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

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.

Response to Sea Ice Melt Indicates High Seeding Potential of the Ice Diatom Thalassiosira to Spring Phytoplankton Blooms: A Laboratory Study on an Ice Algal Community From the Sea of Okhotsk

“…High sinking rates are more often observed at low growth rates and in senescent cells when the energy becomes less sufficient (Smetacek, 1985;Waite et al, 1992;Durkin et al, 2016). Silicic acid limitation terminated the spring diatom bloom and triggered cell death, thus a fast sinking out of the photic layer in the Arctic near Greenland and Svalbard islands (Krause et al, 2019;Agustí et al, 2020). Our data suggests that the low energy availability and cell death due to inefficient photosynthesis under melt stress contributed to the fast sedimentation of some pennate ice diatoms, hence reducing their seeding effect to the open-water bloom.…”

Carbon Export in the Seasonal Sea Ice Zone North of Svalbard From Winter to Late Summer

“…Black lines: rough visual interpolation of seasonal trend using diatoms (solid) and Phaeocystis (dashed) blooms. Agustí et al, 2020). The increase in export fluxes from stations T1 to T4 (Figure 8), which were separated in time but similar in location, may suggest that once silicic acid became limiting (Figure 2), the growth conditions for these diatoms were reduced and subsequently increased their sinking velocity (Agustí et al, 2020).…”

“…This was the case at station C3, which had relatively low vertical export accounting for loss rates of only 0.7% POC day −1 and 0.6% Chl a day −1 at 100 m, allowing for the buildup of the highest Chl a (532 mg m −2 ) and POC (23 g C m −2 ) standing stocks observed in the study. On the other hand, the highest vertical export fluxes are generally associated with intense blooms at the end of their growth phase when nutrients are exhausted (Agustí et al, 2020), grazing pressure is low, and the community composition allows for aggregation or mass flux of ungrazed cells (Wassmann et al, 2006a;Henson et al, 2019). The vertical export and loss rates varied greatly between the stations sampled during blooms, which highlights the need to examine the composition and vertical profiles of the exported material in order to outline the processes involved in the sedimentation of the blooms.…”

“…Agustí et al, 2020). The increase in export fluxes from stations T1 to T4 (Figure 8), which were separated in time but similar in location, may suggest that once silicic acid became limiting (Figure 2), the growth conditions for these diatoms were reduced and subsequently increased their sinking velocity (Agustí et al, 2020). This is further supported by the deepening of the Chl a maxima from 10 to 25 m during the 8 days between sampling at stations T1 and T4, a reduction in surface silicic acid, and a very low mean light transmittance of 0.4 and 0.2% at these stations, respectively (Katlein et al, 2019).…”

Effect of Visible Light on Surface-Attached and Suspended Heterotrophic Bacteria in a Typical Household Rainwater Harvesting Tank

“…Visible light induces the production of organic compounds necessary for the metabolism of heterotrophic bacteria as seen in Figure 3. Augusti et al ( 2020) and Music et al ( 2019) have emphasized that light is a primary energy source for autotrophic organisms, linking photosynthesis directly to the growth of biomass and uptake of nutrients [37,38]. In addition, the secretion of the extracellular substance bu SAB might also contribute to the disparity between the amount of total biomass in TES and TNES.…”