Dust Deposition Events Research Articles

Microorganisms Associated With Dust on Alpine Snow

We investigated snow microstructure and microbial composition from snow samples collected from western Colorado, a region that experiences frequent dust deposition events. We developed a methodology to quantify the amount, size and location of dust particles within the snow matrix through analysis of X-ray micro-computed tomography data. Concurrently, we determined the microbial composition in sampled dust layers through DNA sequencing. We found that dust particles were generally embedded in the snow grains, with a small fraction of the dust particles’ surface area exposed to air. Microbial community composition varied more by alpine site rather than residence time of the dust within the snowpack, with the recently deposited dust layer harboring both the highest diversity and highest concentration of dust particles. The presence of microbiota in the dust-impacted snowpack has important implications for snowpack stability, melting and biogeochemistry.

Changing nature of Saharan dust deposition in the Carpathian Basin (Central Europe): 40 years of identified North African dust events (1979–2018)

Several billion tonnes of mineral dust is emitted, and transported through winds every year from arid-semiarid areas. North African dust hot spots located in the Sahara are responsible for 50–70% of the global mineral dust budget. Dust-loaded air-masses originated from these sources can be transported over long distances and can also affect remote areas, such as North and South Americas, Europe, and the Middle East.In this study, we analysed 218 identified Saharan dust events (SDEs) in the Carpathian Basin (Central Europe) during 1979 to 2018. Systematic identification of SDEs and analyses of dust emission, dust source area activity, dust transporting wind systems, and transport routes revealed that different synoptic meteorological patterns are responsible for SDEs, and these are occurring mostly in spring and summer. The characteristic synoptic meteorological background of episodes was also identified, and three major types of atmospheric pressure-system patterns were distinguished.In recent years, several intense wintertime dust deposition events have been recorded in Central Europe. All of the identified unusual episodes were characterised by severe washout of mineral dust material and were related to very similar synoptic meteorological situations. Enhanced southward propagation of a high-latitude upper-level atmospheric trough to north-western Africa and orographic blocking of Atlas Mountains played an essential role in the formation of severe dust storms, whereas the long-range transport was associated with the northward branch of the meandering jet. The occurrence and southerly penetration of high-latitude upper-level atmospheric trough to low-latitudes and the increased meridionality of the dominant flow patterns may be associated with enhanced warming of the Arctic, leading to more meandering jet streams.Particles size of sampled dust material of some intense deposition episodes were very coarse with a considerable volumetric proportion of > 20 µm particles.

Analysis of a major Aeolian dust input event and its impact on element fluxes and inventories at the DYFAMED site (Northwestern Mediterranean)

Continental dust can be suspended and transported by the wind, reaching seawater masses far away from its source. The deposition of the aerosols on the ocean surface can alter the abundance of chemical species in the water column and contribute to element inventories in seafloor sediments. A major Saharan dust intrusion into the Western Mediterranean Sea was recorded at the DYFAMED site (Ligurian Sea) in 20th February 2004. We determined the influence of this dust event on the concentration of 30 minor and trace level elements (TE) in sinking particles collected by sediment traps deployed at 200 m and 1000 m depth, and how a dust flux event like this contributed to the exchange of TE, including Fe, with the water column during major dust events. With coupled sediment traps and aerosol samples, we assessed the short-term implications of dust events in the water column. The event produced a flux of fast (>111 m d−1) and slow (<20 m d−1) sinking dust particles, detected during 3 weeks at 200 m and 4 weeks at 1000 m depth. Additionally, the obtained results of element concentrations and particles flux show that a single dust deposition event can produce a sinking flux equivalent to annual deposition rates of elements relevant to biogeochemical cycles and/or pollution studies: (>60% for Cr and Cu, >70% for Al, >80% for Ni and Zn, >90% for V and Mn, >100% for Fe and Pb). The corresponding Enrichment Factors (EF) for the minor and TE analyzed in the sediment traps during the dust event were calculated. EF was used to determine how minor and trace element concentrations in sinking particles vary. The values ranged between 0.35 and 421 in both 200 m and 1000 m sediment traps. For most of the analyzed elements, the obtained EF values were higher than 1. On the contrary, V, Y, Zr, Nb, and Ce showed EF ~ 1 while Cr, Ni, Cu, Zn, Sn, and Pb showed EF < 1. Despite the variability in the EF values, vertical fluxes integrated during the dust deposition event increased from 200 m to 1000 m, except for I, which decreased. This contrasts strongly with the element fluxes integrated for the complete sampling period, which decrease or increase from 200 m to 1000 m, depending on the element. This suggests that sinking dust particles were acting generally as sinks of the TE. We conclude that, apart from I, a substantial portion of the elements from atmospheric dust input from a single deposition event can reach the mesopelagic layer of the Western Mediterranean basin without increasing the budget of those elements in the water column.

