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Micro-phytoplankton variability at the equatorial Pacific (140W̊) during the JGOFS EQPAC Time Series Studies 1992
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Micro-phytoplankton (>20 gm 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 Studies, in order to determine the changes in the micro-phytoplanlcton 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 Nifto 1992 (S ST = 28-29'C), low abundance (< 3000 cells liter-1) and carbon biomass (< 1 lAg C liter-1) characterized the micro-phytoplankton structure, accompanied by low numbers of diatoms and coccolithophorids. The pennate diatom, Pseudonitzschia delicatissima, and the thecate dinoflageflate, Oxytoxum variabile, were the most abundant organisms observed during the March-April 1992 Time Series study. The microphytoplankton assemblage during El Niflo conditions in March-April 1992 exhibited lower micro-phytoplankton species richness and abundance compared with October 1992. Also in contrast to the spring, in the autumn the micro-phytoplankton assemblage showed large variability, mainly due to the passage of a instability wave through the study site. During this period, the coldest temperatures (SST = 25'C) were associated with increased abundance (range = 2xIO2 to 12xlo3 cells liter-1), and richness in rnicro-phytoplankton species assemblage, which was again dominated by a colonial pennate diatom P. delicatissima. On the average, micro-phytoplanlaon carbon ranged from 0.5 to 4.0 Ag C liter-1, where the diatom group consistently comprised the major part of the microphytoplanlaon autotrophic biomass in the upper 60 m. Large centric diatoms, such as Rhizosolenia species, as well as chains of P. delicatissima, and heavily silicified species of the 77zalassionemal77wlassiothrix spp. complex were important groups contributing to the total micro-phytoplankton carbon biomass. At the equator, diatoms and dinoffagenates were restricted to the surface and to the upper 60 m, respectively, during both Time Series cruises, while coccolithophorid cells where concentrated at 90 m during Time Series I. The presence of an El Niflo event and an 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.
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Includes bibliographical references.
Iriarte, Jose Luis (1994). Micro-phytoplankton variability at the equatorial Pacific (140W̊) during the JGOFS EQPAC Time Series Studies 1992. Master's thesis, Texas A&M University. Available electronically from
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