Climate variability and phytoplankton composition in the Pacific Ocean

The effect of climate variability on the phytoplankton community was assessed for the tropical and sub‐tropical Pacific Ocean between 1998 and 2007 using an established biogeochemical assimilation model. The tropical and sub‐tropical phytoplankton communities exhibited a wide range of responses to climate variability, from radical shifts in the Equatorial Pacific, to changes of only a couple of phytoplankton groups in the North Central Pacific, to no significant changes in the South Pacific. In the Equatorial Pacific, climate variability represented by ENSO dominated the variability of phytoplankton. Here, nitrate, chlorophyll and all of the 4 phytoplankton types (diatoms, cyanobacteria, chlorophytes and coccolithophores) were strongly correlated (p < 0.05) with the Multivariate El Niño Southern Oscillation Index (MEI). During La Niña events, diatoms increased and expanded westward along the cold tongue (correlation with MEI, r = −0.87, p < 0.05), while cyanobacteria concentrations decreased significantly (r = 0.69, p < 0.05). El Niño produced the reverse pattern, with cyanobacteria populations increasing while diatoms plummeted. In the North Central Pacific, the MEI was significantly correlated with diatoms (r = −0.40) and chlorophytes (r = −0.43). Ocean biology in the South Pacific was not significantly correlated with MEI. The phytoplankton composition from the assimilation model was compared to that from a new empirical algorithm using satellite data. Despite differences in the absolute concentration, the relative abundance from the model and the satellite‐derived approach showed a similar shift in phytoplankton community in the Equatorial Pacific. These results highlight the spatially variable nature of the relationship between phytoplankton community structure and climate variability within the Pacific Ocean.