Holocene atmospheric dust deposition in NW Spain

Atmospheric dust plays an important role in terrestrial and marine ecosystems, particularly those that are nutrient limited. Despite that most dust originates from arid and semi-arid regions, recent research has shown that past dust events may have been involved in boosting productivity in nutrient-poor peatlands. We investigated dust deposition in a mid-latitude, raised bog, which is surrounded by a complex geology (paragneiss/schist, granite, quartzite and granodiorite). As proxies for dust fluxes, we used accumulation rates of trace (Ti, Zr, Rb, Sr and Y) as well as major (K and Ca) lithogenic elements. The oldest, largest dust deposition event occurred between ~8.6 and ~7.4 ka BP, peaking at ~8.1 ka BP (most probably the 8.2 ka BP event). The event had a large impact on the evolution of the mire, which subsequently transitioned from a fen into a raised bog in ~1500 years. From ~6.7 to ~4.0 ka BP, fluxes were very low, coeval with mid-Holocene forest stability and maximum extent. In the late Holocene, after ~4.0 ka BP, dust events became more prevalent with relatively major deposition at ~3.2–2.5, ~1.4 ka BP and ~0.35–0.05 ka BP, and minor peaks at ~4.0–3.7, ~1.7, ~1.10–0.95 ka BP and ~0.74–0.58 ka BP. Strontium fluxes display a similar pattern between ~11 and ~6.7 ka BP but then became decoupled from the other elements from the mid Holocene onwards. This seems to be a specific signal of the granodiorite batholith, which has an Sr anomaly. The reconstructed variations in dust fluxes bear a strong climatic imprint, probably related to storminess controlled by North Atlantic Oscillation conditions. Complex interactions also arise because of increased pressure from human activities.

Saharan dust events in the European Alps: role in snowmelt and geochemical characterization

Abstract. The input of mineral dust from arid regions impacts snow optical properties. The induced albedo reduction generally alters the melting dynamics of the snowpack, resulting in earlier snowmelt. In this paper, we evaluate the impact of dust depositions on the melting dynamics of snowpack at a high-elevation site (2160 m) in the European Alps (Torgnon, Aosta Valley, Italy) during three hydrological years (2013–2016). These years were characterized by several Saharan dust events that deposited significant amounts of mineral dust in the European Alps. We quantify the shortening of the snow season due to dust deposition by comparing observed snow depths and those simulated with the Crocus model accounting, or not, for the impact of impurities. The model was run and tested using meteorological data from an automated weather station. We propose the use of repeated digital images for tracking dust deposition and resurfacing in the snowpack. The good agreement between model prediction and digital images allowed us to propose the use of an RGB index (i.e. snow darkening index – SDI) for monitoring dust on snow using images from a digital camera. We also present a geochemical characterization of dust reaching the Alpine chain during spring in 2014. Elements found in dust were classified as a function of their origin and compared with Saharan sources. A strong enrichment in Fe was observed in snow containing Saharan dust. In our case study, the comparison between modelling results and observations showed that impurities deposited in snow anticipated the disappearance of snow up to 38 d a out of a total 7 months of typical snow duration. This happened for the season 2015–2016 that was characterized by a strong dust deposition event. During the other seasons considered here (2013–2014 and 2014–2015), the snow melt-out date was 18 and 11 d earlier, respectively. We conclude that the effect of the Saharan dust is expected to reduce snow cover duration through the snow-albedo feedback. This process is known to have a series of further hydrological and phenological feedback effects that should be characterized in future research.

