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Micro-phytoplankton of the equatorial Pacific Ocean

by Jose Luis Iriarte

The equatorial Pacific Ocean is an interesting environment in terms of its oceanographic biogeochemical processes. This unique ecosystem is characterized by complex ocean-atmosphere interactions and a physically dynamic oceanic circulation pattern. Frequent upwelling of cold, nutrient-rich water at the Equatorial Divergence supports a highly productive phytoplankton community. Micro-phytoplankton productivity is an important measure of the oceans' food supplies. Also called primary productivity, these rates largely regulate fishery cycles and may be significant in the global carbon cycle. High nutrient concentrations in this area are not accompanied by high phytoplanktonic biomass and primary productivity values, however, indicating that those parameters are somehow limited. Processes such as grazing control, iron limitation and lack of coastal bloom-forming diatoms have been invoked to explain this paradoxical high-nutrient and low-chlorophyll (HNLC) condition.

[43K] Scanning electron micrograph (1000x) of the dinoflagellate, Ornithocercus magnificus. (Micrography by Jose Luis Iriarte)

[34K] Scanning electron micrograph (1000x) of the diatom, Asteromphalus sp. (Micrograph by Jose Luis Iriarte)

The structure and function of marine pelagic food webs can be described in terms of organisms' body sizes. Size determines the rates of life processes in planktonic organisms including nutrient uptake, ingestion, respiration, and photosynthesis. The central equatorial Pacific Ocean is dominated by ubiquitous small phytoplankton, including cyanobacteria, monads, phytoflagellates, and small pennate diatoms, while the larger phytoplankton exhibit more variable populations. These include large centric and pennate diatoms and dinoflagellates. Several studies note the important role diatoms play in new production and biogenic fluxes, but an understanding of factors that control phytoplankton community structure in a highly variable ecosystem is of fundamental concern to a carbon flux balance of the Equatorial Pacific Ocean.

Plankton speciation and its variation in time and space is one of the keys to understanding food web processes. As part of the U.S. Equatorial Pacific Joint Global Ocean Flux Studies, I quantified the abundance and biomass of micro-phytoplankton, including centric and pennate diatoms, thecate dinoflagellates and large coccolithophorids. Working under Dr. Greta Fryxell, I investigated temporal changes in the micro-phytoplankton community over two time-series studies carried out at the equator from the R.V. Thomas G. Thompson during March-April and October, 1992. The physical complexity of the equatorial Pacific Ocean was evident during those two cruises. During the first period, El Niño phenomena were detected; sea surface temperature had risen and nutrient concentrations were low. The second period was characterized by relatively low temperatures and high nutrient concentrations produced by a tropical instability wave passing through the equator. The tropical instability wave is a westward propagating wave along the equator.

Temporal results showed that during the abnormal warmth of the 1991-1992 El Nino micro-phytoplankton were generally small and scarce. In contrast, during the "cold condition" their numbers increased and large centric diatoms such as Rhizosolenia spp., chains of Pseudonitzschia spp., and heavily silicified species of Thalassiothrix spp. were important components contributing to total biomass. Furthermore, the micro-phytoplankton assemblage during the El Nino event exhibited fewer species compared with the "cold condition." In general, the presence of an El Nino event during March and April, and a tropical instability wave during October, 1992 may explain most variability in cell numbers and species diversity found in the Equatorial Pacific Ocean.

Editor's Note: Jose Luis Iriarte is now an Assistant Professor at the Universidad de Magallanes in Punta Arenas, Chile. He teaches and conducts research in phytoplankton ecology with special emphasis on cold-water ecosystemsin fjords and in the Antarctic.

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Updated July 24, 1995