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: As part of the HAUSGARTEN long-term observatory, sediment trap deployments were carried out before, during, and after the anomalously warm Atlantic Water inflow observed from 2005 to 2007 in the eastern Fram Strait. Downward export of particulate organic carbon (POC), zooplankton fecal pellet carbon (FPC), and biogenic particulate silica (bPSi) were measured from August 2002 to June 2003 and from July 2004 to July 2008 to indirectly assess the impact of the warm anomaly on phytoplankton and zooplankton communities in the region. Lower and less frequent bPSi fluxes were observed during most of the warm anomaly period, reflecting a shift in phytoplankton community composition towards dominance of small-sized phytoplankton under warmer conditions. Lower FPC fluxes observed concurrently with the lower bPSi fluxes may indicate a decrease in fecal pellet production due to changing feeding conditions. In addition, the export of smaller fecal pellets in fall 2005 and spring 2006 suggests a dominance of smaller zooplankton during the warm anomaly. Nonetheless, bPSi and FPC export always increased in the presence of ice cover in the area above the sediment trap, even during the warm anomaly period, suggesting that sea ice is a key factor influencing the frequency of export events in the eastern Fram Strait. The scarcity of ice over the sampling area in 2005 and 2006 may partly be due to the warm anomaly, although solar radiation and ice drift due to wind stress also govern ice cover extent in the region. Overall, the warm anomaly resulted in a shift in the composition of the export fluxes when associated with an absence of ice cover in the eastern Fram Strait.