Exploring Possible Influence of Dust Episodes on Surface Marine Chlorophyll Concentrations

Desert dust deposition is thought to act as fertilizer for phytoplankton growth, since it is rich in the required nutrients. The Mediterranean Sea is a nutrient poor marine environment—with its eastern part being the most oligotrophic—which is subject to dust transport. The Hellenic Seas are part of this low-nutrient, low-chlorophyll environment and they are also affected by dust deposition events. Thus, the dust fertilizing effect can be particularly important, especially during the stratification period, when the nutrients needed for phytoplankton growth are not imported from deeper layers. Some individual dust events are examined here in respect of their possible influence on phytoplankton, through the observed variations of satellite derived chlorophyll concentrations. Two strong dust events that were also extreme weather events and three events in the June–September stratification period are examined for the Hellenic Seas as well as a strong dust event in the Central Mediterranean Sea. The results, only when based on absolute chlorophyll differences above 50%, show that dust events seem to favour phytoplankton abundance mainly during the low productive period; however, these differences are area-limited. The difficulty of reaching safe results through specific dust events and discriminating between other meteorological factors favouring phytoplankton growth are also discussed.

Fluxes and sources of nutrient and trace metal atmospheric deposition in the northwestern Mediterranean

Abstract. Total atmospheric deposition was collected on a weekly basis over 3.5 years (March 2008–October 2011) at a remote coastal site on the west coast of Corsica. Deposition time series of macro- and micronutrients (N, P, Si, Fe) and trace metals (As, Cr, Cu, Mn, Ni, V, Zn) are investigated in terms of variability and source apportionment (from fluxes of proxies for aerosol sources (Al, Ti, Ca, Na, Mg, S, Sr, K, Pb)). The highest fluxes are recorded for Si, P, and Fe for nutrients and Zn and Mn for trace metals. For the majority of elements, data show some weeks with high episodic fluxes, except for N, Cr, and V, which present the lowest variability. A total of 12 intense mineral dust deposition events are identified during the sampling period. The contribution of these events to the fluxes of Fe and Si represents 52 % and 57 % of their total fluxes, respectively, confirming the important role of these sporadic dust events in the inputs of these elements in the Mediterranean. For N and P, the contribution of these intense dust deposition events is lower and reaches 10 % and 15 %, respectively. Out of these most intense events, positive matrix factorization (PMF) was applied to our total deposition database in order to identify the main sources of nutrients and trace metals deposited. Results show that P deposition is mainly associated with anthropogenic biomass burning inputs. For N deposition, inputs associated with marine sources (maybe associated with the reaction of anthropogenic N on NaCl particles) and anthropogenic sources are quasi-similar. A good correlation is obtained between N and S fluxes, supporting a common origin associated with inorganic secondary aerosol, i.e., ammonium sulfate. For trace metals, their origin is very variable: with a large contribution of natural dust sources for Ni or Mn and conversely of anthropogenic sources for V and Zn.

Size‐Resolved Dry Deposition Velocities of Dust Particles: In Situ Measurements and Parameterizations Testing

AbstractBulk and size‐resolved particle concentrations were measured at 2.1 and 6.5 m above the soil surface during an intense dust deposition event that occurred in June 2006 in Niger. Bulk concentration measurements were performed using two tapered element oscillating microbalance instruments, and the size‐resolved particle concentrations (from 0.3 to 20 μm) using two optical particle counters. The deposition fluxes derived from the bulk concentrations and those derived from the size‐resolved ones are in very good agreement. The largest deposition fluxes are recorded when the dust concentrations are maximal. The temporal evolution of the dust deposition flux follows that of the dust concentration. This is not the case of the dry deposition velocities that are most of the time controlled by the wind friction velocity. The results also show that large particles are strongly sensible to rebound when the wind friction velocity is the highest. Size‐resolved dry deposition velocities are compared with the rare measurements of dust deposition velocities over bare sandy soils and are confronted to existing parameterizations of the dry deposition velocity. The parameterization of Zhang and Shao (2014, https://doi.org/10.5194/acp‐14‐12429‐2014) appears to be the only one able to reproduce satisfyingly the measured dry deposition velocities on sandy soils in the 1‐ to 10‐μm particle size range. Indeed, unlike others, this scheme considers the desert sandy surfaces as rough surfaces, allowing the interception of dust particles by the sand grains and/or by the small roughness elements present on the surface.

