The fate of a southwest Pacific bloom: gauging the impact of submesoscale vs. mesoscale circulation on biological gradients in the subtropics

Abstract

Abstract. The temporal evolution of a surface chlorophyll a bloom sampled in the western tropical South Pacific during the 2015 Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise is examined. This region is usually characterized by largely oligotrophic conditions, i.e. low concentrations of inorganic nutrients at the surface and deep chlorophyll a maxima. Therefore, the presence of a surface bloom represents a significant perturbation from the mean ecological state. Combining in situ and remote sensing datasets, we characterize both the bloom's biogeochemical properties and the physical circulation responsible for structuring it. Biogeochemical observations of the bloom document the bloom itself, a subsequent decrease of surface chlorophyll a, significantly reduced surface phosphate concentrations relative to subtropical gyre water farther east, and a physical decoupling of chlorophyll a from a deep nitracline. All these characteristics are consistent with nitrogen fixation occurring within the bloom. The physical data suggest surface mesoscale circulation is the primary mechanism driving the bloom's advection, whereas balanced motions expected at submesoscales provide little contribution to observed flow. Together, the data provide a narrative where subtropical gyre water can produce significant chlorophyll a concentrations at the surface that is stirred, deformed, and transported great distances by the mesoscale circulation. In this case, for the time period considered, the transport is in an easterly direction, contrary to both the large-scale and mean mesoscale flow. As a result, future studies concerning surface production in the region need to take into account the role complex mesoscale structures play in redistributing subtropical gyre water.

Microbial community structure in the Western Tropical South Pacific

“…To describe the longitudinal and vertical distribution of different groups of pico-and nanoplankton, we sampled 15 short 35 duration stations (SD1 to SD15, occupied during 8h, Fig 1). Three long duration stations (LDA, LDB, and LDC, occupied during 7 days), chosen for their contrasted biogeochemical conditions (Table 1), were sampled in Lagrangian mode (de Verneil et al, 2017a). All stations were sampled at 12 different depths from the surface down to 200 m for microbial characterization of the extended photic layer.…”

“…Abundances, biomass, and nutrient values are reported either as depth-integrated totals or as depth-normalized averages. In order to account for large vertical gradients in both abundances and nutrient concentrations within the Z e , integrations were performed across two depth ranges: the upper photic zone (Z u ), integrating from the surface to the recorded 2.7 % isolume (hereafter PAR 2.7 ) and the lower euphotic zone (Z l ), integrating from PAR .2.7 to the recorded 0.1 % 15 isolume (hereafter PAR 0.1 ).The mixed layer depth was measured continuously along the transect, as described by de Verneil, et al (2017a). The mixed layer was entirely within Z u at all stations ( Fig.…”

Aphotic N<sub>2</sub> fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean

“…The purified products were barcoded for Illumina (San Diego, CA, USA) MiSeq sequencing with Nextera indexes, using the manufacturer's protocol. The indexed products were purified again with 15 magnetic beads, then quantified with a plate reader (Molecular Devices, Sunnyvale, CA, USA) using Picogreen (Thermo Fisher). The indexed samples were adjusted to equal concentrations and pooled for multiplexing during sequencing.…”

“…In the sunlit layer of the warm oligotrophic tropical and subtropical oceans, cyanobacterial diazotrophs such as Trichodesmium, UCYN-B and diatom-diazotroph associations (DDAs) dominate fixed nitrogen inputs via N 2 fixation (Capone et al, 1997;Montoya et al, 2004). In colder and less 15 oligotrophic waters at higher latitudes, other diazotrophs including UCYN-A and non-cyanobacterial groups such as Gamma A may be more competitive Langlois et al, 2015;Moisander et al, 2010;, expanding considerably the latitudinal range where N 2 fixation is considered significant in predictive biogeochemical models. In the past decade, several studies have retrieved nifH sequences from the dark ocean, some also accompanied by low N 2 fixation rates (Bonnet 20 et al, 2013;Hamersley et al, 2011;Rahav et al, 2013).…”

“…Unfortunately, the current lack of genome information from noncyanobacterial aphotic diazotrophs does not allow us to assess how they are affected by DOM composition and lability. 15 The WTSP has been recently recognized as a global hotspot of photic N 2 fixation, harboring among the highest N 2 fixation rates ever recorded (~600 µmol N m -2 d -1 ; , mostly attributed to Trichodesmium and UCYN-B (Berthelot et al, 2017;Stenegren et al, 2017). To the eastern border of this region, the ultraoligotrophic South Pacific Gyre (GY) has low photic N 2 fixation rates (Raimbault and Garcia, 2008), which have been mainly attributed to small 20 unicellular diazotrophs such as UCYN-A and Gammaproteobacteria (Bonnet et al, 2008;Halm et al, 2012;Stenegren et al, 2017).…”