Agusti, Susana Krause, Jeffrey W. Marquez, Israel A. Wassmann, Paul Kristiansen, Svein Duarte, Carlos M.
Biogeosciences, (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 aphotic zone, fueling carbon sequestration to the
mesopelagic and material to benthic ecosystems.