Hg in snow cover and snowmelt waters in high-sulfide tailing regions (Ursk tailing dump site, Kemerovo region, Russia)

Gold-bearing polymetallic Cu-Zn deposits of sulphur-pyrite ores were discovered in the Novo-Ursk region in the 1930s. The average content of mercury (Hg) was approximately 120 μg/g at the time.A comprehensive study of Hg distribution in waste of metal ore enrichment industry was carried out in the cold season on the tailing dump site and in adjacent areas. Mercury concentration in among snow particulate, dissolved and colloid fractions was determined. The maximal Hg content in particulate fraction from the waste tailing site ranged 230–573 μg/g. Such indices as the frequency of aerosol dust deposition events per units of time and area, enrichment factor and the total load allowed to establish that the territory of the tailing waste dump site had a snow cover highly contaminated with dust deposited at a rate of 247–480 mg/(m2∙day). Adjacent areas could be considered as area with low Hg contamination rate with average deposition rate of 30 mg/(m2∙day). The elemental composition of the aerosol dust depositions was determined as well, which allowed to reveal the extent of enrichment waste dispersion throughout adjacent areas. The amount of Hg entering environment with snowmelt water discharge was estimated. As a result of snowmelting, in 2014 the nearest to the dump site hydrographic network got Hg as 7.1 g with colloids and as 5880 g as particles. The results obtained allowed to assess the degree of Hg contamination of areas under the impact of metal enrichment industry.

Estimating chemical composition of atmospheric deposition fluxes from mineral insoluble particles deposition collected in the western Mediterranean region

Abstract. In order to measure the mass flux of atmospheric insoluble deposition and to constrain regional models of dust simulation, a network of automatic deposition collectors (CARAGA) has been installed throughout the western Mediterranean Basin. Weekly samples of the insoluble fraction of total atmospheric deposition were collected concurrently on filters at five sites including four on western Mediterranean islands (Frioul and Corsica, France; Mallorca, Spain; and Lampedusa, Italy) and one in the southern French Alps (Le Casset), and a weighing and ignition protocol was applied in order to quantify their mineral fraction. Atmospheric deposition is both a strong source of nutrients and metals for marine ecosystems in this area. However, there are few data on trace-metal deposition in the literature, since their deposition measurement is difficult to perform. In order to obtain more information from CARAGA atmospheric deposition samples, this study aimed to test their relevance in estimating elemental fluxes in addition to total mass fluxes. The elemental chemical analysis of ashed CARAGA filter samples was based on an acid digestion and an elemental analysis by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mass spectrometry (MS) in a clean room. The sampling and analytical protocols were tested to determine the elemental composition for mineral dust tracers (Al, Ca, K, Mg and Ti), nutrients (P and Fe) and trace metals (Cd, Co, Cr, Cu, Mn, Ni, V and Zn) from simulated wet deposition of dust analogues and traffic soot. The relative mass loss by dissolution in wet deposition was lower than 1 % for Al and Fe, and reached 13 % for P due to its larger solubility in water. For trace metals, this loss represented less than 3 % of the total mass concentration, except for Zn, Cu and Mn for which it could reach 10 %, especially in traffic soot. The chemical contamination during analysis was negligible for all the elements except for Cd, which has a very low concentration in dust. Tests allowed us to conclude that the CARAGA samples could be used to estimate the contents of nutrients and trace metals in the limits of loss by dissolution. Chemical characterization of CARAGA deposition samples corresponding to the most intense dust deposition events recorded between 2011 and 2013 has been performed and showed elemental mass ratios consistent with the ones found in the literature for Saharan dust. However, the chemical analysis of CARAGA samples revealed the presence of some anthropogenic signatures, for instance high Zn concentrations in some samples in Lampedusa, and also pointed out that mineral dust can be mixed with anthropogenic compounds in the deposition samples collected on Frioul. Results showed that the chemical analysis of CARAGA ashed samples can be used to trace the origins of elemental deposition. The elemental atmospheric fluxes estimated from these chemical analyses of samples from the CARAGA network of weekly deposition monitoring constitute the first assessment of mass deposition fluxes of trace metals and P during intense dust deposition events at the scale of the western Mediterranean Basin. The mass fluxes strongly depend on the distance from dust sources and the most intense events, while proximity from anthropogenic sources strongly impacted the masse fluxes of Zn and Cu at Lampedusa and Frioul.

