Description: The Fram Strait is the main gateway for water, heat and sea-ice exchanges between the Arctic Ocean and the North Atlantic. The complex physical environment results in a highly variable primary production in space and time. Previous regional studies have defined key bottom-up (ice cover and stratification from melt water controlling the light availability, and wind mixing and water transport affecting the supply of nutrients) and top-down processes (heterotrophic grazing). In this study, in situ field data, remote sensing and modeling techniques were combined to investigate in detail the influence of melting sea-ice and ocean properties on the development of phytoplankton blooms in the Fram Strait region for the years 1998–2009. Satellite-retrieved chlorophyll-a concentrations from temporarily ice-free zones were validated with contextual field data. These were then integrated per month on a grid size of 20 × 20 km, resulting in 10 grids/fields. Factors tested for their influence on spatial and temporal variation of chlorophyll-a were: sea-ice concentration from satellite and sea-ice thickness, ocean stratification, water temperature and salinity time-series simulated by the ice-ocean model NAOSIM. The time series analysis for those ten ice-free fields showed a regional separation according to different physical processes affecting phytoplankton distribution. At the marginal ice zone the melting sea-ice was promoting phytoplankton growth by stratifying the water column and potentially seeding phytoplankton communities. In this zone, the highest mean chlorophyll concentration averaged for the productive season (April–August) of 0.8 mgC/m3 was observed. In the open ocean the phytoplankton variability was correlated highest to stratification formed by solar heating of the upper ocean layers. Coastal zone around Svalbard showed processes associated with the presence of coastal ice were rather suppressing than promoting the phytoplankton growth. During the twelve years of observations, chlorophyll concentrations significantly increased in the southern part of the Fram Strait, associated with an increase in sea surface temperature and a decrease in Svalbard coastal ice. Highlights • We used combination of satellite, simulated and in situ data for 1998–2009. • Stratification from sea-ice melt resulted in largest CHL at the marginal ice zone. • Stratification caused by solar warming promoted open ocean blooms. • Late retreat of Svalbard shelf ice delayed coastal blooms.

Description: After a short introduction to the physical setting and the history of biological research the pelagic ecosystem of the Kara Sea is described. Main emphasis is on regional aspects of the plankton communities and their seasonal dynamics using mostly data collected between 1996 and 2001. In the zooplankton, for which most data were available, four regional aggregations were separated: (1) the rivers and estuaries of the Southern Kara Sea, (2) the south-western and (3) the central Kara Sea, and (4) the northern troughs and slope. The phytoplankton communities had a similar distribution. To provide components for detailed carbon budgets the regional dynamics of bacterial, phytoplankton and zooplankton biomass and production are described and carbon requirements of bacteria and zooplankton are estimated. For completeness a short literature review on higher trophic levels is included. Finally, recent observations of the pelago-benthic coupling are considered. Estimates of the carbon requirements from the plankton and benthos reveal a large underestimation of primary production, which to date, together with seasonal aspects, shows the largest gap in our knowledge.

Description: The biogeochemistry of the river-sea interface was studied in the Kem' River (the largest river flowing to the White Sea from Karelian coast) estuary and adjacent area of the White Sea onboard the RV "Ekolog" in summer 2001, 2002 and 2003. The study area can be divided into 3 zones: I - the estuary itself, with water depth from 1 to 5m and low salinity in the surface layer (salinity is lower than 0.2psu in the Kem' River and varies from 15 to 20psu in outer part of this zone); II - the intermediate zone with depths from 5 to 10m and salinity at the surface from 16 to 22psu; III - the marine zone with depths from 10 to 29 m and salinity 21-24.5psu. Highest concentrations of the suspended particulate matter (SPM) were registered in the Kem' mouth (5-7mg/l). They sharply decreased to values 〈1mg/l towards the sea. At beginning of July 2001, particulate organic carbon (POC) concentration in the river mouth was 404µg/l and POC content in total SPM was 5.64%. In the marine part of the studied area the POC concentration varied from 132 to 274µg/l and the POC contents in suspended matter increased to 19-52.6%. These studies show, that the majority of riverborne suspended matter in the Kem' estuary deposits near the river mouth within the 20psu isohaline, where sedimentation of the suspended matter takes place. The role of fresh-water phytoplankton species decreases and the role of marine species increases from the river to sea and the percentage of green algae decreases and the role of diatoms increases. The organic carbon (Corg) to nitrogen (N) ratio (Corg/N) in both suspended matter and bottom sediments decreases from the river to the marine part of the mixing zone (from 8.5 to 6.1 in the suspended matter and from 14.6 to 7.5 in the bottom sediments), demonstrating that content of terrestrial-derived organic matter decreases and content of marine organic matter increases from the river mouth to the sea. The Kem' estuary exhibits a similar character of biogeochemial processes as in the large Arctic estuaries, but the scale of these processes (amount of river input of SPM, POC, area of estuaries) is different.

