Phytoplankton response to a weak El Niño event

Abstract

The western coast of South America is a very productive upwelling zone. However, during El Niño events, phytoplankton productivity in the equatorial Pacific declines sharply. A weak El Niño event occurred in 2014. Oceanographic anomalies included an increase in sea surface temperature and water transparency and a decrease in salinity. Phytoplankton composition also changed throughout time reflecting different El Niño stages. Before the El Niño, the community was mainly dominated by species typical of nutrient rich waters, such as large diatoms or silicoflagellates. At the onset of the event, the number of species and the turnover component of beta diversity increased, indicating that previously predominant species now co-occurred with new species transported by warmer surface water masses. At this initial stage, warm water dinoflagellates were indicator species, while a diatom bloom of Dactyliosolen fragilissimus indicated nutrient availability and some degree of turbulence in the water column. Finally, the overall abundance of phytoplankton was similar to the homologous months in previous years, but under El Niño conditions the community was dominated by smaller cells. Consequently, primary production attained a minimum, as inferred from the highest values of water transparency. There was no evidence of seasonality. It is concluded that the effect of El Niño on the phytoplankton community in the eastern equatorial Pacific is appreciable even during weak events.