Atmospheric Deposition Effects on Plankton Communities in the Eastern Mediterranean: A Mesocosm Experimental Approach

The effects of atmospheric deposition on plankton community structure were examined during a mesocosm experiment using water from the Cretan Sea (Eastern Mediterranean), an area with a high frequency of atmospheric aerosol deposition events. The experiment was carried out under spring-summer conditions (May 2012). The main objective was to study the changes induced from a single deposition event, on the autotrophic and heterotrophic surface microbial populations, from viruses to zooplankton. To this end, the effects of Saharan dust addition were compared to the effects ofmixed aerosol deposition on the plankton community over 9 days. The effects of the dust addition seemed to propagate throughout the food-web, with changes observed in nearly all of the measured parameters up to copepods. The dust input stimulated increased productivity, both bacterial and primary. Picoplankton, both autotrophic and heterotrophic capitalized on the changes in nutrient availability and microzooplankton abundance also increased due to increased availability of prey. Five days after the simulated deposition, copepods also responded, with an increase in egg production. The results suggest that nutrients were transported up the food web through autotrophs, which were favored by the Nitrogen supplied through both treatments. Although, the effects of individual events are generally short lived, increased deposition frequency and magnitude of events is expected in the area, due to predicted reduction in rainfall and increase in temperature, which can lead to more persistent changes in plankton community structure. Here we demonstrate how a single dust deposition event leads to enhancement of phytoplankton and microzooplankton and can eventually, through copepods, transport more nutrients up the food web in the Eastern Mediterranean Sea.

Potential effect of atmospheric dissolved organic carbon on the iron solubility in seawater

Atmospheric aerosols are an important source of soluble iron (sol-Fe) to the global oceans. After deposition to seawater, sol-Fe will either complex with Fe-binding marine organic ligands and enter the ocean dissolved Fe (DFe) pool, or form oxyhydroxide particles (PFe) and precipitate out. Since oceanic DFe is commonly assumed to be bioavailable, the importance of atmospheric sources of sol-Fe for ocean biogeochemistry is determined by both: total fluxes of sol-Fe and the fraction of sol-Fe that is converted to DFe in the ocean. The results from these laboratory studies show that in 20 min from the time of mixing with seawater, nearly all sol-Fe gets oxidized and converted to PFe. The addition of dicarboxylic acids (oxalic and malic) had minor influence on the conversion rate of sol-Fe to PFe. However, the addition of α-hydroxy-carboxylic acids (citric and tartaric) to Fe solutions prior to mixing with seawater was found to considerably increase the concentration of seawater DFe. After 10 days of the experiment, 15% and 50% of the sol-Fe in citric and tartaric acid solutions, respectively, remained in a DFe form. Numerical simulations for sol-Fe laden dust deposition events to the ocean show that if α-hydroxy acids are present in aerosol solution upon deposition to the surface ocean, over 95% of sol-Fe can potentially bind with marine organic ligands; this fraction reduces to < 20% in the absence of atmospheric organics. A possible mechanism is provided to explain the differences in binding strength between aerosol sol-Fe and atmospheric dissolved organic carbon species commonly found in maritime aerosols.

Responses of heterotrophic bacteria abundance and activity to Asian dust enrichment in the low nutrients and low chlorophyll (LNLC) region of the Northwestern Pacific Ocean

Bacteria, as an essential part of microbial food web, play a significant role in the marine ecosystem. Dust deposits into the surface ocean carrying with vital nutrient such as Inorganic nitrogen and phosphorus etc., which has an important influence on the life activities of heterotrophic bacteria. The microcosm experiments with Asian dust deposition was carried out on board in the station K3 (26.18°N, 136.73°E) in April 2015, aiming to estimate the impact of dust deposition on the oligotrophic Northwestern pacific Sea, the main goal of the present paper was to assess how dust deposition events affect the abundance and activity of heterotrophic bacteria in low nutrient and low chlorophyll (LNLC) sea area. Station K3 located in the central northwestern Pacific Ocean, which has the characteristic of low nutrient and low chlorophyll. The study shows that there was an N–P co-limitation in station K3, and the deposition of Asian dust can increase the abundance, and promote the activity of heterotrophic bacteria in the station K3.

