Plankton Assemblage Estimated with BGC‐Argo Floats in the Southern Ocean: Implications for Seasonal Successions and Particle Export

Abstract

The Southern Ocean (SO) hosts plankton communities that impact the biogeochemical cycles of the global ocean. However, weather conditions in the SO restrict mainly in situ observations of plankton communities to spring and summer, preventing the description of biological successions at an annual scale. Here, we use shipboard observations collected in the Indian sector of the SO to develop a multivariate relationship between physical and bio‐optical data, and, the composition and carbon content of the plankton community. Then we apply this multivariate relationship to five biogeochemical Argo (BGC‐Argo) floats deployed within the same bio‐geographical zone as the ship‐board observations to describe spatial and seasonal changes in plankton assemblage. The floats reveal a high contribution of bacteria below the mixed layer, an overall low abundance of picoplankton and a seasonal succession from nano‐ to microplankton during the spring bloom. Both naturally iron‐fertilized waters downstream of the Crozet and Kerguelen Plateaus show elevated phytoplankton biomass in spring and summer but they differ by a nano‐ or microplankton dominance at Crozet and Kerguelen, respectively. The estimated plankton group successions appear consistent with independent estimations of particle diameter based on the optical signals. Furthermore, the comparison of the plankton community composition in the surface layer with the presence of large mesopelagic particles diagnosed by spikes of optical signals provides insight into the nature and temporal changes of ecological vectors that drive particle export. This study emphasizes the power of BGC‐Argo floats for investigating important biogeochemical processes at high temporal and spatial resolution.

The Intraseasonal Dynamics of the Mixed Layer Pump in the Subpolar North Atlantic Ocean: A Biogeochemical‐Argo Float Approach

“…In addition, taxonomic changes in the phytoplankton community could further increase the Chla signal relative to the b bp signal. Indeed, the local restratification could enhance the light environment and stimulate larger phytoplankton, such as diatoms, with higher Chla to b bp ratio (Cetinić et al, ; Lacour et al, ; Rembauville et al, ). In the twilight remnant layer, change in the balance between production and consumption leads to a decrease in both Chla and b bp .…”

Diatoms shape the biogeography of heterotrophic prokaryotes in early spring in the Southern Ocean

“…Gradients in environmental parameters, determined in seawater collected at 10 m, were particularly pronounced between the STZ and the SAZ, while overall minor variations within surface waters of the PFZ and AAZ were detectable (see Supporting Information Table S1 for dissolved oxygen and salinity). Diatom carbon biomass accounted for 2%-14% of total phytoplankton biomass in subtropical waters, while its contribution was on average 59% for all other stations (Rembauville et al, 2017). Concentrations of DOC dropped from roughly 70 μM in the subtropical zone to 50 μM within and south of the SAZ.…”

“…Diatoms dominated in terms of abundance over dinoflagellates at all sampling sites except at station O22 in the STZ. Diatom carbon biomass accounted for 2%-14% of total phytoplankton biomass in subtropical waters, while its contribution was on average 59% for all other stations (Rembauville et al, 2017).…”

“…Concentrations of chlorophyll a (Chl a) were determined by High-Performance Liquid Chromatography (HPLC) analyses (Uitz et al, 2009). Observations were done in an Utermöhl counting chamber (24 h, dark) using an inverted microscope with phase contrast (Olympus IX70) with 400× magnification as described in Rembauville et al (2017). For the identification and enumeration of the diatom and dinoflagellate assemblages, 100 ml of seawater were fixed with Lugol solution (1% final concentration) and stored in the dark at 4 C for 2 months.…”

“…For the identification and enumeration of the diatom and dinoflagellate assemblages, 100 ml of seawater were fixed with Lugol solution (1% final concentration) and stored in the dark at 4 C for 2 months. Observations were done in an Utermöhl counting chamber (24 h, dark) using an inverted microscope with phase contrast (Olympus IX70) with 400× magnification as described in Rembauville et al (2017). For the analyses of the prokaryotic community composition, three 6 L biological replicates were collected at each site, filtered on 60 μm nylon screens, and then through 0.8 μm polycarbonate filters (47-mm diameter, Nuclepore, Whatman, Sigma Aldrich, St Louis, MO).…”