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.

Description: In the Arctic Ocean sea ice volume and extent currently experience massive reduction due to global warming. These rapid changes may have unforeseen consequences for pelagic biodiversity and biogeochemistry, particularly CO2 sequestration by the biological carbon pump. The production and export of carbonate-bearing organisms from the surface ocean can influence effective CO2 sequestration. However, the impact of rising temperatures and changing ice conditions on the export of carbonate-bearing organisms remains poorly constrained. Here we use sediment trap records to describe biogenic carbonate flux in relation to total matter flux and the export of foraminiferans, pteropods and ostracods. Samples were obtained from two sites: the Long-Term Ecological Research (LTER) observatory HAUSGARTEN in seasonally ice-covered eastern Fram Strait (since 2000), and over one annual cycle in the almost entirely ice-covered Nansen and Amundsen Basins (2011-2012). The flux of carbonate shell-bearing organisms showed large variations seasonally and annually at both Fram Strait and in the Central Arctic Ocean (CAO). Whereas the flux of foraminiferans was comparable at both sites, pteropod flux was high in the Fram Strait and negligible in Nansen and Amundsen Basin pointing towards a higher share of aragonite flux on total carbonate flux in the Fram Strait than in the CAO. Total matter and carbonate export flux were about one order of magnitude lower in the CAO compared to the eastern Fram Strait. Together, these results suggests that the two regions are characterized by different ecosystems and export mechanisms. The Fram Strait is changing from a partially seasonal ice-covered region towards an ice-free region with strong pelagic production and might provide valuable insights to the future of a summertime ice-free CAO.

Description: Arctic Ocean sea ice cover is shrinking due to warming. Long-term sediment trap data shows higher export efficiency of particulate organic carbon in regions with seasonal sea ice compared to regions without sea ice. To investigate this sea-ice enhanced export, we compared how different early summer phytoplankton communities in seasonally ice-free and ice-covered regions of the Fram Strait affect carbon export and vertical dispersal of microbes. In situ collected aggregates revealed two-fold higher carbon export of diatom-rich aggregates in ice-covered regions, compared to Phaeocystis aggregates in the ice-free region. Using microbial source tracking, we found that ice-covered regions were also associated with more surface-born microbial clades exported to the deep sea. Taken together, our results showed that ice-covered regions are responsible for high export efficiency and provide strong vertical microbial connectivity. Therefore, continuous sea-ice loss may decrease the vertical export efficiency, and thus the pelagic-benthic coupling, with potential repercussions for Arctic deep-sea ecosystems.