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

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.

Anoxia markers in the photic layer of the sedimentation basin in the oils of Tatarstan

It is established that the source rocks for the oils of Tatarstan was formed under similar conditions in the presence of anoxia in the photic layer of the sedimentation basin that existed during the whole time of accumulation of the initial organic matter at a considerable thickness of the layer infected with hydrogen sulphide. It is not excluded that disturbances in the existence of anoxia existed or thickness of the infected layer was reduced that resulted in decreasing of anoxia markers content for some oils. Biocenoses of microflora in the sedimentation layer contaminated by hydrogen sulfide demonstrate high stability. Transformation conditions of the buried organic matter in diagenesis and catagenesis were uniform, which manifests itself in approximately the same ratio between the depth of the complete hydrogenation and cyclization of the initial polyene aromatic carotenoids, the closeness of the molecular mass distributions of the C-C bond destruction products of components C40 and relatively small differences in the total concentration of monoaromatic compounds and the sum of hydrogenated analogs of the initial aromatic carotenoids. Analysis of content of the components – anoxia markers throughout the whole Devonian section is needed to establish the complexes that generated oils of Tatarstan.

Longitudinal contrast in turbulence along a ∼ 19° S section in the Pacific and its consequences for biogeochemical fluxes

Microstructure measurements were performed along the OUTPACE longitudinal transect in the tropical Pacific (Moutin and Bonnet, 2015). Small-scale dynamics and turbulence in the first 800 m surface layer were characterized based on hydrographic and current measurements at fine vertical scale and turbulence measurements at centimeter scale using a vertical microstructure profiler. The possible impact of turbulence on biogeochemical budgets in the surface layer was also addressed in this region of increasing oligotrophy to the east. The dissipation rate of turbulent kinetic energy, ϵ, showed an interesting contrast along the longitudinal transect with stronger turbulence in the west, i.e., the Melanesian Archipelago, compared to the east, within the South Pacific Subtropical Gyre, with a variation of ϵ by a factor of 3 within [100–500 m]. The layer with enhanced turbulence decreased in vertical extent travelling eastward. This spatial pattern was correlated with the energy level of the internal wave field, higher in the west compared to the east. The difference in wave energy mostly resulted from enhanced wind power input into inertial motions in the west. Moreover, three long-duration stations were sampled along the cruise transect, each over three inertial periods. The analysis from the western long-duration station gave evidence of an energetic baroclinic near-inertial wave that was responsible for the enhanced ϵ, observed within a 50–250 m layer, with a value of 8×10-9 W kg−1, about 8 times larger than at the eastern long-duration stations. Averaged nitrate turbulent diffusive fluxes in a 100 m layer below the top of the nitracline were about twice larger west of 170∘ W due to the higher vertical diffusion coefficient. In the photic layer, the depth-averaged nitrate turbulent diffusive flux strongly decreased eastward, with an averaged value of 11 µmolm-2d-1 west of 170∘ W compared with the 3 µmolm-2d-1 averaged value east of 170∘ W. Contrastingly, phosphate turbulent diffusive fluxes were significantly larger in the photic layer. This input may have an important role in sustaining the development of N2-fixing organisms that were shown to be the main primary contributors to the biological pump in the area. The time–space intermittency of mixing events, intrinsic to turbulence, was underlined, but its consequences for micro-organisms would deserve a dedicated study.

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

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−1. 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.

N₂ fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

We performed nitrogen (N) budgets in the photic layer of three contrasting stations representing different trophic conditions in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (February–March 2015). Using a Lagrangian strategy, we sampled the same water mass for the entire duration of each long-duration (5 days) station, allowing us to consider only vertical exchanges for the budgets. We quantified all major vertical N fluxes both entering (N2 fixation, nitrate turbulent diffusion, atmospheric deposition) and leaving the photic layer (particulate N export). The three stations were characterized by a strong nitracline and contrasted deep chlorophyll maximum depths, which were lower in the oligotrophic Melanesian archipelago (MA, stations LD A and LD B) than in the ultra-oligotrophic waters of the South Pacific Gyre (SPG, station LD C). N2 fixation rates were extremely high at both LD A (593 ± 51 µmol N m−2 d−1) and LD B (706 ± 302 µmol N m−2 d−1), and the diazotroph community was dominated by Trichodesmium. N2 fixation rates were lower (59 ± 16 µmol N m−2 d−1) at LD C, and the diazotroph community was dominated by unicellular N2-fixing cyanobacteria (UCYN). At all stations, N2 fixation was the major source of new N (> 90 %) before atmospheric deposition and upward nitrate fluxes induced by turbulence. N2 fixation contributed circa 13–18 % of primary production in the MA region and 3 % in the SPG water and sustained nearly all new primary production at all stations. The e ratio (e ratio = particulate carbon export ∕ primary production) was maximum at LD A (9.7 %) and was higher than the e ratio in most studied oligotrophic regions (< 5 %), indicating a high efficiency of the WTSP to export carbon relative to primary production. The direct export of diazotrophs assessed by qPCR of the nifH gene in sediment traps represented up to 30.6 % of the PC export at LD A, while their contribution was 5 and < 0.1 % at LD B and LD C, respectively. At the three studied stations, the sum of all N input to the photic layer exceeded the N output through organic matter export. This disequilibrium leading to N accumulation in the upper layer appears as a characteristic of the WTSP during the summer season.