Description: The succession of phytoplankton in the Fram Strait was investigated at the AWI LTER (Long Term Ecological Research) observatory HAUSGARTEN at a fixed station (HG-IV; ~ 79°N , 4° E) for the first time. For this purpose, a moored automated sampling system ( at ~25 m water depth) was collecting samples every other week except during winter month from August 15 2017 to July 8 2018. Phytoplankton was identified, counted, and the size was measured, in a total of 17 samples. Growth started in the middle of May with diatoms of the genus Chaetoceros and Thalassiosira directly followed by the haptophyte Phaeocystis during June. The group of dinoflagellates showed two growth phases; one during August to mid-September and another one during May. Cryptophytes exhibited their growth phase in September, while Dictyocha speculum dominated in November. Rhizosolenia spp. and Pseudo-nitzschia spp. were more frequent during late summer. The common bloom forming diatoms Chaetoceros spp. and Thalassiosira spp. as well as the haptophyte Phaeocystis sp. were the most abundant species predominating the cell counts and also made up the majority of the phytoplankton carbon (PPC) in the course of that year in eastern Fram Strait.

Description: The Arctic Ocean is highly susceptible to climate change as evidenced by rapid warming and the drastic loss of sea ice during summer. The consequences of these environmental changes for the microbial cycling of organic matter are largely unexplored. Here, we investigated the distribution and composition of dissolved organic matter (DOM) along with heterotrophic bacterial activity in seawater and sea ice of the Eurasian Basin at the time of the record ice minimum in 2012. Bacteria in seawater were highly responsive to fresh organic matter and remineralized on average 55% of primary production in the upper mixed layer. Correlation analysis showed that the accumulation of dissolved combined carbohydrates (DCCHO) and dissolved amino acids (DAA), two major components of fresh organic matter, was related to the drawdown of nitrate. Nitrate‐depleted surface waters at stations adjacent to the Laptev Sea showed about 25% higher concentrations of DAA than stations adjacent to the Barents Sea and in the central Arctic basin. Carbohydrate concentration was the best predictor of heterotrophic bacterial activity in sea ice. In contrast, variability in sea‐ice bacterial biomass was largely driven by differences in ice thickness. This decoupling of bacterial biomass and activity may mitigate the negative effects of biomass loss due to ice melting on heterotrophic bacterial functions. Overall, our results reveal that changes in DOM production and inventories induced by sea‐ice loss have a high potential to enhance the bacterial remineralization of organic matter in seawater and sea ice of the Arctic Ocean.

Description: The biological carbon pump removes CO2 from the atmosphere via phytoplankton growth in the sunlit stratified upper ocean. This is followed by a partial export to deeper water layers and finally deposition on the sea floor. In polar regions, sea ice affects stratification and light availability. However, it remains unclear to what extent sea ice cover affects the biological carbon pump. Additionally, climate change is expected to increase the seasonal ice zone (i.e., the part of the ocean that is ice-covered for part of the year). Observational time series of the seasonal cycle contrasting production and export in ice-covered and ice-free conditions are still scarce. Here, we present multidisciplinary time series observations from the marginal ice zone in Fram Strait (located between Greenland and Svalbard) of all ocean compartments from the sea surface to the seafloor. Data are from two contrasting years when our measurement site was either partially ice-covered (2016-2017) or when the ice edge was located further to the north (2017-2018). We start by introducing the study site followed by a description of the atmospheric forcing and its effects on the upper ocean stratification and hydrography. In mostly ice-free conditions, the mixed layer went from deep to stratified in a single event, whereas the presence of sea ice resulted in a number of alternating events of shallow and deep mixed layers. These patterns changed phytoplankton and bacterial dynamics and upper ocean chemistry. The onset of the bloom was much more gradual in the presence of sea ice. We also explore the different trophic levels in the upper that contributed to the export recorded at the seafloor which fueled biological benthic activity a few weeks after the bloom. Our observations reveal an impact of the respective sea ice patterns on concomitant species and biogeochemical reactions in the upper water column and for the export of organic matter to the deep sea in the Fram Strait.

Description: Two mooring arrays carrying sediment traps were deployed from September 2011 to August 2012 at ∼83°N on each side of the Gakkel Ridge in the Nansen and Amundsen Basins to measure downward particle flux below the euphotic zone (approx. 250m) and approximately 150 m above seafloor at approximately 3500 and 4000m depth, respectively. In a region that still experiences nearly complete ice cover throughout the year, export fluxes of total particulate matter (TPM), particulate organic carbon (POC), particulate nitrogen (PN), biogenic matter, lithogenic matter, biogenic particulate silica (bPSi), calcium carbonate (CaCO3 ), protists and biomarkers only slightly decreased with depth. Seasonal variations of particulate matter fluxes were similar on both sides of the Gakkel Ridge. Somewhat higher export rates in the Amundsen Basin and differences in the composition of the sinking TPM and bPSi on each side of the Gakkel Ridge probably reflected the influence of the Lena River/Transpolar Drift in the Amundsen Basin and the influence of Atlantic water in the Nansen Basin. Low variations in particle export with depth revealed a limited influence of lateral advection in the deep barren 2 Eurasian Basin. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.

