Dynamics of organic matter and bacterial activity in the Fram Strait during summer and autumn

The Arctic Ocean is considerably affected by the consequences of global warming, including more extreme seasonal fluctuations in the physical environment. So far, little is known about seasonality in Arctic marine ecosystems in particular microbial dynamics and cycling of organic matter. The limited characterization can be partially attributed to logistic difficulties of sampling in the Arctic Ocean beyond the summer season. Here, we investigated the distribution and composition of dissolved organic matter (DOM), gel particles and heterotrophic bacterial activity in the Fram Strait during summer and autumn. Our results revealed that phytoplankton biomass influenced the concentration and composition of semi-labile dissolved organic carbon (DOC), which strongly decreased from summer to autumn. The seasonal decrease in bioavailability of DOM appeared to be the dominant control on bacterial abundance and activity, while no temperature effect was determined. Additionally, there were clear differences in transparent exopolymer particles (TEP) and Coomassie Blue stainable particles (CSP) dynamics. The amount of TEP and CSP decreased from summer to autumn, but CSP was relatively enriched in both seasons. Our study therewith indicates clear seasonal differences in the microbial cycling of organic matter in the Fram Strait. Our data may help to establish baseline knowledge about seasonal changes in microbial ecosystem dynamics to better assess the impact of environmental change in the warming Arctic Ocean

Drivers of pelagic and benthic microbial communities on Central Arctic seamounts

Seamounts are abundant features on the seafloor that serve as hotspots and barriers for the dispersal of benthic organisms. The primary focus of seamount ecology has typically been on the composition and distribution of faunal communities, with far less attention given to microbial communities. Here, we investigated the microbial communities in the water column (0-3400 m depth) and sediments (619-3883 m depth, 0-16 cm below seafloor) along the ice-covered Arctic ridge system called the Langseth Ridge. We contextualized the microbial community composition with data on the benthic trophic state (i.e., organic matter, chlorophyll-a content, and porewater geochemistry) and substrate type (i.e., sponge mats, sediments, basaltic pebbles). Our results showed slow current velocities throughout the water column, a shift in the pelagic microbial community from a dominance of Bacteroidia in the 0-10 m depth towards Proteobacteria and Nitrososphaeria below the epipelagic zone. In general, the pelagic microbial communities showed a high degree of similarity between the Langseth Ridge seamounts to a northern reference site. The only notable differences were decreases in richness between ~600 m and the bottom waters (~10 m above the seafloor) that suggest a pelagic-benthic coupling mediated by filter feeding of sponges living on the seamount summits. On the seafloor, the sponge spicule mats, and polychaete worms were the principal source of variation in sedimentary biogeochemistry and the benthic microbial community structure. The porewater signature suggested that low organic matter degradation rates are accompanied by a microbial community typical of deep-sea oligotrophic environments, such as Proteobacteria, Acidimicrobiia, Dehalococcoidia, Nitrospira, and archaeal Nitrososphaeria. The combined analysis of biogeochemical parameters and the microbial community suggests that the sponges play a significant role for pelagic-benthic coupling and acted as ecosystem engineers on the seafloor of ice-covered seamounts in the oligotrophic central Arctic Ocean

Quantifying permafrost organic carbon remineralization after redeposition on the ocean floor, using δ13C and F14C.

Arctic permafrost is a critical global tipping element in a warming climate. Annually, the erosion of coastal permafrost discharges an estimated 5 to 14 Tg of organic carbon (OC) into the Arctic Ocean. Although this previously stored OC has the potential to be reintroduced into the atmosphere, thus accelerating human-induced climate change, little is known about the benthic remineralization processes of permafrost OC after erosion and redeposition on the ocean floor. Our research quantified fluxes of dissolved inorganic carbon (DIC) and analyzed its isotopic composition of nearshore sediments in the Canadian Beaufort Sea, specifically off Herschel Island. Our findings showed a DIC release of 0.217 mmo/m&amp;#178;/d, with an average signature of &amp;#948;13C = -22.44 &amp;#177; 72 &amp;#8240; and F14C = 0.548 &amp;#177; 0.007. Utilizing a model that combines two carbon isotopes, we estimate that approximately 38 &amp;#177; 10% of the released DIC is a result of subsurface degradation of redeposited permafrost OC, with an additional 15 &amp;#177; 12% originating from redeposited active layer OC. Additionally, isotopic endmember analysis was utilized on bacterial membrane lipids from live sedimentary bacteria to determine the relative utilization of OC sources in bacterial communities within shallow subsurface sediment (&lt;25 cm). Our results indicate that, on average, these communities obtain 73 &amp;#177; 10% of their OC from recent marine primary production, 11 &amp;#177; 6% from permafrost OC, and 16 &amp;#177; 11% from active layer OC. This study is the first direct quantitative assessment of the release of permafrost OC into the active carbon cycle after it has been redeposited on the ocean floor, as far as we know. The data suggest that the redeposited permafrost OC is easily accessible and utilized by subsurface bacteria. Considering the immense size and vulnerability of the eroding coastal permafrost OC pool, 27 to 53% of it contributing to benthic DIC fluxes could have a prolonged effect on the world's climate, worsening the climate emergency.</jats:p

