In Situ Rates of Carbon and Nitrogen Uptake by Phytoplankton and the Contribution of Picophytoplankton in Kongsfjorden, Svalbard

Plankton community changes during the last 124 000 years in the subarctic Bering Sea derived from sedimentary ancient DNA

Current global warming results in rising sea-water temperatures, and the loss of sea ice in Arctic and subarctic oceans impacts the community composition of primary producers with cascading effects on the food web and potentially on carbon export rates. This study analyzes metagenomic shotgun and diatom rbcL amplicon sequencing data from sedimentary ancient DNA of the subarctic western Bering Sea that records phyto- and zooplankton community changes over the last glacial-interglacial cycles, including the last interglacial period (Eemian). Our data show that interglacial and glacial plankton communities differ, with distinct Eemian and Holocene plankton communities. The generally warm Holocene period is dominated by picosized cyanobacteria and bacteria-feeding heterotrophic protists, while the Eemian period is dominated by eukaryotic picosized chlorophytes and Triparmaceae. By contrast, the glacial period is characterized by microsized phototrophic protists, including sea ice-associated diatoms in the family Bacillariaceae and co-occurring diatom-feeding crustaceous zooplankton. Our deep-time record of plankton community changes reveals a long-term decrease in phytoplankton cell size coeval with increasing temperatures, resembling community changes in the currently warming Bering Sea. The phytoplankton community in the warmer-than-present Eemian period is distinct from modern communities and limits the use of the Eemian as an analog for future climate scenarios. However, under enhanced future warming, the expected shift toward the dominance of small-sized phytoplankton and heterotrophic protists might result in an increased productivity, whereas the community's potential of carbon export will be decreased, thereby weakening the subarctic Bering Sea's function as an effective carbon sink.

Marine Nitrogen Fixation and Phytoplankton Ecology

Many oceans are currently undergoing rapid changes in environmental conditions such as warming temperature, acidic water condition, coastal hypoxia, etc [...]

Spatial distribution and origin of organic matters in an Arctic fjord system based on lipid biomarkers (n-alkanes and sterols)

The concentration of n-alkanes (C₁₇-C₃₅) and sterols in marine particulate matter were investigated to trace the origin of organic carbon in Kongsfjorden in early spring (April). The spatial distributions of environmental factors (seawater temperature, salinity, density, turbidity, chlorophyll a (chl. a) and particulate organic carbon (POC) concentrations) and the cell density of phytoplankton differed between the inner and outer fjord regions. In addition, brassicasterol, diatom biomarker, showed a high concentration in the outer fjord and positive correlations with the chl. a and POC concentrations in the water column. In contrast, some sterols originating from terrestrial organic matter (OM), such as stigmasterol and campesterol, showed relatively higher concentrations in the inner fjord than in the outer fjord. Based on the distance-based redundancy analysis (db-RDA) result, the distributions of organic compounds are predominantly controlled by the water density and the POC and chl. a concentrations, and these distributions allowed us to divide the inner and outer fjord regions. However, the hierarchical clustering of principal components (HCPC) results obtained based on principal component analysis (PCA) using lipid biomarkers (C₁₇-C₃₅ alkanes and sterols) and environmental factors indicated that the clusters were distinguished by surface (0 m) and subsurface (>4 m) seawater samples rather than by any regional division. Notably, the concentration of relatively short-chain alkanes (average chain length (ACL): 24.6 ± 3.7) without a carbon preference for odd numbers (carbon preference index (CPI): 0.97 ± 0.11) in the sea surface layer was significantly higher than that of subsurface seawater (ACL: 31.1 ± 0.5 and CPI: 1.06 ± 0.03) in the early spring. This suggests the potential of these compounds as indicators for tidewater glacier-derived OM and freshwater input by snow melt into the fjord system. Hence, these results demonstrate that the distributions of lipid biomarkers in the water column possibly provide important information for a comprehensive understanding of the origin and transport of OM in an Arctic fjord.

The Interplay of Mycosporine-like Amino Acids between Phytoplankton Groups and Northern Krill (Thysanoessa sp.) in a High-Latitude Fjord (Kongsfjorden, Svalbard)

We investigated pigment and mycosporine-like amino acid (MAA) concentrations of phytoplankton and Northern krill (Thysanoessa sp.) in sub-Arctic Kongsfjorden. Chlorophyll a (Chl-a) concentrations in the surface and middle-layer water were 0.44 μg L⁻¹ (±0.17 μg L⁻¹) and 0.63 μg L⁻¹ (±0.25 μg L⁻¹), respectively. Alloxanthin (Allo, a marker of cryptophytes) was observed at all stations, and its mean values for surface and middle-layer water were 0.09 μg L⁻¹ (±0.05 μg L⁻¹) and 0.05 (±0.02 μg L⁻¹), respectively. The mean MAA-to-Chl-a ratios at the surface (3.31 ± 2.58 μg (μg Chl-a)⁻¹) were significantly higher than those in the middle-layer water (0.88 ± 0.49 μg (μg Chl-a)⁻¹), suggesting that these compounds play an important role in reducing UV photodamage. In gut pigment levels of Northern krill, the most abundant accessory pigment was Allo (2.79 ± 0.33 μg g⁻¹ dry weight; d.w.), as was the accumulation of Chl-a (8.29 ± 1.13 μg g⁻¹ d.w.). The average concentration of MAAs was 1.87 mg g⁻¹ d.w. (±0.88 mg g⁻¹ d.w.) in krill eyes, which was higher than that in all other body parts (0.99 ± 0.41 mg g⁻¹ d.w.), except for the gut. Thysanoessa sp. was found to contain five identified MAAs (shinorine, palythine, porphyra-334, mycosporine-glycine, and M-332) in the krill eye, whereas shinorine and porphyra-334 were only observed in the krill body, not the eyes and gut. These findings suggest that Northern krill accumulate MAAs of various compositions through the diet (mainly cryptophytes) and translocate them among their body parts as an adaptation for photoprotection and physiological demands.