Changes in phytoplankton groups composition in the Equatorial Pacific during ENSO cycles

Abstract

The El Niño Southern Oscillation (ENSO) drives important changes in marine productivity of the Equatorial Pacific, in particular during major El Niño/La Niña transitions. Changes in environmental conditions associated with these climatic events also likely impact the phytoplankton composition. In this work, the distribution of four major phytoplankton groups (nanoeucaryotes, Prochlorococcus, Synechococcus and diatoms) was monitored between 1996 and 2007 by applying the PHYSAT algorithm to the ocean color data archive from the Ocean Color and Temperature Sensor (OCTS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Coincident with the decrease in chlorophyll concentrations, a large-scale change in the Equatorial Pacific phytoplankton composition that was characterized by a decrease in Synechococcus and an increase in nanoeucaryotes, was observed during the early stages of both the strong El Niño of 1997 and the moderate El Niño of 2006. A significant increase in diatoms in the Equatorial Pacific was observed during the 1998 La Niña and is associated with elevated marine productivity. An analysis of the environmental variables using a coupled physical-biogeochemical model (NEMO-PISCES) suggests that the decrease in Synechococcus during the two El Niño events was associated with an abrupt decrease in nutrient availability (-0.9 to -2.5 μM NO3 month-1), while an increase in nutrient availability (3 μM NO3 month-1) during the 1998 La Niña led to the development of diatom blooms in the Equatorial Pacific.Our results show a rapid resilience of these changes of the structure of phytoplankton communities which come back to a mean composition after a few months. Such rapid modifications of the composition of phytoplankton groups have thus to be considered in future modeling approaches to represent variability of the marine productivity in the Equatorial Pacific and to quantify its potential implications on food-web and on global carbon cycle.