Unravelling the dynamics of pelagic ecosystems by quantitative observation of morphological attributes of marine snow: a case study in the Arctic.

The pulse of organic matter and the subsequent transfer of carbon out of the photic zone to the deep ocean is limited to a very short time period in the Arctic, especially in areas temporarily covered by sea ice. The quantity and quality of matter produced in the form of marine snow results from many processes related to the dynamics and relationships among ice, phytoplankton and zooplankton. Those are extremely difficult to assess due to technical limitations of their collection and in situ observations. Using the Underwater Vision Profiler (UVP), we were able to observe the spatio-temporal dynamics (190 stations) of marine snow formation, transformation and downward transport in two crucial deep water Arctic basins (Baffin Bay and Fram Strait). Each marine snow particle in the water column was photographed and described by 24 morphological traits, referring to its size, shape, transparency and structure. Those descriptors were used to cluster marine snow into a few morphological groups, such as filaments, faecal pellets, Phaeocystis flocs, and various forms of aggregates. The spatio-temporal mapping of those morphological groups enabled to clearly follow the succession of different forms of marine snow particles and their probable export to the deep sea. Under sea ice marine snow was dominated by dark and elongated forms, while with progressing bloom and the switch from diatoms to Phaeocystis, the importance of lighter and more heterogeneous forms increased, which in turn were mostly avoided by zooplankton. The size range of marine snow was wider in the upper 250 m than in deeper layers, where the increase in its circularity was observed. We believe that the approach of analysing the morphological traits of marine snow will broaden our understanding of not only particle vertical fluxes, but also of the role of phytoplankton and zooplankton activity for marine snow formation and transformation.