|Heterotrophic prokaryote distribution along a 2300 km transect in the North Pacific subtropical gyre during a strong La Nina conditions: relationship between distribution and hydrological conditions |
(Article) Publié: Biogeosciences, vol. 12 p.3607-3621 (2015)
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The spatial distribution of heterotrophic prokaryoteswas investigated during the Tokyo–Palau cruise in thewestern part of the North Pacific subtropical gyre (NPSG)along a north–south transect between 33.60 and 13.25◦ N.The cruise was conducted in three different hydrological areasidentified as the Kuroshio region, the subtropical gyrearea and the transition zone. Two eddies were crossed alongthe transect: one cold-core cyclonic eddy and one warmcoreanticyclonic eddy and distributions of the heterotrophicprokaryotes were recorded. By using analytical flow cytometryand a nucleic acid staining protocol, heterotrophicprokaryotes were discriminated into three subgroups dependingon their nucleic acid content (low, high and very high nucleicacid contents labelled LNA, HNA and VHNA, respectively).Statistical analyses performed on the data set showedthat LNA, mainly associated with low temperature and lowsalinity, were dominant in all the hydrological regions. Incontrast, HNA distribution seemed to be associated with temperature,salinity, Chl a and silicic acid. A latitudinal increasein the HNA / LNA ratio was observed along the north–southtransect and was related to higher phosphate and nitrate concentrations.However, the opposite relationship observed forthe VHNA / HNA ratio suggested that the link between nucleicacid content and oligotrophic conditions is not linear,underlying the complexity of the biodiversity in the VHNA,HNA and LNA subgroups. In the Kuroshio Current, it is suggestedthat the high concentration of heterotrophic prokaryotesobserved at station 4 was linked to the path of the coldcyclonic eddy core. In contrast, it is thought that low concentrationsof heterotrophic prokaryotes in the warm coreof the anticyclonic gyre (Sta. 9) are related to the low nutrientconcentrations measured in the seawater column. Ourresults showed that the high variability between the variousheterotrophic prokaryote cluster abundances depend both onthe mesoscale structures and the oligotrophic gradient.