines. This devastating impact on schooling fish directly affects apex predators.
Seabirds, including guano-producing species like the Peruvian booby and pelicans, experience widespread reproductive failure and high mortality rates. Marine mammals, such as sea lions and fur seals, also suffer from famine and population drops because their primary prey is unavailable. The warm, nutrient-poor water causes a wholesale shift in the distribution and survival rates of all marine life.
Impact on Global Fisheries
The biological devastation caused by El Niño translates directly into severe economic consequences for nations dependent on Eastern Pacific fisheries. The Peruvian anchoveta fishery, historically the world’s largest single-species fishery by volume, is highly vulnerable. Since this species relies heavily on the upwelling-driven productivity of the Humboldt Current, its population plummets when nutrients are suppressed.
During strong El Niño events, the anchoveta catch can be reduced to less than a third of its normal level, creating severe economic instability for Peru and other fishing nations. Authorities often implement temporary closures of the fishing season to mitigate the biological collapse. While cold-water species decline, certain warm-water species, like some types of tuna, may shift their distribution eastward, disrupting established fishing patterns. This widespread decline in commercially important fish stocks affects food security and the livelihoods of coastal communities.