Plankton community structure in response to hydrothermal iron inputs along the Tonga-Kermadec arc

Abstract

The Western Tropical South Pacific (WTSP) basin has been identified as a hotspot of atmospheric dinitrogen fixation due to the high dissolved iron ([DFe]) concentrations (up to 66 nM) in the photic layer linked with the release of shallow hydrothermal fluids along the Tonga-Kermadec arc. Yet, the effect of such hydrothermal fluids in structuring the plankton community remains poorly studied. During the TONGA cruise (November-December 2019), we collected micro- (20-200 μm) and meso-plankton (>200 μm) samples in the photic layer (0-200 m) along a west to east zonal transect crossing the Tonga volcanic arc, in particular two volcanoes associated with shallow hydrothermal vents (< 500 m) in the Lau Basin, and both sides of the arc represented by Melanesian waters and the South Pacific Gyre. Samples were analyzed by quantitative imaging (FlowCam and ZooScan) and then coupled with acoustic observations, allowing us to study the potential transfer of phytoplankton blooms to higher planktonic trophic levels. We show that micro- and meso-plankton exhibit high abundances and biomasses in the Lau Basin and, to some extent, in Melanesian waters, suggesting that shallow hydrothermal inputs sustain the planktonic food web, creating productive waters in this otherwise oligotrophic region. In terms of planktonic community structure, we identified major changes with high [DFe] inputs, promoting the development of a low diversity planktonic community dominated by diazotrophic cyanobacteria. Furthermore, in order to quantify the effect of the shallow hydrothermal vents on chlorophyll a concentrations, we used Lagrangian dispersal models. We show that chlorophyll a concentrations were significantly higher inside the Lagrangian plume, which came into contact with the two hydrothermal sites, confirming the profound impact of shallow hydrothermal vents on plankton production.

Composition of the sinking particle flux in a hot spot of dinitrogen fixation revealed through polyacrylamide gel traps

“…Here we show that the POC fluxes were primarily driven by zooplankton derived particles (fecal pellets and aggregates, and zooplankton carcasses), with fecal aggregates dominating at 1000 m, while cylindrical fecal pellets and zooplankton carcasses dominated at 170 m (Table 3; Figure 5). The dominance of zooplankton derived particles is in line with the plankton community composition present in surface waters during the same expedition (Meŕiguet et al, 2023). This latter study reports that grazers, dominated by gelatinous carnivores, chaetognatha, copepoda, and other larger species, account for 34% (average abundance of 329 ind.…”

“…The high POC fluxes derived from zooplankton particles suggest that the Fe-rich hydrothermal inputs from the Tonga arc have stimulated N 2 fixation (Bonnet et al, 2023b), that in turn stimulated primary and secondary productions in this LNLC ecosystem. Meŕiguet et al (2023) showed that diazotrophs dominated the microplankton (20-200 μm) community in the area of trap deployment (69% of the total community abundance of 5.10 5 ind.m -3 ) and that >80% of diazotrophs were Trichodesmium spp. In this ecosystem, diazotrophy supports >80% of export production (Bonnet et al, 2023b).…”

“…Thus, considering sinking velocities of filamentous and UCYN diazotroph aggregates (100 and 400 m d -1 respectively) measured in the laboratory by Ababou et al (2023), they would reach the depth of 270 m within 0.67 to 3 days, and the depth of 1000 m within 2 to 10 days, which supports our hypothesis. Another hypothesis that could explain the higher contribution of diazotrophs to POC fluxes at 1000 m compared to that at is the high abundance of zooplankton carcasses (Table 3) at 170 m (thus increasing total POC, and decreasing the contribution of POC associated with diazotrophs) and gelatinous organisms (329 ind m -3 ) in shallow (0-200 m) waters (Meŕiguet et al, 2023). This suggests that these organisms may have ingested diazotrophs between 0 and 200 m, consequently decreasing their export flux at 170 m. Trichodesmium spp.…”