Arctic phytoplankton may act as a sink for carbon dioxide (CO2) and a major producer of dimethylsulfide (DMS). Maximum effects are expected in the marginal ice zone where phytoplankton blooms extend from May to September as the ice recedes northwards. These productive waters can cover more than half the area of arctic shelf seas, such as the Barents Sea, in any one growth period. One of the main components of the phytoplankton at the marginal ice zone is Phaeocystis pouchetii which, in its colonial form, is a high producer of DMS. This species is known to excrete a high proportion of newly fixed carbon as high molecular weight polymers in order to form the colony. Later, it sinks in mass in the latter stages of the bloom when nitrate becomes limiting. Diatoms, such as Thalassiosira gravida, may dominate the bloom at the ice edge late in the summer and may also precede the P. pouchetii bloom in early spring (i.e. Chaetoceros socialis). The different patterns of dissolved organic carbon formation and cell sedimentation between P. pouchetii and diatoms need to be included in productivity models for an understanding of the production and sink of CO2 and DMS in arctic shelf areas. The objectives of this field and laboratory study, in collaboration with Norwegian scientists, are to: (1) to determine the quantitative differences between P. pouchetii and diatoms blooms in the ice edge with respect to primary production, proportion of carbon fixed that is extracellular, DMS production, and sedimentation of particulate carbon and sulfur to depth: (2) to determine which are the main environmental variables that favor each type of bloom using a 10-year time series from the Barents Sea; and (3) to estimate a 10-year time series of DMS production and CO2 uptake by combining the existing time series with carbon and sulfur budgets of the ice edge blooms. This research will provide a first approximation to the seasonal and interannual variability of the phytoplankton contribution to the carbon and sulfur cycle during the ice edge bloom in arctic shelf areas.
