Micro-phytoplankton at the equatorial Pacific (140°W) during the JGOFS EqPac Time Series studies: March to April and October 1992

Micro-phytoplankton (>20 um cell size) was sampled in the upper 200 m of the water column at the Pacific equator, 140°W during two JGOFS EqPac Time Series (TS) Studies, in order to determine the changes in the micro-phytoplankton assemblage between March–April and October 1992, to find the vertical distribution of micro-phytoplankton taxa, and to relate any changes in the environmental factors to micro-phytoplankton structure. Cell abundance and carbon biomass of three major taxonomic classes: diatoms, dinoflagellates and coccolithophorids were examined. During the abnormal warmth of El Niño 1992 (SST = 28−29°C), low abundance (<3000 cells liter−1) and carbon biomass (<1,μg C liter−1) characterized the micro-phytoplankton structure, accompanied by low numbers of diatoms and coccolithophorids. Pennate diatoms, Pseudonitzschia delicatissima and thecate dinoflagellate, Oxytoxum variabile, were the most abundant organisms observed during March–April 1992 Time Series study. The microphytoplankton assemblage during El Niño conditions in March–April 1992 exhibited lower microphytoplankton species richness and abundance compared with October 1992. Also in contrast to the spring, in October 1992 the micro-phytoplankton assemblage showed large variability, mainly due to the passage of an instability wave through the study site. During this period, the coldest temperatures (SST = 25°C) were associated with increased abundance (range = 2 × 102 to 12 × 103 cells liter−1) and richness in micro-phytoplankton species assemblage, which was again dominated by a colonial pennate diatom P. delicatissima. On the average, micro-phytoplankton carbon ranged from 0.5 to 4.0 μg C liter−1, where the diatom group consistently comprised the major part of the micro-phytoplankton autotrophic biomass in the upper 60 m. Large centric diatoms, such as Rhizosolenia species, as well as chains of P. delicatissima united in stepped colonies, and heavily silicified species of the Thalassionema/Thalassiothrix spp. complex were important groups contributing to the total micro-phytoplankton carbon biomass. At the equator, diatoms and dinoflagellates were restricted to the surface and to the upper 60 m, respectively, during both Time Series cruises, while coccolithophorid cells were concentrated at 90 m during TS I. The presence of an El Niño event and a instability wave during March–April and October 1992, respectively, may explain most of the variability in abundance and species richness found in the equatorial Pacific at 140°W during the study periods.