Modeling the impacts of atmospheric deposition of nitrogen and desert dust-derived phosphorus on nutrients and biological budgets of the Mediterranean Sea

Atmospheric deposition represents a significant source of nutrients at the Mediterranean basin scale. We apply aerosol deposition fields simulated from atmospheric models into the high resolution oceanic biogeochemical model NEMOMED12/PISCES with nutrient ratios used for plankton growth set to Redfield ratio. We perform 3 simulations to determine the impact of nutrients on productivity over the period 1997–2012: (i) without atmospheric deposition, (ii) with nitrogen deposition from anthropogenic and natural sources, and (iii) with deposition of both nitrogen (from anthropogenic and natural sources) and phosphate from desert dust. Time series of modeled deposition fluxes are compared to available measurements. This comparison with measurements shows that both variability and intensity ranges are realistic enough for our main purpose of estimating the atmospheric deposition impact on Mediterranean biogeochemical tracers such as surface nutrient concentrations, chlorophyll a and plankton concentrations. Our results show that atmospheric deposition is one of the major sources of nitrogen and phosphorus for some regions of the oligotrophic Mediterranean Sea. More than 18·109gNmonth−1 are deposited to the whole Mediterranean Sea. This deposition is responsible for an average increase of 30–50% in primary production over vast regions. Natural dust-derived deposition of phosphorus is sparser in space and time (0.5·109gmonth−1 on average over the entire basin). However, dust deposition events can significantly affect biological production. We calculate fertilizing effects of phosphate from dust to be low on average (6–10%) but up to 30% increase in primary productivity can be observed during the months when surface water stratification occurs. Finally, these fertilizing effects are shown to be transmitted along the biological chain (primary production, Chl a, phytoplankton, zooplankton, grazing). We also perform a preliminary study on the maximal biological response of the Mediterranean by simulating extreme deposition events throughout the basin over a full year period. We show that nitrogen deposition effects observed in our long-term simulations (1997–2012) are close to maximal effects (i.e. those produced by high intensity deposition events) whereas dust-derived phosphate effects are substantially weaker than the effect on productivity reached when an extreme deposition event occurs.

Impact of dust deposition on the albedo of Vatnajökull ice cap, Iceland

Abstract. Deposition of small amounts of airborne dust on glaciers causes positive radiative forcing and enhanced melting due to the reduction of surface albedo. To study the effects of dust deposition on the mass balance of Brúarjökull, an outlet glacier of the largest ice cap in Iceland, Vatnajökull, a study of dust deposition events in the year 2012 was carried out. The dust-mobilisation module FLEXDUST was used to calculate spatio-temporally resolved dust emissions from Iceland and the dispersion model FLEXPART was used to simulate atmospheric dust dispersion and deposition. We used albedo measurements at two automatic weather stations on Brúarjökull to evaluate the dust impacts. Both stations are situated in the accumulation area of the glacier, but the lower station is close to the equilibrium line. For this site ( ∼ 1210 m a.s.l.), the dispersion model produced 10 major dust deposition events and a total annual deposition of 20.5 g m−2. At the station located higher on the glacier ( ∼ 1525 m a.s.l.), the model produced nine dust events, with one single event causing ∼ 5 g m−2 of dust deposition and a total deposition of ∼ 10 g m−2 yr−1. The main dust source was found to be the Dyngjusandur floodplain north of Vatnajökull; northerly winds prevailed 80 % of the time at the lower station when dust events occurred. In all of the simulated dust events, a corresponding albedo drop was observed at the weather stations. The influence of the dust on the albedo was estimated using the regional climate model HIRHAM5 to simulate the albedo of a clean glacier surface without dust. By comparing the measured albedo to the modelled albedo, we determine the influence of dust events on the snow albedo and the surface energy balance. We estimate that the dust deposition caused an additional 1.1 m w.e. (water equivalent) of snowmelt (or 42 % of the 2.8 m w.e. total melt) compared to a hypothetical clean glacier surface at the lower station, and 0.6 m w.e. more melt (or 38 % of the 1.6 m w.e. melt in total) at the station located further upglacier. Our findings show that dust has a strong influence on the mass balance of glaciers in Iceland.

