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Sub-chapter 1.4.3. Atmospheric deposition to nutrient depleted seawater

p. 165-168

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1Atmospheric deposition is the removal from the atmosphere of particles and gas by sedimentation (dry deposition) and by rainfall (wet deposition). Atmospheric inputs of pollutants and nutrients are of the same order of magnitude or larger than the riverine inputs for the Mediterranean region (Koçak et al. 2010; Christodoulaki et al. 2013; Moon et al. 2016), and hence of primary importance for marine ecosystems in the particularly oligotrophic seawater (i.e. with very low nitrogen and phosphorus nutrient concentrations) of the Mediterranean basin (Dugdale AND Wilkerson, 1988). Indeed, owing to the small size (2.51 106 km²) of the Mediterranean Sea and to many intense land-based sources of emissions, open waters receive significant loads of nutrients through atmospheric deposition (Fig. 6). Some deposition events qualified as ‘extreme events’, such as dust inputs as high as 22 g m-2 (Bonnet and Guieu, 2006), can occur at very short time scales (hours to days).

2Atmospheric deposition is a significant source of major nutrients including inorganic (Kouvarakis et al. 2001) and organic nitrogen (Violaki et al. 2010), and phosphorous (Markaki et al. 2003). Even if water convection generally supplies most of the nitrogen (N) and phosphorus (P) available for biology in surface waters (where photosynthesis occurs) from deep waters, atmospheric inputs of inorganic N and P may be the most intense source of nutrients in summer when thermal stratification of the surface water column prevents vertical mixing (Pasqueron de Fommervault et al. 2015). Atmospheric deposition could also partly explain the increasing N:P ratio in the seawater column from the western to the eastern Mediterranean basin, since a similar N: P trend is observed in atmospheric deposition in these areas (Markaki et al. 2010). It has been shown that atmospheric deposition of iron (Bonnet and Guieu, 2006) and trace metals (Theodosi et al. 2010) represents significant inputs to support the primary production in surface waters. Deposition also transfers different atmospheric organic contaminants from the lower atmosphere to the ocean surface, including organochlorine compounds or polycyclic aromatic hydrocarbons (PAH), but their low deposition fluxes and their degradation in the water column limit their impact on marine ecosystems (Castro-Jimenez et al. 2012; Berrojalbiz et al. 2014).


Figure 6
Main processes occurring at the air-sea interface driven by different atmospheric inputs (adapted from Law et al. 2013).

3Deposited nutrients come from two main sources: anthropogenic sources and soil dust, the latter mainly from the Sahara but also from the Middle East in the easternmost Mediterranean basin. Anthropogenic inputs control the deposition flux of N, inorganic nitrogen being mainly supplied by dry deposition (Markaki et al. 2010), and organic nitrogen by wet deposition (Violaki et al. 2010). The deposition of desert dust plays an important role in the fluxes of P and trace metals due to sporadic but intense deposition events (Özsoy and Örnektekin, 2009; Guieu et al. 2010; Morales-Baquero and Perez-Martinez, 2016), even if the contribution of anthropogenic aerosol deposition is significant (between 10% for Fe to 90% for Zn; Guieu et al. 2010). Dust deposition releases dissolved inorganic phosphorous (DIP) and nitrate in N-and P-depleted surface waters (Ridame et al. 2014). The atmospheric deposition of mineral dust also determines enrichment of the sea-surface microlayer in dissolved trace metal micro-nutrients such as Cd, Co, Cu, Fe (Tovar Sanchez et al. 2014). However, it has been shown that dust deposition can result either in a net release or in scavenging of DIP and nitrate (Louis et al. 2015) and trace elements (Wagener et al. 2010; Wuttig et al. 2013) in seawater, depending on the quantity and quality of in situ dissolved organic matter at the time of the deposition. Indeed, the dissolved organic matter can control the dissolution of nutrients carried by dust particles.

