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Impact of climate change on light-related carbon fluxes in the Arctic Ocean

Final Report Summary - MALINA (Impact of climate change on light-related carbon fluxes in the Arctic Ocean)

The general objective of this project was to determine the absolute significance of organic matter photo-oxidation and of primary production, and their relative balance in the Arctic Ocean. The fellow's first objective for the proposed training was be to become familiar with the Arctic Ocean environment and reach a level of expertise high enough to lead research programs in Arctic on processes, and using tools he is familiar with.

Part I: Primary production and CDOM photo-oxidation have opposing impacts on carbon fluxes in the ocean
The balance between the two processes may be significantly affected in the near future by climate change. This is especially true for the Arctic Ocean, which is increasingly exposed to light because perennial ice recedes, and which receives increasing amounts of terrigenous dissolved organic matter (tDOM) as the permafrost thaws and river discharges increase.

In this study, we used remote sensing data to estimate the pan-Arctic distributions of primary production and CDOM photo-oxidation, and how they evolved from 1998 to now. Our results provide the first pan-Arctic combined estimates of primary production and CDOM photo-oxidation based on remote sensing, and allow determining how these two processes compare. They indicate that CDOM photo-oxidation accounts for a major fraction of allochthonous organic carbon mineralization in the Arctic Ocean, and is comparable in magnitude to the fraction of gross primary production that ends up sequestered within the ocean bottom sediments. The ratio between photo-oxidation and primary production turns out being highly variable, which indicates significant competition for light between CDOM and phytoplankton. As a response to sea ice decline, both photo-oxidation and primary production showed increasing trends from 1998 to 2007. Primary production increased significantly more that CDOM photo-oxidation.

Part II: Tides and wind-driven mixing play a major role in promoting post-bloom productivity in subarctic shelf seas
Whether this is also true in the high Arctic remains unknown. This question is particularly relevant in a context of increasing Arctic stratification in response to global climatic change. We have used a three-dimensional ocean-sea ice-ecosystem model to assess the contribution of tides and strong wind events to summer (June - August 2001) primary production in the Barents Sea. Tides are responsible for ~20 % (~60 % locally) of the areal post-bloom production above Svalbard Bank and east of the Kola Peninsula. By contrast, more than 9 % of the areal primary production is due to winds larger than 8 m/s in the central Barents Sea. Locally, this contribution reaches ~25 %. Removing tides or winds faster than 8 m/s promotes a regime more sustained by regenerated production, with a f-ratio that decreases by ~20 % and 40 %, respectively. In the marginal ice zone, both tides and wind events have a limited effect (< 2 %) on primary production. When integrated over all the Barents Sea, strong wind events account for 4.1 % (0.93 TgC) of the areal post-bloom production (22.7 TgC), which is 5 times more than the fraction accounted for by tides (0.8 %). The contribution of winds faster than 8 m/s is equivalent to a spring bloom event integrated over the Svalbard area.

In part II, a comprehensive climatology was also developed for nutrients and dissolved organic carbon brought to the Arctic Ocean by major rivers. During the returning period, the modelling activities were pursued with the development and validation of a 1-D biological model. This model is the premise of the biogeochemical model we are coupling with the MIT-GCM ocean-ice model to predict carbon fluxes in the Arctic Ocean. Additionally, the Malina project kept on expanding from an international France-Canada-USA project to an even more ambitious partnership between CNRS (and LOV) and Université Laval in the form of an 'International Joint Laboratory' dedicated to Arctic research.