Experimental evidence of formation of transparent exopolymer particles (TEP) and POC export provoked by dust addition under current and high pCO2 conditions.

The evolution of organic carbon export to the deep ocean, under anthropogenic forcing such as ocean warming and acidification, needs to be investigated in order to evaluate potential positive or negative feedbacks on atmospheric CO2 concentrations, and therefore on climate. As such, modifications of aggregation processes driven by transparent exopolymer particles (TEP) formation have the potential to affect carbon export. The objectives of this study were to experimentally assess the dynamics of organic matter, after the simulation of a Saharan dust deposition event, through the measurement over one week of TEP abundance and size, and to evaluate the effects of ocean acidification on TEP formation and carbon export following a dust deposition event. Three experiments were performed in the laboratory using 300 L tanks filled with filtered seawater collected in the Mediterranean Sea, during two ‘no bloom’ periods (spring at the start of the stratification period and autumn at the end of this stratification period) and during the winter bloom period. For each experiment, one of the two tanks was acidified to reach pH conditions slightly below values projected for 2100 (~ 7.6–7.8). In both tanks, a dust deposition event of 10 g m-2 was simulated at the surface. Our results suggest that Saharan dust deposition triggered the abiotic formation of TEP, leading to the formation of organic-mineral aggregates. The amount of particulate organic carbon (POC) exported was proportional to the flux of lithogenic particles to the sediment traps. Depending on the season, the POC flux following artificial dust deposition ranged between 38 and 90 mg m-2 over six experimental days. Such variability is likely linked to the seasonal differences in the quality and quantity of TEP-precursors initially present in seawater. Finally, these export fluxes were not significantly different at the completion of the three experiments between the two pH conditions.

Phytoplankton Response to Saharan Dust Depositions in the Eastern Mediterranean Sea: A Mesocosm Study

The response of phytoplankton populations from surface ultra-oligotrophic waters of the Eastern Mediterranean Sea to Saharan dust additions was studied during a 10-day mesocosm experiment in May 2014. A set of triplicate mesocosms entitled ‘Single Addition’ treatment (SA) was amended with Saharan dust once, while another triplicate set entitled ‘Repetitive Addition’ treatment (RA) received the same amount of dust divided into three consecutive daily doses administered within the first three experimental days, both simulating patterns of dust deposition events taking place in the field. In both treatments, dust particles released small amounts of dissolved inorganic nitrogen and phosphorus which stimulated by two-fold both chlorophyll-a concentration and primary production for a time period of six days, as compared to a set of control mesocosms carried out without dust addition. Phytoplankton response was similar in both treatments, regardless of the dust addition pattern, and it evolved through two distinct phases in both cases. The first phase (i.e. 1 to 2 days after initial addition) was characterized by enhancement of picoplankton chlorophyll-normalized production rates as a result of elevated orthophosphate concentrations while the second phase (i.e. 3 to 4 days after initial dust addition), was characterized by elevated chlorophyll-normalized production rates corresponding to larger cells (> 5 μm) as a result of increased mineral nitrogen concentrations. The stimulated primary production of larger cells was not accompanied by a respective increase in carbon biomass suggesting important top-down control. The major phytoplankton taxa detected during the experiment were Synechococcus, Pelagophytes and Prymnesiophytes. Estimations of cellular pigment concentrations and carbon-to-chlorophyll ratios of identified groups and differences between prokaryotic and eukaryotic cells are discussed.

Trace elements in the sea surface microlayer: rapid responses to changes in aerosol deposition

Natural and anthropogenic aerosols are a significant source of trace elements to oligotrophic ocean surface waters, where they provide episodic pulses of limiting micronutrients for the microbial community. However, little is known about the fate of trace elements at the air-sea interface, i.e. the sea surface microlayer. In this study, samples of aerosols, sea surface microlayer, and underlying water column were collected in the Florida Keys during a dusty season (July 2014) and non-dusty season (May 2015) and analyzed for the dissolved and particulate elements Al, Fe, Ni, Cu, Zn, and Pb. Microlayer samples were collected using a cylinder of ultra-pure SiO2 (quartz glass), a novel adaptation of the glass plate technique. A significant dust deposition event occurred during the 2014 sampling period which resulted in elevated concentrations of trace elements in the microlayer. Residence times in the microlayer from this event ranged from 12 to 94 minutes for dissolved trace elements and from 1.3 to 3.4 minutes for particulate trace elements. These residence times are potentially long enough for the atmospherically derived trace elements to undergo chemical and biological alterations within the microlayer. Characterizing the trace element distributions within the three regimes is an important step towards our overall goals of understanding the rates and mechanisms of the solubilization of trace elements following aeolian dust deposition and how this might affect microorganisms in surface waters.

