Which of the following describes a significant biological change due to El Niño?

The correct choice highlights a key impact of El Niño events on marine ecosystems. During an El Niño, the usual patterns of ocean currents and upwelling are disrupted, leading to warmer surface waters. This warming effect can inhibit the upwelling of nutrient-rich waters from deeper regions of the ocean to the surface. In typical conditions, these nutrients support phytoplankton growth, which forms the basis of the marine food web.

With the reduction in nutrient upwelling, there is a decrease in primary productivity, meaning that the overall amount of plant biomass, particularly phytoplankton, diminishes. This decline can have cascading effects throughout the food chain, impacting fish populations and other marine organisms that depend on phytoplankton for nourishment.

The other options present scenarios that do not accurately reflect the biological changes initiated by El Niño. For instance, increased upwelling of nutrients is contrary to what typically occurs during an El Niño, as upwelling is generally suppressed. The enhanced growth of cold-water species and stabilization of ocean temperatures across different zones also do not align with the observed biological responses during these climatic events.