LTER HAUSGARTEN 2018 - Long-Term Ecological Research in the Fram Strait, Cruise No. MSM77, September 15 - October 13, 2018, Longyearbyen (Svalbard) - Edinburgh (Scotland)

The 77th cruise of the RV MARIA S. MERIAN contributed to various large national and international research and infrastructure projects (FRAM, ARCHES, INTAROS, ICOS, SIOS) as well as to the research programme PACES-II (Polar Regions and Coasts in the changing Earth System) of the Alfred-Wegener-Institute Helmholtz-Center for Polar and Marine Research (AWI). Investigations within Work Package 4 (Arctic sea ice and its interaction with ocean and ecosystems) of the PACES-II programme, aim at assessing and quantifying ecosystem changes from surface waters to the deep ocean in response to the retreating sea ice, and at exploring the most important (feedback) processes determining temporal and spatial variability. Contributions to the PACES-II Work Package 6 (Large scale variability and change in polar benthic biota and ecosystem functions) include the identification of spatial patterns and temporal trends in relevant benthic community functions, and the development of a comprehensive science community reference collection of observational data. Work carried out within WPs 4 and 6 will support the time-series studies at the LTER (Long-Term Ecological Research) observatory HAUSGARTEN (Fig. 1.1), where we document Global Change induced environmental variations on a polar deep-water ecosystem. This work is carried out in close co-operation between the HGF-MPG Joint Research Group on Deep-Sea Ecology and Technology and the PEBCAO Group (Phytoplankton Ecology and Biogeochemistry in the Changing Arctic Ocean) at AWI as well as the working group Microbial Geochemistry at the GEOMAR and the HGF Young Investigators Group SEAPUMP (Seasonal and regional food web interactions with the biological pump)

Seasonal Dynamics of Organic Matter Turnover in the Arctic Ocean

The Arctic Ocean is extremely susceptible to climate change, which has led to warmer air temperatures, accelerated sea ice loss, and an intensified inflow of Atlantic water masses into the Arctic. Furthermore, the reduction in sea ice has been linked to a prolonged phytoplankton growth season. Fewer days of sea ice cover and thinner sea ice increases the light availability for ice algae and expands the habitat for pelagic phytoplankton. An extension of the growth season prolongs the release of phytoplankton-derived organic matter with consequences for the seasonal carbon cycle. However, the seasonal carbon stock and microbial processes are largely unexplored because many parameters require in situ sampling. A systematic strategy to enhance microbial observations is through time series stations. The long-term ecological research observatory HAUSGARTEN was established in 1999 and is the only pelagic microbial observatory in the Arctic to date. The observatory is sampled annually during the summer months, which limits its explanatory power to assess the seasonal carbon cycling and microbial dynamics. To close this gap, the goals of this doctoral project were to (a) investigate the seasonal variability of organic matter, (b) explore seasonal organic matter turnover and the microbial community, and (c) evaluate the inter-annual variability in the Fram Strait

Seasonal Dynamics of Organic Matter Turnover in the Arctic Ocean

The Arctic Ocean is extremely susceptible to climate change, which has led to warmer air temperatures, accelerated sea ice loss, and an intensified inflow of Atlantic water masses into the Arctic. Furthermore, the reduction in sea ice has been linked to a prolonged phytoplankton growth season. Fewer days of sea ice cover and thinner sea ice increases the light availability for ice algae and expands the habitat for pelagic phytoplankton. An extension of the growth season prolongs the release of phytoplankton-derived organic matter with consequences for the seasonal carbon cycle. However, the seasonal carbon stock and microbial processes are largely unexplored because many parameters require in situ sampling. A systematic strategy to enhance microbial observations is through time series stations. The long-term ecological research observatory HAUSGARTEN was established in 1999 and is the only pelagic microbial observatory in the Arctic to date. The observatory is sampled annually during the summer months, which limits its explanatory power to assess the seasonal carbon cycling and microbial dynamics. To close this gap, the goals of this doctoral project were to (a) investigate the seasonal variability of organic matter, (b) explore seasonal organic matter turnover and the microbial community, and (c) evaluate the inter-annual variability in the Fram Strait

