Modelling primary production in seasonally ice-covered regions of the Arctic Ocean and its response to climate change




Lavoie, Diane

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I developed a 1D coupled sea ice-ocean-biological (including ice algae) model to study the controlling effect of sea ice on primary and biogenic particle export production in the western Arctic and the impacts of climate change (reduction in sea ice cover duration and thickness, and in surface freshwater fluxes) on these productions. The model was developed in two steps to maximize validation of model results with as much data as possible. I first developed a coupled snow-ice-ice algae model for bottom landfast ice in Resolute (Canadian Archipelago). Next, I developed and coupled a pelagic component (NPZD type) to the ice algal model. The coupled model was implemented on the Mackenzie shelf in the Canadian Beaufort Sea. And finally, I used simulations of future climate change from the Canadian Global Climate Model (CGCM2) to force the 1D model and obtain projections of future primary production on the Beaufort Sea shelf for two 18-year periods (2042-2059, and 2082-2099). The model results show that ice algae are light limited at the beginning of the bloom, then fluctuate between light and nutrient limitation, to finally remain nutrient limited toward the end of the bloom. The bottom ice melt rate regulates the maximum biomass attained in Resolute, while biomass accumulation remains low in the Beaufort Sea due to nutrient iv limitation. The termination of the bloom is triggered by melting of the snow cover and results from (i) increased ice algal losses due to high bottom ice melt rate and (ii) decreased ice algal growth due to nutrient limitation caused by the formation of a meltwater lens below the ice. The snow and sea ice cover melt and/or break-up also controls the timing of the phytoplankton bloom. However, primary producers on the Beaufort Sea outer shelf are essentially nutrient limited and total annual primary production is controlled in part by nutrient “pre-conditioning” in the previous fall and winter and by the depth of winter convective mixing, that are controlled in part by the supply of fresh water from runoff and ice melt. The spring bloom sometimes represents an important fraction of the total annual primary production, which occurs in great part at the base of the mixed layer. Future projections show an increase in average annual primary production of 6% between the periods 1975-1992 and 2042-2059, and of 9% between 1975-1992 and 2082-2099. The relative contribution of the ice algal and spring phytoplankton blooms to annual primary production is reduced in the future runs due to a reduction in the length of the ice algal growth season (resulting from earlier snow and ice melt) and to a reduction in the replenishment of nutrient to the mixed layer in winter. The duration of the summer subsurface phytoplankton bloom increases, which favours the development of the main copepod species and leads to an increase in export production (16% between 1975-1992 and 2082-2099) that is greater than the increase in primary production. This leads to an increase in averaged simulated e-ratio of 10% between the first and last period.



Climatic changes, Algal blooms, Marine