Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions

Abstract

The distribution of ultraphytoplankton was investigated in the western North Pacific Subtropical Gyre (NPSG) during La Niña, a cold phase of El Niño Southern Oscillation (ENSO). Observations were conducted in a north-south transect (33.6–13.25° N) along the 141.5° E meridian in order to study the ultraplankton assemblages in various oligotrophic conditions. Analyses were performed at the single cell level by analytical flow cytometry. Five ultraphytoplankton groups (<i>Prochlorococcus</i>, <i>Synechococcus</i>, picoeukaryotes, nanoeukaryotes and nanocyanobacteria-like) defined by their optical properties were enumerated in three different areas visited during the cruise: the Kuroshio region, the subtropical Pacific gyre and a transition zone between the subtropical Pacific gyre and the Warm pool. <i>Prochlorococcus</i> outnumbered the other photoautotrophs in all the investigated areas. However, in terms of carbon biomass, an increase in the relative contribution of <i>Synechococcus</i>, picoeukaryotes and nanoeukaryotes was observed from the centre of the subtropical gyre to the Kuroshio area. In the Kuroshio region, a peak of abundance of nanoeukaryotes observed at the surface suggested an increase in nutrients likely due to the vicinity of a cold cyclonic eddy. In contrast, in the salinity front along the isohaline 35 and anticyclonic eddy located around 22.83° N, the mainly constant distribution of <i>Prochlorococcus</i> from the surface down to 150 m characterised the dominance by these microorganisms in high salinity and temperature zone. Results suggested that the distribution of nanocyanobacteria-like is also closely linked to the salinity front rather than low phosphate concentration. The maximum abundance of ultraphytoplankton was located above the SubTropical Counter Current (STCC) at depths > 100 m where higher nutrient concentrations were measured. Finally, comparison of the ultraphytoplankton concentrations during El Niño (from the literature) and La Niña (this study) conditions seems to demonstrate that La Niña conditions lead to higher concentrations of <i>Synechococcus</i> in the Subtropical gyre and a lower abundance of <i>Synechococcus</i> in the Kuroshio region. Our results suggest that the west part of NPSG is a complex area, where different water masses, salinity fronts and eddies lead to a heterogeneous distribution of ultraphytoplankton assemblages in the upper layer of the water column

Anticyclonic Eddy Edge Effects on Phytoplankton Communities and Particle Export in the Northern South China Sea

“…First, although the TChl a concentrations were very similar at the center and edge, Prochlorococcus contributed~37% and 18% of the elevated TChl a concentrations, respectively, relative to the reference. There have been some reports of a positive response of Prochlorococcus to ACEs (Baltar et al, 2010;Fernández et al, 2008;Girault et al, 2013;Lasternas et al, 2013), presumably because Prochlorococcus benefits from the warmer, oligotrophic water in ACEs (Agawin et al, 2000). The enhancement of the Prochlorococcus Chl a concentration is dramatic considering the higher cell mortality and lysis rates at the centers of ACEs (Lasternas et al, 2013).…”

Characterization of ultraphytoplankton pigments and functional community structure in Xiangxi Bay, China, using HPLC-CHEMTAX

“…They consist of both prokaryotic and eukaryotic phytoplankton (Sieburth et al 1978). The global distribution of ultraphytoplankton is extremely wide, and they are found in polar, temperate, and tropical waters (Dishon et al 2012;Girault et al 2013). Their quantity, biomass, and productivity can be used as a quality indicator of water ecosystem (Silva et al 2009).…”

Heterotrophic prokaryote distribution along a 2300 km transect in the North Pacific subtropical gyre during a strong La Niña conditions: relationship between distribution and hydrological conditions

“…Analyses were performed in the same period as the heterotrophic prokaryotes and based on their light scatter and fluorescence emission properties. Ultraphytoplankton was discriminated in this study into five flow cytometry clusters (Synechococcus, Prochlorococcus, Picoeukaryotes, Nanoeukaryotes and Nanocyanobacteria-like) as described in Girault et al (2013b).…”

“…The cruise took place along a north-south transect in the western part of the NPSG (141.5 • E) during a strong La Niña climatic event. According to the temperature-salinity diagram presented in the study made by Girault et al (2013b), three main areas corresponding to the Kuroshio region (Sta. 1-4), the subtropical gyre (Sta.…”

“…However, the relationship between heterotrophic prokaryote abundance and oligotrophic conditions is unclear, especially in terms of mesoscale structures such as eddies (Baltar et al, 2010;Lasternas et al, 2013). The differences within the same type of mesoscale circulation reported in the literature highlights that the relationship between heterotrophic prokaryotes and photoautotrophs can be dependent on the identification of the different microorganisms making up the community (Girault et al, 2013b).…”