Climate change has triggered fundamental modifications of marine biotopes in the Arctic Ocean (AO). The decrease in the extent of the ice pack during summer has led to a 20% increase in pan-Arctic primary production (PP) over the last decade. Phytoplankton blooms now occur earlier in several parts of the AO. In other parts, the structure of the phytoplankton community is shifting toward smaller species, typical of more oligotrophic conditions and some species found in warmer waters now migrate into the Arctic Ocean. Phytoplankton grow in the top tens of meters of both ice-free and ice-covered waters. The phytoplankton spring bloom (PSB) that develops at the ice-edge accounts for >50% of annual primary production in the AO, and is generally associated with both large energy transfer to higher trophic levels and export of carbon to the bottom. As well, the culture, health and economic capacity building of Northerners are closely associated with marine resources supported by the PSB. The Arctic PSB develops in the seasonally-covered ice zone (SIZ), the extent of which is expected to increase significantly during the next years, possibly over the whole AO as early as in 2030. How the PSB will actually evolve in this context is unknown. Will it span over the entire AO, and thereby make the AO ecosystems more productive? Will the ongoing modifications in physical properties of the AO rather limit the PSB and PP in general? How will biodiversity respond to and/or impact on those changes? To be able to answer these questions, it is necessary to understand in great detail and quantitatively the physical, chemical and biological processes involved in the preconditioning, development and decline of the PSB. Because this is a transient phenomenon occurring in a remote, complex and harsh environment, such a detailed understanding has not yet been achieved. The general objective of this research project is to understand the dynamics of the PSB and determine its role in the Arctic Ocean of tomorrow, including for human populations. More specifically, we want to 1) understand the key physical, chemical and biological processes that govern the PSB, 2) identify the key phytoplankton species involved in the PSB and model their growth under various environmental conditions, and 3) predict the fate of the PSB and related carbon transfer through the food web and toward the bottom sediments over the next decades. First, a PSB event will be monitored during 2015 in the Baffin Bay from its onset under melting sea ice in May to its conclusion within the seasonal ice zone in July. The distribution of relevant physical, chemical and biological properties will be described at various time and space scales using a fleet of profiling floats and gliders and an autonomous underwater vehicle, all equipped with a suite of physical and bio-optical sensors. Process studies will be conducted from an ice camp and then from a research icebreaker to document phytoplankton growth, nutrient assimilation and the transfer of carbon through the food web and toward the sediment. Second, key phytoplankton species will be isolated and grown in the laboratory under various conditions to model their response to environmental factors and to understand their succession during spring. Third, a coupled physical-biological model will be optimized for simulating the PSB in the Arctic Ocean and for predicting changes in phytoplankton communities and food web dynamics. In parallel, past and present trends in the intensity and spatial distribution of the PSB will be documented using a paleoceanography approach, and using remote sensing. Finally, interviews and bilateral discussion with local Inuit communities will enable the documentation of changing marine productivity from a social perspective and feed into a multi-scale integrated analysis of environment-human interactions.