Organic matter, water-isotopes and cell abundances of Arctic Century cruise

The discrete biogeochemical measurements from RV Akademik Tryoshnikov AT21 sampled for chlorophylla, delta 18O isotopes, dissolved organic matter and microbial cell abundances. The parameters include dO, d18O, Chla, DOC, cDOM, TDN, DIN, combined carbohydrates, hydrolysable amino acids, and microbial cell abundances. The samples were aquired within 0-10 m of the Barents and Laptev Sea, lake, and snow between 76 N to 82 N and 59 E to 106 E. The data was collected on board Arctic Century cruise from 09th August 2021-2nd September 2021. The water collection occured using a CTD, bucket (lake) and gloves (snow, thawed). The laboratory methods varied by parameter: chlorophyll using Turner Designs Trilogy fluorometer, oxygen isotopes using a Finnigan Delta-S mass spectrometer, DOC/TDN was analysed using TOC-VCSH, and cDOM were analyzed spectrophotometrically, dissolved carbohydrates using high performance anion exchange chromatography coupled with pulsed amperometric detection, dissolved amino acids using ortho-phthaldialdehyde derivatization by high-performance liquid chromatography, and microbial cell abundance (phytoplankton, bacteria) was analysed using flow cytometery. The purpose of data collection was to understand spatial variability of organic mater and associated dynamics of the microbial loop

Organic matter and bacteria measurements of POLARSTERN cruise PS114 and Maria S. Merian cruise MSM77

The discrete biogeochemical measurements from RV Polarstern PS114 and RV Maria S. Merian MSM77 sampled for dissolved organic matter, heterotrophic bacteria and gel particles. The parameters include chlorophyll, DOC/TDN/TDP/DOP, dissolved hydrolysable amino acids, dissolved combined carbohydrates, heterotrophic bacterial abundance and production, gel particles like TEP and CSP. The samples were aquired within 0-100 m of the Greenland Sea between 78°N to 80°N and 2°W to 14°E. The data was collected daily on board PS114 from 16th July 2018-23rd July 2018 and on board MSM77 from 16th September 2018-4th October 2018. The water collection occured using a CTD and laboratory methods varied by parameter. Chlorophyll was extracted using acetone, DOC/TDN was analysed using High-Temperature Catalytic Oxidation (TOC-VCSH), TDP/DOP was analysed colorimetrically using Acidic Molybdate Solution, dissolved hydrolysable amino acids were analysed using High Performal Liquid Chromatography (HPLC) Ortho-phthaldialdehyde Derivatization, dissolved combined carbohydrates were analysed using High Performance Anion Exchange Chromatography (HPAEC) coupled with Pulsed Amperometric Detection (PAD), cell abundance was analysed using flow cytometery, bacterial production was analysed using radioactively labelled 3H-Leucine and apllication of the microcentrifuge method, gel particles were analysed microscopically followed by image analysis. The purpose of data collection was to understand seasonal cycling of organic mater and heterotrophic bacteria dynamics within microbial loop

Organic matter and phytoplankton measurements of POLARSTERN cruise PS114

The discrete biogeochemical measurements from RV Polarstern PS114 sampled for organic matter, primary production and phytoplankton abundances. The parameters include POC, phytoplankton abundances and production. The samples were acquired within 0-100 m of the Greenland Sea between 78°N to 80°N and 3°E to 10°E. The data was collected daily on board PS114 from 16th July 2018-23rd July 2018 . The water collection occurred using a CTD and laboratory methods varied by parameter. POC was analyzed using a EURO EA CHNS-O Elemental Analyzer, carbohydrates were analyzed using High performance anion exchange chromatography with pulsed amperometric detection, amino acids were analyzed using High Performance Liquid Chromatography (HPLC) ortho-phthaldialdehyde derivatization, rates of primary production (PP) were measured in situ using the 14C method and subsampled as total PP (PP-TOC), particulate PP (PP-POC), and dissolved PP (PP-DOC), cell abundances were measured using orange and red autofluorescence detected by flow cytometry. The purpose of data collection was to understand seasonal cycling of organic mater