Spatial and temporal variability of phytoplankton pigment distributions in the central equatorial Pacific Ocean

During the 1992 U.S. JGOFS Equatorial Pacific (EqPac) study, we participated in survey (12°N-12°S, along 135°–140°W) and time-series (0°, 140°W) cruises to identify the factors that control spatial and temporal variations in phytoplankton pigment biomass, taxonomic composition and size structure. To achieve this goal, we determined pigment marker distributions in conjunction with collaborative measurements performed by EqPac investigators. Distributions of phytoplankton pigments measured during early 1992 (El Niño conditions) were different from those measured during late 1992 (“normal” conditions). Most notably, the accessory pigment distributions revealed that the 1991–1992 El Niño event produced a significant reduction in eukaryotic phytoplankton biomass, especially that contributed by prymnesiophytes, pelagophytes and diatoms. This variability was most likely caused by a combination of physical (Kelvin waves, tropical instability waves and advection), chemical (iron limitation) and biological (growth-grazing imbalances) processes. The results of this pigment study underscore the need for sampling biological properties on the appropriate time and space scales, and the necessity of physical measurements for interpreting their distributions.