Arctic (Svalbard islands) active and exported diatom stocks and cell health status

• Published in: Biogeosciences
• Main Authors: Agusti, Susana, Krause, Jeffrey W., Marquez, Israel A., Wassmann, Paul, Kristiansen, Svein, Duarte, Carlos M.
• Other Authors: Biological and Environmental Sciences and Engineering (BESE) Division, Marine Science Program, Red Sea Research Center (RSRC), Dauphin Island Sea Lab, Dauphin Island, AL 36528-4603, USA, Department of Marine Sciences, University of South Alabama, Mobile, AL 36688-0002, USA, Department of Arctic and Marine Biology, UiT The Arctic University of Norway, 9037 Tromsø, Norway, Arctic Research Centre, Department of Bioscience, Aarhus University, C.F. Mollers Alle 8, 8000 Aarhus C, Denmark
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Copernicus GmbH 2018
• Subjects: Arctic; Phytoplankton; Svalbard
• Online Access: http://hdl.handle.net/10754/629932; https://doi.org/10.5194/bg-17-35-2020

Diatoms tend to dominate the Arctic spring phytoplankton bloom, a key event in the ecosystem including a rapid decline in surface-water pCO2. While a mass sedimentation event of diatoms at the bloom terminus is commonly observed, there are few reports on the status of diatoms' health during Arctic blooms and its possible role on sedimentary fluxes. Thus, we examine the idea that the major diatom-sinking event which occurs at the end of the regional bloom is driven by physiologically deteriorated cells. Here we quantify, using the Bottle-Net, Arctic diatom stocks below and above the photic zone and assess their cell health status. The communities were sampled around the Svalbard islands and encompassed pre- to post-bloom conditions. A mean of 24.2±6.7 % SE (standard error) of the total water column (max. 415 m) diatom standing stock was found below the photic zone, indicating significant diatom sedimentation. The fraction of living diatom cells in the photic zone averaged 59.4±6.3 % but showed the highest mean 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 diatoms' standing 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. Shipboard manipulation experiments demonstrated that (1) dead diatom cells sank faster than living cells, and (2) diatom cell mortality increased in darkness, showing an average half-life among diatom groups of 1.025±0.075 d. The results conform to a conceptual model where diatoms grow during the bloom until resources are depleted and supports a link between diatom cell health status (affected by multiple factors) and sedimentation fluxes in the Arctic. Healthy Arctic phytoplankton communities remained at the photic layer, whereas the physiologically compromised (e.g., dying) communities exported a large fraction of the biomass to the ...