Abstract
Abstract. 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.
Highlights
Diatoms can support most of the Arctic primary production during the spring phytoplankton bloom (Krause et al, 2018), the key event setting the ecosystem and driving the intense carbon-uptake characteristic of the Arctic (Vaquer-Sunyer et al, 2013)
Healthy Arctic phytoplankton communities remained at the photic layer, whereas the physiologically compromised communities exported a large fraction of the biomass to the aphotic zone, fueling carbon sequestration to the mesopelagic and material to benthic ecosystems
The results presented confirm that active and healthy diatom populations, as those actively growing during the spring bloom, are associated with relatively small stocks of fastsinking diatoms
Summary
Diatoms can support most of the Arctic primary production during the spring phytoplankton bloom (Krause et al, 2018), the key event setting the ecosystem and driving the intense carbon-uptake characteristic of the Arctic (Vaquer-Sunyer et al, 2013). Silicic acid concentrations [Si(OH)4] are characteristically low in the European sector of the Arctic, due to the inflow of Si-depleted Atlantic water (Rey, 2012). S. Agustí et al.: Arctic active and exported diatoms southern Greenland, whereby diatom depletion of [Si(OH)4] collapsed the bloom with ∼ 4 μmol L−1 remaining nitrate (Krause et al, 2019)
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