Arctic (Svalbard Islands) Active and Exported Diatom Stocks and Cell Health Status

Abstract

<p><strong>Abstract.</strong> Diatoms tend to dominate the Arctic spring bloom, a key event in the ecosystem. Large sinking of diatoms is expected at the end of the bloom driven by deteriorated cell status associated to nutrients (silicon) depletion. However, there are few reports on the status of diatoms' health during Arctic blooms and its possible role on sedimentary fluxes. Here we quantify, using the Bottle-Net, Arctic diatom stocks below and above the photic layer and assess their cell health status. The communities were sampled around the Svalbard Islands and encompassed a broad diversity of conditions and bloom stages. About 1/4 (mean±SE 24.2 ± 6.7 %) of the total water column (max. 415 m) diatom stock was found below the photic layer, indicating significant sinking of diatoms in the area. The fraction of living diatom cells in the photic layer averaged 59.4 ± 6.3 % but showed the highest percentages (72.0 %) in stations supporting active blooms. In contrast, populations below the photic layer were dominated by dead cells (20.8 ± 4.9 % living cells). The percentage of diatom’s stock found below the photic layer was negatively related to the percentage of living diatoms in the surface, indicating that healthy populations remained in the surface layer. An experiment on board in a tall (1.35 m) sedimentation column confirmed that dead diatom cells from the Arctic community sink faster that living ones. Also, diatoms cell mortality increased in darkness, showing an averaged half life of 1.025 ± 0.075 d<sup>−1</sup>. The results conform to a conceptual model where diatoms grow during the bloom until silicic acid stocks are depleted, and support a link between diatom cell health status and sedimentation fluxes in the Arctic. Healthy arctic phytoplankton communities remained at the photic layer, whereas dying communities exported a large fraction of the biomass to the aphotic zone, fuelling carbon sequestration and benthic ecosystems.</p>

The Influence of Sea Ice Cover and Atlantic Water Advection on Annual Particle Export North of Svalbard

“…(Figure 5). Diatoms are often associated with high export events in the Arctic Ocean as their ability to aggregate increases their sinking speed, especially under nutrient‐limited conditions (Agustí et al., 2020; Roca‐Martí et al., 2017; Smetacek, 1985). Previous studies in the Arctic seasonal ice zone where ice melt induces stratified conditions reported the build‐up of large algal biomasses and high vertical POC and chl a fluxes (Andreassen & Wassmann, 1998; Dybwad et al., 2021; Wassmann & Reigstad, 2011).…”

“…Algal bloom build‐up and composition has direct consequences on the export of organic carbon to depth. Single‐celled and chain‐forming diatoms sink rapidly due to their silicified cell walls and are often associated with high export fluxes of carbon (Boyd & Newton, 1999; Dybwad et al., 2021), especially on termination of their growth phase (Agustí et al., 2020; Smetacek, 1985). By contrast, the small flagellate Phaeocystis pouchetii sinks slowly and contributes to lower carbon export efficiency (Reigstad & Wassmann, 2007; Wolf et al., 2016).…”