Modeling aerosol deposition impacts on the Mediterranean biogeochemical cycles

Observations and experiments showed that aerosol deposition can increase the amount of bioavailable nutrients and favor biological production of the Mediterranean Sea. In this context, the present study yields for the first time a quantification of the effects of aerosol deposition from various sources thanks to the coupled physical-biogeochemical model NEMOMED12/PISCES. This study consists in modeling and analyzing the effects on the Mediterranean biogeochemistry of atmospheric deposition of nitrogen and phosphate from various natural and anthropogenic sources. For this purpose, regional and global atmospheric models representing aerosol deposition were evaluated and selected. The NEMOMED12/PISCES model was modified to take into account these new nutrient sources. The analysis of the simulations showed that atmospheric deposition accounts for approximately 10 % of total external nitrate supply and 5 to 30 % of phosphate supply on average over the entire basin. Aerosol deposition can also increase biological production up to 50 % thanks to the lowering of nutrient limitations. The maximal fertilizing effects are observed during the stratified period which, in the Mediterranean region, is summer. The effects of climate change may be particularly important in sensitive regions such as the Mediterranean. Therefore, the evolutions of basin scale biogeochemistry were evaluated under a climate change scenario. The NEMOMED8/PISCES model was used with physical and biogeochemical forcings for the IPCC A2 climate change scenario. This study shows a reduction in basin scale surface productivity by approximately 10 % triggered by warming and stratification. Nutrient limitations are modified and the Mediterranean Sea sensibility to atmospheric deposition changes. The results of this thesis underline the importance of atmosphere as a nutrient source, in particular for nitrogen and phosphate. Deposition effects vary according to the season and the location. They are more important during the stratified period, when surface water is nutrient limited. Also, any change in biological productivity is quickly transferred along the biological chain. To refine the results, the atmospheric models could be improved and more knowledge on deposition fluxes and physical and chemical transformations of aerosols before and after deposition would be necessary. Moreover, more precise scenarios concerning climate change effects would be necessary in order to study the future evolutions of biogeochemical conditions in the Mediterranean. Finally, the recent developments on the PISCES model make new studies possible in a non-redfieldian context. Preliminary results indicate that the productivity of the different phyto-planktonic groups varies with intracellular C/N/P ratios. (author)