Particulate matter stoichiometry driven by microplankton community structure in summer in the Indian sector of the Southern Ocean

Abstract

Microplankton community structure and particulate matter stoichiometry were investigated in a late summer survey across the Subantarctic and Polar Front in the Indian sector of the Southern Ocean. Microplankton community structure exerted a first order control on PON:POP stoichiometry with diatom‐dominated samples exhibiting much lower ratios (4–6) than dinoflagellate and ciliate‐dominated samples (10–21). A significant fraction of the total chlorophyll a (30–70%) was located beneath the euphotic zone and mixed layer and sub‐surface chlorophyll features were associated to transition layers. Although microplankton community structure and biomass was similar between mixed and transition layers, the latter was characterized by elevated Chl:POC ratios indicating photoacclimation of mixed layer communities. Empty diatom frustules, in particular of Fragilariopsis kerguelensis and Pseudo‐nitzschia, were found to accumulate in the Antarctic Zone transition layer and were associated to elevated BSi:POC ratios. Furthermore, high Si(OH)4 diffusive fluxes (>1 mmol m2 d−1) into the transition layer appeared likely to sustain silicification. We suggest transition layers as key areas of C and Si decoupling through (1) physiological constraints on carbon and silicon fixation (2) as active foraging sites for grazers that preferentially remineralize carbon. On the Kerguelen Plateau, the dominant contribution of Chaetoceros Hyalochaete resting spores to microplankton biomass resulted in a three‐fold enhancement of POC concentration at 250 m, compared to other stations. These findings further highlight the importance of diatom resting spores as a significant vector of carbon export through the intense remineralization horizons characteristing Southern Ocean ecosystems.

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

“…Diatoms dominated the microplankton counts and biomass in the SOCLIM data set used to train the PLSR (supporting information data set). It is likely that the strong microplankton dominance predicted from the float data reflects a major diatom contribution as usually observed in productive environments of the SO (Armand et al, ; Korb et al, ; Lasbleiz et al, ; Rembauville et al, ). Floats 107c and 104c, respectively deployed at low‐ and high biomass sites in the vicinity of the Kerguelen Plateau, clearly reported the succession from a dominance of nano‐ to microplankton during the spring/summer phytoplankton bloom (Figures and ).…”

“…By contrast, the highest d ML values (200 ‐ 500 µm) were observed in summer during high POC, microplankton‐dominated phytoplankton blooms. These blooms are generally composed of a mixture of chain forming (chain length >100 µm) small diatoms ( Pseudo‐Nitzschia , Chaetoceros Hyalochaete, Odontella ), large diatoms such as Corethron pennatum (200 µm), Proboscia and Rhizosolenia (200–500 µm), large dinoflagellates ( Protoperidinium , Gyrodinium ∼100 µm) and even the giant diatom Thalassiothrix (>1,000 µm length) (Armand et al, ; Kopczyńska et al, ; Lasbleiz et al, ; Rembauville et al, ). Interestingly, the late summer nanoplankton‐dominated bloom North of Crozet was not associated with any increase in d ML that remained <50 µm (float 036b, January–March 2016, Figure b).…”

Southern Ocean Phytoplankton in a Changing Climate

“…Stochastic inputs of iron, wind, and storms disrupt stratification; influencing productivity, species composition, and export production through changes in nutrients and light climate. Changes in community composition from diatoms to flagellates also affect particulate matter stoichiometry in this region, causing a decline in nutritional quality for grazing zooplankton (Martiny et al, 2013;Rembauville et al, 2016a) and subsequent flow on effects throughout the food web (Finkel et al, 2010). Ocean acidification will also cause declines in carbonate saturation, affecting coccolithophore calcification, resulting in greater surface pCO 2 uptake and decreased carbon export.…”

Unveiling the Si cycle using isotopes in an iron-fertilized zone of the Southern Ocean: from mixed-layer supply to export

“…A two-step preconcentration procedure adapted from the MAGIC method (Karl and Tien, 1992;Reynolds et al, 2006) was performed on seawater samples to increase H 4 SiO 4 concentration and reduce the anionic matrix that could interfere with Si during isotopic analysis (e.g. sulfates, SO 2− 4 ; Hughes et al, 2011).…”