Pollution, fires, deserts, volcanoes, the transformation of gases into particles... the particles - or aerosols - found in the atmosphere come from many different sources. The same applies to their chemical composition. As these particles fall back to the surface of the sea, either by gravity or by being carried along by raindrops, they dissolve totally or partially, changing the chemical composition of the seawater.
Surrounded by three contrasting continents, the Mediterranean receives several hundred thousand tonnes of aerosols on its surface every year, particularly offshore where the input of matter from rivers does not arrive. Every summer, as surface waters warm up and become isolated from deeper waters, the surface of the Mediterranean naturally becomes depleted of dissolved phosphorus and nitrogen, two major nutrients essential for the photosynthetic growth of phytoplankton, the micro-algae at the base of the marine food chain. The deposition of aerosols can therefore provide a source of nutrients for these depleted zones and maintain the development of micro-organisms. Although in situ and laboratory experiments have shown the potential major impact of aerosol deposition in the Mediterranean, its effect on the scale of the basin as a whole was previously unknown.
For the first time, modelling has been used to simulate the response of the Mediterranean Sea to aerosol deposition. By combining the atmospheric modelling (ChArMEx) and marine biogeochemistry (MerMex) components of the MISTRALS programme, scientists were able to model the emission, transformation in the atmosphere and deposition of aerosols, taking into account the specific biological characteristics and spatial and temporal variability of the Mediterranean Sea.
The results showed that the deposition of desert dust primarily affects the south and east of the basin, while the deposition of phosphorus and nitrogen from particles produced by natural and anthropogenic combustion is more significant in the north of the basin. As expected, the effects of deposition on surface water composition are greatest in summer due to surface stratification. Phosphate concentration increases by up to 30% and nitrogen fluxes can double nitrate concentration in the south of the basin. These deposition events lead to the lifting of nutrient limitations and an increase in biological production, which has repercussions on the first levels of the trophic chain. Following aerosol deposition, primary productivity increases by 5 to 30%, accompanied by an increase in the biomass of phytoplankton, bacteria and zooplankton. Moreover, the effect of sporadic deposition following intense dust storms or heavy rainfall seems to last several days before returning to pre-deposition productivity levels.
These results are all the more interesting given that simulations for the coming decades show that the increase in surface water temperature in summer will make biological production in the Mediterranean increasingly dependent on aerosol deposition.
Authors
Camille Richon1, François Dulac, Jean-Claude Dutay