4Recent experiments in realistic conditions showed that, by providing P and N to the marine biosphere, wet Saharan dust deposition strongly stimulates primary production and phytoplankton biomass for several days after deposition (Ridame et al. 2014; Guieu et al. 2014b; Fig. 7). From such studies, the inputs of atmospheric trace metals into the Mediterranean Sea associated with dust deposition are also suspected of stimulating bacteria and phytoplankton species such as cyanobacteria that are able to assimilate atmospheric dinitrogen N 2 (Ridame et al. 2011). The extent of the fertilizing effect of dust deposition events in the Mediterranean was revealed by statistically positive correlations between dust deposition and surface chlorophyll concentrations in combined remote sensing and modeling approaches (Gallisai et al. 2014). However, a negative effect of atmospheric deposition on chlorophyll was observed in the regions under the influence of aerosols of European origin (Gallisai et al. 2014). Indeed, inputs of anthropogenic aerosols, such as Cu-rich aerosol, are suspected of inhibiting phytoplankton growth (Jordi et al. 2012). Dust deposition has also been shown to modify the structure of the bacterial community by selectively stimulating and inhibiting certain types (Pulido-Villena et al. 2014). By stimulating predominantly heterotrophic bacteria (i.e. that use organic carbon for their growth), atmospheric dust deposition can increase the recycling of carbon, thereby reducing net atmospheric CO 2 drawdown and the fraction of dissolved organic carbon that can be mixed and exported to deep waters (Pulido-Villena et al. 2008). In contrast, Saharan dust deposition in the Mediterranean can enhance the export of particulate organic carbon to the deep ocean by acting as ballast and facilitating aggregation processes (i.e. Bressac et al. 2014; Desboeufs et al. 2014).

5To tackle these questions at the scale of the Mediterranean basin, numerical models need to be developed. The main challenges are to quantify the relative contributions of anthropogenic and natural deposition of nutrients and contaminants in this region in a context of Mediterranean climate change, and to improve our ability to simulate the chemical elements and their soluble fraction in 3-D atmospheric transport and chemistry models. Modeling the size distribution of desert dust particles and its evolution from emission to deposition contributes to the difficulty. In situ measurements by aircraft suggest a coarse mode of large soil dust particles over the Mediterranean (with a volume mean effective diameter in the range of 3.8-14.2 µ m) as large as that observed close to the Saharan and Sahelian source regions (Denjean et al. 2016). Balloon-borne aerosol counters have also shown the frequent presence of large particles (> 20 µm) inside airborne desert dust plumes over the western Mediterranean (Renard et al. 2016). These particles, which appear to be transported for several days without significant gravitational sedimentation, in contradiction to their size, probably control the mass flux of deposition of dust.


Figure 7
Stocks (green) and fluxes (blue) measured during mesocosm experiments after the simulation of a realistic atmospheric input in a water body large enough to be representative of natural processes (Guieu et al. 2014b; modified from de Leeuw et al. 2014).

6Predicting inputs of atmospheric nutrients in the future is important to understand the vulnerability of the marine ecosystem and carbon fluxes. Climate change simulations predict a hotter dryer climate in the Mediterranean basin (e.g., Hertig & Jacobeit, 2008), a situation that could increase dust emissions and transport due to increased aridity (Moulin & Chiapello, 2006) but also intense deposition events (Beaulant et al. 2011). However, the most recent measurements of dust deposition in Corsica (2011-2013) show that deposition fluxes are much lower than in the three past decades shown by existing records in the area (11-14gm-2 yr-1) (Vincent et al. 2016). But it is not yet understood if the reduction in dust deposition is related to a decrease in the frequency of dust events and/or to a change in deposition processes and patterns in the Mediterranean region. The temporal dynamics of marine N and P concentrations since 1985 showed high sensitivity to anthropogenic atmospheric deposition and are expected to decline in the coming decades due to mitigation/control of pollutant emissions (Moon et al. 2016). In the same way, Christodoulaki et al. (2016) showed that even if the human-driven atmospheric deposition of N and P has led to a 16% increase in total phytoplankton biomass over the past one and a half centuries, small changes in carbon fluxes and planktonic biomasses are predicted for the near future with the projected inputs of N and P. The regulation of inputs of anthropogenic nutrients could be a key driver of seawater nutrient cycles and hence marine ecosystems in the future. However, findings concerning the sources of the atmospheric trace metals and effects need to be completed to enable high quality projections in a context of future changes.

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