Description: 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 2 in microbial ecosystem dynamics to better assess the impact of environmental change in the warming Arctic Ocean. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.

Description: Microalgal cells collected with moored sediment traps deployed during three to five annual cycles at three sites in the Beaufort Sea were identified to investigate variations in the timing, abundance and composition of microalgal fluxes in relation to snow and sea ice cover. The investigation period encompassed two extremes in snow and sea ice conditions: a delayed melt due to an ice rebound in 2013 and a premature snowmelt and sea ice breakup that led to an ice-free Beaufort Sea in 2016. Diatoms dominated the microalgal fluxes, with the pelagic centric diatoms Thalassiosira spp. and the ice-associated pennate diatoms Fragilariopsis spp. consistently collected at the three sites. The export of the ice-obligated algae Nitzschia frigida indicated the release of sea ice algae at the onset of snowmelt. Early snowmelt and sea ice breakup in 2016 contributed to an early start of ice algae release accompanied with early peaks in diatom fluxes and higher diatom and phytoplankton carbon (PPC) fluxes during spring and summer. Conversely, delayed sea ice algae release, low diatom fluxes, and low PPC fluxes were observed when snowmelt and sea ice breakup occurred late over the Mackenzie shelf break. The amount of diatoms exported at ~100–300 m also likely depended on a match or mismatch between algal production and zooplankton grazing. Variations in the snow and sea ice regimes at the regional scale therefore directly impact the timing and magnitude of microalgal export and its contribution to particulate organic carbon flux in the Arctic Ocean. With global warming, the ongoing sea ice reduction in the Arctic Ocean may increase PPC fluxes to the seafloor and potential carbon sequestration at depth.

Description: Half of the Arctic Ocean is deep sea (〉1000 m), and this area is currently transitioning from being permanently ice-covered to being seasonally ice-free. Despite these drastic changes, it remains unclear how organisms are distributed in the deep Arctic basins, and particularly what feeds them. Here, we summarize data on auto- and heterotrophic organisms in the benthic, pelagic, and sympagic realm of the Arctic Ocean basins from the past three decades and put together an organic carbon budget for this region. Based on the budget, we investigate whether our current understanding of primary and secondary production and vertical carbon flux are balanced by the current estimates of the carbon demand by deep-sea benthos. At first glance, our budget identifies a mismatch between the carbon supply by primary production (3–46 g C m−2 yr−1), the carbon demand of organisms living in the pelagic (7–17 g C m−2) and the benthic realm (〈 5 g C m−2 yr−1) versus the low vertical carbon export (at 200 m: 0.1–1.5 g C m−2 yr−1, at 3000–4000 m: 0.01–0.73 g C m−2 yr−1). To close the budget, we suggest that episodic events of large, fast sinking ice algae aggregates, export of dead zooplankton, as well as large food falls need to be quantified and included. This work emphasizes the clear need for a better understanding of the quantity, phenology, and the regionality of carbon supply and demand in the deep Arctic basins, which will allow us to evaluate how the ecosystem may change in the future.

Description: Sea ice in the Arctic Ocean (AO) has been undergoing dramatic changes during the last two decades. In addition, the water temperature of the inflow of Atlantic water masses at the gateway Fram Strait has recently increased. Long-term data may help to evaluate the impact of these physical changes on the biological processes in surface waters. Over a 25-year period, and mostly in summer, water samples were collected at discrete depths within the uppermost 100 m of the Fram Strait and other regions of the AO to investigate chlorophyll a (Chl a) and particulate organic carbon (POC) standing stocks. Stations sampled from 1991 to 2015 were located in the Fram Strait, Barents Sea (BS), on the Eurasian shelf, and over the Nansen, Amundsen, and parts of the Amerasian basins (AB). Discrete Chl a and POC measurements obtained during 33 and 24 expeditions, respectively, were integrated over the upper 100 m of the water column to monitor spatial and interannual variations in distribution patterns of standing stocks. In general, the highest Chl a and POC standing stocks were observed in the eastern Fram Strait (EFS) and in the BS, while the lowest biomasses were observed in the heavily ice-covered regions of the central AO, mainly in the Amundsen and ABs. Whereas summertime Chl a stocks sharply decreased northward from the Fram Strait and Barents Sea toward high latitudes, the decline in POC standing stocks was less pronounced. Over the sampling period, summertime Chl a stocks slightly increased in the EFS but remained more or less constant in the central AO. In contrast to Chl a, standing stocks of POC eventually increased over the last 25 years in the central AO, possibly as an effect of increasing air temperatures, decreasing sea ice extent and thickness, and increasing light availability. Moreover, variations in riverine discharge and in sea ice export within the Transpolar Drift may have contributed to the enhanced POC stock in the central AO surface waters. Overall, the objective of the present study was to provide baseline datasets of Chl a and POC to better track the effects of environmental changes due to global warming on the Arctic pelagic system.