MICRO-PHYTOPLANKTON AT THE EQUATORIAL PACIFIC (140-DEGREES-W) DURING THE JGOFS EQPAC TIME-SERIES STUDIES - MARCH TO APRIL AND OCTOBER 1992

Micro-phytoplankton (> 20 mu m cell size) was sampled in the upper 200 m of the water column at the Pacific equator, 140 degrees W during tw o JGOFS EqPac Time Series (TS) Studies, in order to determine the chan ges in the micro-phytoplankton assemblage between March-April and Octo ber 1992, to find the vertical distribution of micro-phytoplankton tax a, and to relate any changes in the environmental factors to micro-phy toplankton structure. Cell abundance and carbon biomass of three major taxonomic classes: diatoms, dinoflagellates and coccolithophorids wer e examined. During the abnormal warmth of El Nino 1992 (SST = 28-29 de grees C), low abundance (< 3000 cells liter(-1)) and carbon biomass (< 1 mu g C liter(-1)) characterized the micro-phytoplankton structure, accompanied by low numbers of diatoms and: coccolithophorids. Pennate diatoms, Pseudonitzschia delicatissima and thecate dinoflagellate, Oxy toxum variabile, were the most abundant organisms observed during Marc h-April 1992 Time Series study. The micro-phytoplankton assemblage dur ing El Nino conditions in March-April 1992 exhibited lower micro-phyto plankton species richness and abundance compared with October 1992. Al so in contrast to the spring, in October 1992 the micro-phytoplankton assemblage showed large variability, mainly due to the passage of an i nstability wave through the study site. During this period, the coldes t temperatures (SST = 25 degrees C) were associated with increased abu ndance (range = 2 x 10(2) to 12 x 10(3) cells liter(-1)) and richness in micro-phytoplankton species assemblage, which was again dominated b y a colonial pennate diatom P. delicatissima. On the average, micro-ph ytoplankton carbon ranged from 0.5 to 4.0 mu g C liter(-1), where the diatom group consistently comprised the major part of the micro-phytop lankton autotrophic biomass in the upper 60 m. Large centric diatoms, such as Rhizosolenia species, as well as chains of P. delicatissima un ited in stepped colonies, and heavily silicified species of the Thalas sionema/Thalassiothrix spp. complex were important groups contributing to the total micro-phytoplankton carbon biomass. At the equator, diat oms and dino-flagellates were restricted to the surface and to the upp er 60 m, respectively, during both Time Series cruises, while coccolit hophorid cells were concentrated at 90 m during TSI. The presence of a n El Nino event and a instability wave during March-April and October 1992, respectively, may explain most of the variability in abundance a nd species richness found in the equatorial Pacific at 140 degrees W d uring the study periods.