Janssens Julie

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Janssens
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Julie
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  • Article
    Sea ice meltwater and Circumpolar Deep Water drive contrasting productivity in three Antarctic polynyas
    (American Geophysical Union, 2019-03-28) Moreau, Sebastien ; Lannuzel, Delphine ; Janssens, Julie ; Arroyo, Mar C. ; Corkill, Matthew ; Cougnon, Eva ; Genovese, Cristina ; Legresy, Benoit ; Lenton, Andrew ; Puigcorbé, Viena ; Ratnarajah, Lavenia ; Rintoul, Stephen R. ; Roca-Martí, Montserrat ; Rosenberg, Mark ; Shadwick, Elizabeth H. ; Silvano, Alessandro ; Strutton, Peter G. ; Tilbrook, Bronte
    In the Southern Ocean, polynyas exhibit enhanced rates of primary productivity and represent large seasonal sinks for atmospheric CO2. Three contrasting east Antarctic polynyas were visited in late December to early January 2017: the Dalton, Mertz, and Ninnis polynyas. In the Mertz and Ninnis polynyas, phytoplankton biomass (average of 322 and 354 mg chlorophyll a (Chl a)/m2, respectively) and net community production (5.3 and 4.6 mol C/m2, respectively) were approximately 3 times those measured in the Dalton polynya (average of 122 mg Chl a/m2 and 1.8 mol C/m2). Phytoplankton communities also differed between the polynyas. Diatoms were thriving in the Mertz and Ninnis polynyas but not in the Dalton polynya, where Phaeocystis antarctica dominated. These strong regional differences were explored using physiological, biological, and physical parameters. The most likely drivers of the observed higher productivity in the Mertz and Ninnis were the relatively shallow inflow of iron‐rich modified Circumpolar Deep Water onto the shelf as well as a very large sea ice meltwater contribution. The productivity contrast between the three polynyas could not be explained by (1) the input of glacial meltwater, (2) the presence of Ice Shelf Water, or (3) stratification of the mixed layer. Our results show that physical drivers regulate the productivity of polynyas, suggesting that the response of biological productivity and carbon export to future change will vary among polynyas.
  • Article
    Distribution and export of particulate organic carbon in East Antarctic coastal polynyas
    ( 2022-11-03) Ratnarajah, Lavenia ; Puigcorbé, Viena ; Moreau, Sébastien ; Roca-Martí, Montserrat ; Janssens, Julie ; Corkill, Matthew ; Duprat, Luis ; Genovese, Cristina ; Lieser, Jan ; Masqué, Pere ; Lannuzel, Delphine
    Polynyas represent regions of enhanced primary production because of the low, or absent, sea-ice cover coupled with the proximity of nutrient sources. However, studies throughout the Southern Ocean suggest elevated primary production does not necessarily result in increased carbon export. Three coastal polynyas in East Antarctica and an off-shelf region were visited during the austral summer from December 2016 to January 2017 to examine the vertical distribution and concentration of particulate organic carbon (POC). Carbon export was also examined using thorium-234 (234Th) as a proxy at two of the polynyas. Our results show that concentrations and integrated POC stocks were higher within the polynyas compared to the off-shelf sites. Within the polynyas, vertical POC concentrations were higher in the Mertz and Ninnis polynyas compared to the Dalton polynya. Similarly, higher carbon export was measured in the diatom-dominated Mertz polynya, where large particles (>53 μm) represented a significant fraction of the particulate 234Th and POC (average 50% and 39%, respectively), compared to the small flagellate-dominated Dalton polynya, where almost all the particulate 234Th and POC were found in the smaller size fraction (1–53 μm). The POC to Chlorophyll-a ratios suggest that organic matter below the mixed layer in the polynyas consisted largely of fresh phytoplankton at this time of the year. In combination with a parallel study on phytoplankton production at these sites, we find that increased primary production at these polynyas does lead to greater concentrations and export of POC and a higher POC export efficiency.Elevated primary production leads to enhanced vertical concentration and export of particulate organic carbon (POC) in coastal polynyas in austral summer.POC inventories, export fluxes and export efficiencies vary significantly between polynyas. POC:Chl-a ratios within polynyas suggest that organic matter below the mixed layer consists largely of fresh phytoplankton at this time of the year.