Saved in:
| Main Authors: | , |
|---|---|
| Format: | Dataset Open Access |
| Language: | en |
| Published: |
PANGAEA
1996
|
| Subjects: | |
| Online Access: | https://doi.org/10.1594/PANGAEA.807552 |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
Table of Contents:
- Natural populations of phytoplankton from the western tropical Pacific Ocean were analyzed by flow cytometry from a transect along 165°E between 20°S and 7°N. The abnormal hydrological situation corresponded to a weak El Niño event, with no equatorial upwelling and a marked nutrient ridge centered on 10°S. Prochlorophytes dominated numerically everywhere along the vertical, whatever the depth, in the 0-160-m layer (96% of cell abundance). Paradoxically, the highest concentrations, up to 4.4 x 10**5 cells/ml, were found in oligotrophic waters (< 0.1 µM NO3). In contrast, the highest concentrations of orange cyanobacteria and redfluorescing picoeukaryotes were observed when nitrate was present in the photic layer, i.e. around 10°S (up to 6.4 x 10**4 cells/ml and 1.3 x 10**4 cells/ml), and, to a lesser extent in the vicinity of the deep nitracline north of 8°S. Along the transect we encountered two hydrological situations, characterized by different community structures. The first one, found from 15°S to 7°N, except at 10°S, was a two-layer structure (Typical Tropical Structure, TTS) defined by a strong pycnocline in the upper 180 m and a well-marked nitracline. In this region, Prochlorococcus and picoeukaryotes co-dominated the 180-m integrated fluorescence and carbon biomass, but Prochlorococcus were the major component in the upper nitrate-depleted layer, while picoeukaryotes dominated the underlying rich layer. Inversely, Synechococcus were a relatively minor contributor to fluorescence (~4%) and phytoplankton biomass (< 1%) in comparison to the other cell types. The second structure observed in the southernmost part of the transect (20°S-16°S) was defined by the absence of a density gradient, and therefore by deep vertical mixing. In this case, the concentration of Prochlorococcus in the upper nitrate-depleted layer was reduced, whereas Synechococcus percentage contribution in the upper 180 m was significantly higher than in the TTS ( > 30% of total fluorescence and ~4% of carbon biomass). According to our results, we discuss the expected role of each phytoplankton group in the regenerated and new production. Finally, we discuss the importance of cell size as a factor in the expected roles of the different phytoplankton groups in the carbon sink.