Variability of size-fractionated chlorophyll a in the high-latitude Arctic Ocean in summer 2020

The size structure of phytoplankton has considerable effects on the energy flow and nutrient cycling in the marine ecosystem, and thus is important to marine food web and biological pump. However, its dynamics in the high-latitude Arctic Ocean, particularly ice-covered areas, remain poorly understood. We investigated size-fractionated chlorophyll a (Chl a) and related environmental parameters in the highly ice-covered Arctic Ocean during the summer of 2020, and analyzed the relationship between Chl a distribution and water mass through cluster analysis. Results showed that inorganic nutrients were typically depleted in the upper layer of the Canada Basin region, and that phytoplankton biomass was extremely low (mean= 0.05 ± 0.18 mg·m−3) in the near-surface layer (upper 25 m). More than 80% of Chl a values were <0.1 mg·m−3 in the water column (0–200 m), but high values appeared at the ice edge or in corresponding ice areas on the shelf. Additionally, the mean contribution of both nanoplankton (2–20 μm) (41%) and picoplankton (<2 μm) (40%) was significantly higher than that of microplankton (20–200 μm) (19%). Notably, the typical subsurface chlorophyll maximum (0.1 mg·m−3) was found north of 80°N, where the concentration of sea ice reached approximately 100%. The Chl a profile results showed that the deep chlorophyll maximum of total-, micro-, nano-, and picoplankton was located at depth of 40, 39, 41, and 38 m, respectively, indicating that nutrients are the primary factor limiting phytoplankton growth in the ice-covered Arctic Ocean during summer. These phenomena suggest that, despite the previous literatures pointing to significant light limitation under the Arctic ice, the primary limiting factor for phytoplankton in summer is still nutrient

Characteristics of hydrogen/oxygen isotopes in water masses and implications for spatial distribution of freshwater in the Amundsen Sea, Southern Ocean

Antarctica’s marginal seas are of great importance to atmosphere–ocean–ice interactions and are sensitive to global climate change. Multiple factors account for the freshwater budget in these regions, including glacier melting, seasonal formation/decay of sea ice, and precipitation. Hydrogen (H) and oxygen (O) isotopes represent useful proxies for determining the distribution and migration of water masses. We analyzed the H and O isotopic compositions of 190 seawater samples collected from the Amundsen Sea during the 34th Chinese Antarctic Research Expedition in 2017/2018. The upper-oceanic structure (3%) of freshwater generally lie in the upper ~50 m and extend from Antarctica to ~65°S in the meridional direction (anomalously low freshwater proportion occurred between 68°S and 71°S). Winter Water mainly occupied the layer between 50 and 150 m south of 71°S in the western Amundsen Sea. The water structure and spatial distribution of freshwater in the upper Amundsen Sea were found influenced mainly by the rates of basal and surficial melting of ice shelves, seasonal alternation of sea ice melt/formation, wind forcing, and regional bathymetry. Owing to the distance between heavy sea ice boundary (HSIB) and ice shelves is much shorter in the western HSIB than the east HSIB, the western part of the heavy sea ice boundary includes a higher proportion of freshwater than the eastern region. This study, which highlighted the distribution and extent of freshwater derived from ice (ice shelves and sea ice) melt, provides important evidence that the offshore drift pathway of cold and fresh Antarctic continental shelf water is likely interrupted by upwelled UCDW in the Amundsen Sea

Spatial distribution and diversity of the heterotrophic flagellates in the Cosmonaut Sea, Antarctic

As predators of bacteria and viruses and as food sources for microzooplankton, heterotrophic flagellates (HFs) play an important role in the marine micro-food web. Based on the global climate change’s impact on marine ecosystems, particularly sea ice melting, we analyzed the community composition and diversity of heterotrophic flagellates, focusing on the Antarctic Cosmonaut Sea. During the 36th China Antarctic research expedition (2019-2020), we collected seawater samples, subsequently analyzing HFs through IlluminaMiSeq2000 sequencing to assess community composition and diversity. Notable variations in HFs abundance were observed between the western and eastern sectors of the Cosmonaut Sea, with a distinct concentration at a 100-meter water depth. Different zones exhibited diverse indicators and dominants taxa influenced by local ocean currents. Both the northern Antarctic Peninsula and the western Cosmonaut Sea, where the Weddell Eddy and Antarctic Land Slope Current intersect, showcased marine stramenopiles as dominant HFs species. Our findings offer insights into dominant taxa, spatial distribution patterns among heterotrophic flagellates, correlations between taxa distribution and environmental factors, and the exploration of potential indicator taxa

Biogeographic traits of dimethyl sulfide and dimethylsulfoniopropionate cycling in polar oceans