Large Saharan dust storms: Implications for chlorophyll dynamics in the Mediterranean Sea

AbstractWe investigate the large (LDE) and very large (VLDE) Saharan dust deposition events that occurred between 2000 and 2007 and their short‐term impact on the dynamics of marine phytoplankton in the Mediterranean Sea. A total of 153 LDE were identified unevenly distributed over the years. Events were more frequent during winter, in the eastern Mediterranean, and autumn, when they affected both the western and the central Mediterranean. Most of the 31 VLDE occurred during winter and autumn in the central Mediterranean. The dynamics of chlorophyll after VLDE were studied as a proxy for phytoplankton response to atmospheric dust. A significant response of chlorophyll to dust addition was evident; this appeared to be especially true for the western Mediterranean where a chlorophyll increase of up to 345% was recorded, whereas in the central Mediterranean it was up to 146% and in the eastern Mediterranean up to 121%. Chlorophyll response behavior was quite heterogeneous probably as a result of the uniqueness of each VLDE, the differences between Mediterranean areas, the community structure of phytoplankton, and the interaction between bacteria and phytoplankton for new resources. An eastward decreasing trend in chlorophyll response was observed, which is in accordance with the relative importance of bacterial activity with respect to phytoplankton. The increase in mineral aerosols with increased aridity in the region together with the decrease in the depth of the mixed layer of the oceans should boost the importance of aerosols fueling marine production.

Variability of mineral dust deposition in the western Mediterranean basin and south-east of France

Abstract. Previous studies have provided some insight into the Saharan dust deposition at a few specific locations from observations over long time periods or intensive field campaigns. However, no assessment of the dust deposition temporal variability in connection with its regional spatial distribution has been achieved so far from network observations over more than 1 year. To investigate dust deposition dynamics at the regional scale, five automatic deposition collectors named CARAGA (Collecteur Automatique de Retombées Atmosphériques insolubles à Grande Autonomie in French) have been deployed in the western Mediterranean region during 1 to 3 years depending on the station. The sites include, from south to north, Lampedusa, Majorca, Corsica, Frioul and Le Casset (southern French Alps). Deposition measurements are performed on a common weekly period at the five sites. The mean dust deposition fluxes are higher close to the northern African coasts and decrease following a south–north gradient, with values from 7.4 g m−2 year−1 in Lampedusa (35°31′ N, 12°37′ E) to 1 g m−2 year−1 in Le Casset (44°59′ N, 6°28′ E). The maximum deposition flux recorded is of 3.2 g m−2 wk−1 in Majorca with only two other events showing more than 1 g m−2 wk−1 in Lampedusa, and a maximum of 0.5 g m−2 wk−1 in Corsica. The maximum value of 2.1 g m−2 year−1 observed in Corsica in 2013 is much lower than existing records in the area over the 3 previous decades (11–14 g m−2 year−1). From the 537 available samples, 98 major Saharan dust deposition events have been identified in the records between 2011 and 2013. Complementary observations provided by both satellite and air mass trajectories are used to identify the dust provenance areas and the transport pathways from the Sahara to the stations for the studied period. Despite the large size of African dust plumes detected by satellites, more than 80 % of the major dust deposition events are recorded at only one station, suggesting that the dust provenance, transport and deposition processes (i.e. wet vs. dry) of dust are different and specific for the different deposition sites in the Mediterranean studied area. The results tend to indicate that wet deposition is the main form of deposition for mineral dust in the western Mediterranean basin, but the contribution of dry deposition (in the sense that no precipitation was detected at the surface) is far from being negligible, and contributes 10 to 46 % to the major dust deposition events, depending on the sampling site.