Primary Production in the Kara, Laptev, and East Siberian Seas.

Understanding of the primary production of phytoplankton in the Kara Sea (KS), the Laptev Sea (LS), and the East Siberian Sea (ESS) remains limited, despite the recognized importance of phytoplankton in the Arctic Ocean. To address this knowledge gap, we conducted three NABOS (Nansen and Amundsen Basins Observational System) expeditions in 2013, 2015, and 2018 to measure in situ primary production rates using a 13C-15N dual-tracer method and examine their major controlling factors. The main goals in this study were to investigate regional heterogeneity in primary production and derive its contemporary ranges in the KS, LS, and ESS. The daily primary production rates in this study (99 ± 62, 100 ± 77, and 56 ± 35 mg C m-2 d-1 in the KS, LS, and ESS, respectively) are rather different from the values previously reported in each sea mainly because of spatial and regional differences. Among the three seas, a significantly lower primary production rate was observed in the ESS in comparison to those in the KS and LS. This is likely mainly because of regional differences in freshwater content based on the noticeable relationship (Spearman, rs = -0.714, p < 0.05) between the freshwater content and the primary production rates observed in this study. The contemporary ranges of the annual primary production based on this and previous studies are 0.96-2.64, 0.72-50.52, and 1.68-16.68 g C m-2 in the KS, LS, and ESS, respectively. Further intensive field measurements are warranted to enhance our understanding of marine microorganisms and their community-level responses to the currently changing environmental conditions in these poorly studied regions of the Arctic Ocean.

Spatiotemporal transitions in Pseudo-nitzschia species assemblages and domoic acid along the Alaska coast.

The toxic diatom genus Pseudo-nitzschia is distributed from equatorial to polar regions and is comprised of >57 species, some capable of producing the neurotoxin domoic acid (DA). In the Pacific Arctic Region spanning the Bering, Chukchi, and Beaufort seas, DA is recognized as an emerging human and ecosystem health threat, yet little is known about the composition and distribution of Pseudo-nitzschia species in these waters. This investigation characterized Pseudo-nitzschia assemblages in samples collected in 2018 during summer (August) and fall (October-November) surveys as part of the Distributed Biological Observatory and Arctic Observing Network, encompassing a broad geographic range (57.8° to 73.0°N, -138.9° to -169.9°W) and spanning temperature (-1.79 to 11.7°C) and salinity (22.9 to 32.9) gradients associated with distinct water masses. Species were identified using a genus-specific Automated Ribosomal Intergenic Spacer Analysis (ARISA). Seventeen amplicons were observed; seven corresponded to temperate, sub-polar, or polar Pseudo-nitzschia species based on parallel sequencing efforts (P. arctica, P. delicatissima, P. granii, P. obtusa, P. pungens, and two genotypes of P. seriata), and one represented Fragilariopsis oceanica. During summer, particulate DA (pDA; 4.0 to 130.0 ng L-1) was observed in the Bering Strait and Chukchi Sea where P. obtusa was prevalent. In fall, pDA (3.3 to 111.8 ng L-1) occurred along the Beaufort Sea shelf coincident with one P. seriata genotype, and south of the Bering Strait in association with the other P. seriata genotype. Taxa were correlated with latitude, longitude, temperature, salinity, pDA, and/or chlorophyll a, and each had a distinct distribution pattern. The observation of DA in association with different species, seasons, geographic regions, and water masses underscores the significant risk of Amnesic Shellfish Poisoning (ASP) and DA-poisoning in Alaska waters.

Fluvial influence on the biochemical composition of particulate organic matter in the Laptev and Western East Siberian seas during 2015.

Here, we investigated the elemental (C/N ratio) and isotopic signatures (δ13C) and major biomolecules (carbohydrates, proteins, and lipids) and their relative abundance (i.e., the biochemical composition) in particulate organic matter (POM) to assess their origin and fate in the Laptev and western East Siberian seas during late summer/fall of 2015. In addition, we compared our results with the summer data of 2013 collected from Laptev and northwestern East Siberian seas. In accordance with the observed hydrological structure (i.e., a northward, warmer, diluted freshwater plume than previously observed in 2013), the more depleted δ13C (-28.2 ± 0.9‰) and higher C/N ratio (10.8 ± 2.0) than those of 2013 signalled that fluvially released terrestrial organic carbon (TerrOC) was the main source of the POM, unlike in 2013, when phytoplankton was the dominant source (δ13C = -24.9 ± 1.0‰, C/N ratio = 7.6 ± 2.4; Ahn et al., 2019). During the offshore transport of heterogeneous TerrOC, carbohydrates seem to be the primary contributor to the bulk POM as a result of selective degradation and hydrodynamic sorting. Despite the TerrOC-dominated system in 2015, some marine influence was also found. The estimated phytoplankton biomass was low and comparable among the study sites. In addition, the presence of resting spores and high ammonium concentrations within the water column may suggest senescent and, to some extent, degrading conditions of the resident phytoplankton. In this regard, carbohydrate concentrations and freshwater content were significantly correlated (r = 0.79, p < 0.01), suggesting that carbohydrates are useful inferences of freshwater within overall study sites, at least when the marine influence is similar or low.