Description: Since the 1990s the AWI conducts studies on phytoplankton ecology at various locations in the central Arctic Ocean, the Greenland Sea and the Fram Strait onboard the ice going research vessel Polarstern. These studies provide valuable insight into the pelagic Arctic ecosystem. However, plankton abundance and composition were determined only sporadically and only few biogeochemical components were analysed. Considering the rapid changes of environmental conditions due to increasing temperatures, sea ice loss and ocean acidification, a comprehensive view of their impact on pelagic biological processes and the consequences for organic matter cycling is essential for our understanding of Arctic ecosystems. Thus a new group - Plankton Ecology and Biogeochemistry in a Changing Arctic Ocean (PEPBCAO) - established at the AWI studying the pelagic system in greater detail. The aim of this group is to complement the measurements of bulk variables and phyto- and zooplankton abundances by molecular assessment of the phytoplankton diversity including pico- and nanoplankton, allowing to better quantifying the intrusion of invading species into the polar habitat. Point measurements during cruises will serve as ground-truthing data to create basin wide satellite images, focussing on the quantitative estimation of phytoplankton functional types, which can serve as an input for modelling approaches. Changes in the composition of organic matter are investigated by molecular analyses and together with abundance and activity of zooplankton key species this will improve our ability of estimating carbon export under climate change. Beside the Central Arctic, one focus of the group will be the ‘AWI Hausgarten Deep Sea Monitoring Station’ in the Fram Strait off Svalbard. These data will be integrated in the existing data sets of the Arctic Ocean and this will provide a more detailed view of the changing pelagic system in the transition region between the central Arctic Ocean and the North Atlantic Ocean and first results will be presented.

Description: As climate change is expected to be extremely intense in the Arctic Ocean there is an utmost need to study food-web interactions to contribute to a better understanding of the direction and strength of biogeochemical and microbiological feedback processes. Climate change induced alterations will directly affect food-web structures and ecosystem functioning. Recent studies indicate that environmental changes like increasing temperatures as well as freshening of surface waters promote a shift in the phytoplankton community towards a dominance of smaller cells, especially of eukaryotic picoplankton. The response of oceanic ecosystems and marine carbon cycling to these changes is particularly determined by microbial loop activity. Heterotrophic bacteria, as part of the microbial loop and a crucial component of marine food webs, have a key role in controlling carbon fluxes in the oceans. Microbial activities, dynamics and diversity were studied in the area of the deep-sea long-term observatory HAUSGARTEN of the Alfred-Wegener-Institute (Fram Strait) in July 2009. The investigation area is located within a transition zone between the northern North Atlantic and the central Arctic Ocean, which separates the warm and cold water masses originating from the West Spitzbergen and the East Greenland currents. While bacterial abundance and chlorophyll a were tightly coupled, differences of the planktonic and bacterial community structures are most likely due to the heterogeneous hydrography. Warmer water masses comprise a higher genetic diversity of picoplankton, as it is also expected for bacteria. A shift towards a dominance of smaller plankton species can potentially affect the quality of organic matter and subsequently microbial cycling. Here we present data on bacterial abundance, biomass and protein production, hydrolytical enzyme activities and community structure within different size classes with respect to changing biotic and abiotic conditions in the Fram Strait.