Understanding the dynamics of the phytoplankton spring bloom and predicting its fate and impact on marine ecosystems in the Arctic Ocean of tomorrow

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In most parts of the World Ocean, the phytoplankton spring bloom (PSB) provides a large fraction of the annual primary production (PP) and, most importantly, nearly all of the new PP exportable through the food chain and toward the bottom sediments. In the Arctic Ocean (AO) the PSB often develops around the ice-edge. This highly transient phenomenon lasts about 3 weeks at any given location in the seasonal ice zone (SIZ) and accounts for most of the annual PP in the AO. The SIZ is currently increasing in size and may cover the entire AO as of the 2030s. Therefore, one may wonder whether ice-edge blooms will cover a much larger area than they used to through a significant northward expansion. If so, what will be the fate of this additional phytoplankton biomass build-up along the retreating ice cover. Will it sustain higher secondary production and trophic transfer in the pelagic food web, thus benefiting megafauna? Or will it sink rapidly and create new benthic hotspots, possibly contributing increased carbon sequestration in the sediment? The overarching goal of the present project is to understand the processes that control the Arctic PSB as it expands northward and to determine its fate in the ecosystem by investigating its related carbon fluxes. The short-term objective of this project is to understand the dynamics of the Arctic PSB and, more specifically, how it is controlled by physical and chemical properties of the upper ocean and by phytoplankton species succession during the bloom. The long-term objective is to determine the fate of the PSB in a changing AO. We hypothesize that PP at high latitudes will increase in the coming decades because of a northward expansion of the SIZ and an intensification of the phytoplankton spring bloom. Changes in the phytoplankton phenology may have profound consequence on the entire ecosystem and food chain.A PSB event will be monitored in Baffin Bay from its onset under melting ice in May to its conclusion in the SIZ in July. Relevant physical, chemical and biological properties will be described at various scales using profiling floats, gliders and an autonomous underwater vehicle, equipped with physical and bio-optical sensors. Process studies will be conducted to document phytoplankton growth, nutrient assimilation and carbon transfer through the food web and towards the sediment. Key phytoplankton species will be isolated and grown under laboratory conditions to model their species succession and response to environmental factors. We will use ocean color remote sensing to thoroughly study the Arctic PSB, the full phenology of phytoplankton biomass, as well as transient phytoplankton blooms triggered by forcing events such as shelf-break upwellings. The optical properties of seawater in the vicinity of the ice edge and their variations during the spring bloom will be used to optimize our ocean colour algorithms and we will establish a time series of pan-Arctic PP over the period for which remotely sensed ocean colour data are available. Finally, a coupled physical-biological model will be optimized to simulate the Arctic PSB and predict changes in phytoplankton communities and food web dynamics over a variety of temporal scales.The project is an unprecedented effort for gaining a mechanistic understanding of the PSB in the AO through the use of state-of-the-art field and laboratory observation technologies, remote sensing of ocean colour and sophisticated modelling tools. The Knowledge gained from this project will expand our observational capacity and knowledge of the rapid changes taking place in Arctic ecosystems.