Around the equator in much of the Pacific Ocean, concentrations of ocean plant life are generally low. This “marine desert” occurs because this part of the ocean is low in iron, a key nutrient that the ocean’s tiny plants—called phytoplankton—require. However, during the La Niña episode that followed the strong El Niño in 1997, satellites recorded a major phytoplankton bloom in the equatorial Pacific. For about one month, the desert became a garden, where plants flourished, died, and sank into the deep ocean.
These images show satellite-measured chlorophyll concentrations in the Pacific Ocean during the La Niña bloom in August 1998 (top left); in December of 1999 (top right), which was a typical year; and during the 1997 El Niño (bottom right). The graph at lower left shows the chlorophyll concentration of the region from September (S) of 1997 through December (D) of 2000. Chlorophyll concentrations increase from purple (very low) to red (high). During the La Niña bloom an area of very high chlorophyll occurs around 140 degrees West; this area corresponds to the peak in the graph, which reaches its maximum in August 1998. These data were collected by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS).
What is it about El Niño and La Niña cycles that could account for these dramatic swings in plant productivity in the Pacific? Iron content in the remote Pacific is low because the area is far from land-based sources of iron, such as sediment and dust. The iron that is available in the water comes from upwelling of deeper ocean waters. During normal years, winds blowing from east to west across the eastern Pacific off the coast of South America drive the surface water away from land. Cold, nutrient-rich water wells up from the deep to replace it.
During El Niño events, the winds virtually disappear, and upwelling becomes very weak. Without nutrients, phytoplankton concentrations plummet, as do the concentrations of the microscopic animals—zooplankton—that eat the phytoplankton, and the fish that eat the zooplankton. The explosiveness of the August 1998 bloom was the result of two factors. As the strong El Niño of 1997 weakened, the winds returned. Upwelling increased the concentration of iron and other nutrients rapidly, allowing phytoplankton to start recovering.
The second factor that helped the bloom become so large was the scarcity of zooplankton predators. Because the population of zooplankton had also crashed during the El Niño “famine” in 1997, the recovering phytoplankton populations had a short window in which they weren’t being grazed by their predators. Scientists who studied chlorophyll concentrations in the Pacific during and after the 1997 El Niño estimate that the amount of carbon that made its way into the deep ocean in the remains of dead phytoplankton increased by a factor of 8 during the event.
Images courtesty Xiujun Wang, University of Maryland, Earth System Science Interdisciplinary Center