Background: Dimethyl sulfide (DMS) is the dominant volatile organic sulfur in global oceans. The predominant source of oceanic DMS is the cleavage of dimethylsulfoniopropionate (DMSP), which can be produced by marine bacteria and phytoplankton. Polar oceans, which represent about one fifth of Earth’s surface, contribute significantly to the global oceanic DMS sea-air flux. However, a global overview of DMS and DMSP cycling in polar oceans is still lacking and the key genes and the microbial assemblages involved in DMSP/DMS transformation remain to be fully unveiled. Results: Here, we systematically investigated the biogeographic traits of 16 key microbial enzymes involved in DMS/DMSP cycling in 60 metagenomic samples from polar waters, together with 174 metagenome and 151 metatranscriptomes from non-polar Tara Ocean dataset. Our analyses suggest that intense DMS/DMSP cycling occurs in the polar oceans. DMSP demethylase (DmdA), DMSP lyases (DddD, DddP, and DddK), and trimethylamine monooxygenase (Tmm, which oxidizes DMS to dimethylsulfoxide) were the most prevalent bacterial genes involved in global DMS/DMSP cycling. Alphaproteobacteria (Pelagibacterales) and Gammaproteobacteria appear to play prominent roles in DMS/DMSP cycling in polar oceans. The phenomenon that multiple DMS/DMSP cycling genes co-occurred in the same bacterial genome was also observed in metagenome assembled genomes (MAGs) from polar oceans. The microbial assemblages from the polar oceans were significantly correlated with water depth rather than geographic distance, suggesting the differences of habitats between surface and deep waters rather than dispersal limitation are the key factors shaping microbial assemblages involved in DMS/DMSP cycling in polar oceans. Conclusions: Overall, this study provides a global overview of the biogeographic traits of known bacterial genes involved in DMS/DMSP cycling from the Arctic and Antarctic oceans, laying a solid foundation for further studies of DMS/DMSP cycling in polar ocean microbiome at the enzymatic, metabolic, and processual levels. 6bJ8nkA7sq-T64bgHw5GYLVideo Abstrac

Overview of the MOSAiC expedition: Physical oceanography

Arctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present alongdrift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean

Auxiliary data from SIMBA-type sea ice mass balance buoy 2019T56

Geographic position, barometric pressure, tilt and compass

Temperature and heating induced temperature difference measurements from SIMBA-type sea ice mass balance buoy 2019T56, deployed during MOSAiC 2019/20

Temperature and heating-induced temperature differences were measured along a chain of thermistors. SIMBA 2019T56 (a.k.a. FMI_05_06, IRIDIUM number 300234065176750) is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the 1st leg of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) in November 2019. The buoy was deployed at the ~2 km from the ship at the SW direction with initial thicknesses of snow and ice of 0.18 and 0.42 m, respectively, on 2 November 2019. The thermistor chain was 5 m long and included 241 sensors with a regular spacing of 2cm. The depths for the sensors are 80 to -398 cm, referring to the initial interface between snow and ice. The last sensor was used to measure the air temperature at 1 m above the initial snow surface. The resulting time series describes the evolution of temperature and temperature differences after two heating cycles of 30 and 120 s as a function of depth and time between 2 November 2019 and 2 July 2020 in sample intervals of 6 hours for temperature and 24 hours for temperature differences. In addition to temperature, geographic position, barometric pressure, tilt and compass were measured

Temperature measurements from SIMBA-type sea ice mass balance buoy 2019T56

Temperature profile from atmosphere through snow and ice into the ocean

Heating induced temperature difference measurements from SIMBA-type sea ice mass balance buoy 2019T62: 30 s after the heating cycle

Temperature rising around the thermistors after a weak heating applied to each sensor daily after 30 s

Temperature and heating induced temperature difference measurements from SIMBA-type sea ice mass balance buoy 2019T62, deployed during MOSAiC 2019/20

Temperature and heating-induced temperature differences were measured along a chain of thermistors. SIMBA 2019T62 (a.k.a. PRIC_09_01,IRIDIUM number 300234068706290) is an autonomous instrument that was installed on drifting sea ice in the Arctic Ocean during the 1st leg of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) in October 2019. The buoy was deployed at the second year coring site of the MOSAiC central observatory with initial thicknesses of snow and ice of 0.18 and 0.80 m, respectively, on 29 October 2019. The thermistor chain was 5 m long and included 241 sensors with a regular spacing of 2 cm. The depths for the sensors are 96 to -382 cm, referring to the initial interface between snow and ice. The last sensor was used to measure the air temperature at 1 m above the initial snow surface. The resulting time series describes the evolution of temperature and temperature differences after two heating cycles of 30 and 120 s as a function of place, depth and time between 29 October 2019 and 28 July 2020 in sample intervals of 6 hours for temperature and 24 hours for temperature differences. In addition to temperature, geographic position, barometric pressure, tilt and compass were measured until 1 August 2020