# Phytoplanktonic response to contrasted Saharan dust deposition events during mesocosm experiments in LNLC environment

Céline Ridame, J. Dekaezemacker, Cécile Guieu, Sophie Bonnet, Stéphane

L'Helguen, F. Malien

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Céline Ridame, J. Dekaezemacker, Cécile Guieu, Sophie Bonnet, Stéphane L'Helguen, et al.. Phytoplanktonic response to contrasted Saharan dust deposition events during mesocosm experiments in LNLC environment. Biogeosciences Discussions, 2014, 11, pp.753-796. 10.5194/bgd-11-753-2014. hal-00952674

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# Phytoplanktonic response to contrasted Saharan dust deposition events during mesocosm experiments in LNLC environment

C. Ridame<fnref n="1" />,<fnref n="2" />, J. Dekaezemacker<fnref n="3" />,<fnref n="4" />, C. Guieu<fnref n="5" />,<fnref n="6" />, S. Bonnet<fnref n="3" />, S. L'Helguen⁷, and F. Malien⁸

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Abstract

Introduction

Conclusions

References

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<footnote n="1">¹CNRS-INSU/IRD/MNHN/UPMC, LOCEAN: Laboratoire d'Océanographie et du Climat: Expérimentation et Approches Numériques, UMR 7159, 4 Place Jussieu – 75252 Paris Cedex 05, France</footnote>

<footnote n="2">²Sorbonne Universités, UPMC Univ Paris 06, UMR 7159, LOCEAN, 75005, Paris, France</footnote>

<footnote n="3">³Mediterranean Institute of Oceanography, IRD/Aix Marseille Université/CNRS
Marseille/Noumea, New Caledonia, France</footnote>

<footnote n="4">⁴Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany</footnote>

<footnote n="5">⁵LOV: Laboratoire d'Océanographie de Villefranche/Mer, CNRS-INSU UMR7093, Observatoire Océanologique, 06230 Villefranche-sur-Mer, France</footnote>

<footnote n="6">⁶Sorbonne Universités, UPMC Univ Paris 06, UMR 7093, LOV, Observatoire Océanologique, 06230 Villefranche-sur-Mer, France</footnote>
⁷Université de Brest, CNRS/IRD, UMR 6539, Laboratoire des Sciences de l'Environnement Marin, OSU-IUEM, 29280 Plouzané, France

⁸GEOMAR Helmholtz Centre for Ocean Research Kiel, Chemical Oceanography, Düsternbrooker Weg 20, 24105 Kiel, Germany

Received: 16 December 2013 – Accepted: 18 December 2013 – Published: 13 January 2014

Correspondence to: C. Ridame (celine.ridame@upmc.fr)

Published by Copernicus Publications on behalf of the European Geosciences Union.

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The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE "a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem" project. We simulated realistic dust deposition events (10 $gm^{-2}$) into large mesocosms (52 $m^3$). Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition, DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1, -R2: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentration (Chl a) were observed after a dry deposition event while a wet deposition event resulted in a rapid (24 h after dust additions), strong (up 2.4 fold) and long (at least a week duration) increase in PP and Chl a. We show that in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of $NO_3^-$ (net increases up to +9.8 $\mu M$ $NO_3^-$ at 0.1 m depth) to the nutrient depleted surface waters due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of $NO_3^-$.
By transiently increasing DIP and $NO_3^-$ concentrations in P-N starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of $NO_3^-$ relative to DIP, a wet deposition event resulted in a strong increase in the $NO_3^-$ /DIP ratio from initially < 6 to over 150 at the end of the DUNE-2-R1 experiment suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP increased after wet dust deposition events from initially 15% to 60-70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE experiments show that wet and dry dust deposition events induce contrasted responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm

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experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments, changes nutrient stocks and alters the NO₃⁻/DIP ratio leading to a switch in the nutrient limitation of the phytoplanktonic activity.

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# 1 Introduction

The primary nutrients that limit marine phytoplankton productivity include nitrogen (N), phosphorus (P) and iron (Fe). Mineral dust deposition from arid regions provides a source for each of these nutrients to the open ocean surface waters and can have important effects on marine biogeochemical cycles and potentially affect the global climate (e.g., Baker et al., 2003; Jickells et al., 2005; Mahowald et al., 2008). Researchers have suggested that Fe delivered by dust could have stimulated oceanic productivity and may help explain the glacial-interglacial atmospheric pCO₂ transition (Martin, 1990; Falkowski, 1997; Ridgwell and Watson, 2002; Mahowald et al., 2006). While the impact of Fe on productivity has been recognized in high nutrient low chlorophyll (HNLC) oceanic areas (Boyd et al., 2000, 2007; de Baar et al., 2005; Blain et al., 2007), the ecological and biogeochemical effects of aeolian dust deposition of macro-N (N, P) and micro-nutrients (Fe) in oligotrophic low nutrient low chlorophyll (LNLC) environments remain less explored and still poorly understood although LNLC regions represent 60% of the global surface ocean (Longhurst et al., 1995) and over 50% of the global oceanic carbon export (Emerson et al., 1997). The mineral dust can be transported thousands of miles across the Atlantic and Pacific oceans and deliver large quantities of nutrients to the LNLC Atlantic and Pacific gyres which in turn could play a significant role in stimulating primary production (Bishop et al., 2002; Johnson et al., 2003; Duarte et al., 2006; Maranon et al., 2010; Franchy et al., 2013) potentially increasing the efficiency of the biological pump in the sequestration of atmospheric CO₂.

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The Mediterranean Sea is a typical LNLC region particularly well adapted to tackle the question of the planktonic response to atmospheric nutrient inputs. The Mediter-

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ranean Sea is an oligotrophic quasi-enclosed basin which receives a noticeable flux of dust, mainly derived from Sahara Desert, in the form of strong pulses (Guerzoni et al., 1999; Guieu et al., 2010a). After the seasonal phytoplanktonic bloom during spring, the surface mixed layer is isolated from deeper waters by a strong thermal stratification, and becomes N and P depleted (Marty et al., 2002; Pulido-Villena et al., 2010) leading to low primary productivity and phytoplanktonic biomass (Moutin and Raimbault, 2002; Marty and Chiaverini, 2002; Bosc et al., 2004; Lopez-Sandoval et al., 2011). During the whole stratification period, atmospheric inputs are therefore the main source of allochtonous nutrients to the oligotrophic surface waters. By bringing new macro-nutrients (e.g. Herut et al., 1999; Ridame and Guieu, 2002; Pulido-Villena et al., 2010; Markaki et al., 2010) and Fe (e.g. Bonnet and Guieu, 2006; Wuttig et al., 2013) to the Mediterranean surface waters, Saharan dust deposition is strongly suspected to play a key role in the control of primary production. During the BOUM (Biogeochemistry from the Oligotrophic to the Ultra-Oligotrophic Mediterranean) cruise in the summer of 2008, nutrient/dust additions in microcoms showed that primary production and chlorophyll a were N or NP co-limited in the western Mediterranean Sea (Tanaka et al., 2011) and that Saharan dust stimulated primary production (Ternon et al., 2011).

The goal of the DUNE project was to estimate the impact of Saharan dust events on a whole LNLC ecosystem like the Mediterranean Sea from virus to zooplankton over a period of one week and to evaluate the biogeochemical implications associated with this forcing (Guieu et al., 2010b, 2013a). The approach applied in DUNE was, for the first time, to perform realistic dust seedings onto large metal-free mesocosms. In this context, the present paper is focused on the response of the whole phytoplanktonic community to contrasted Saharan dust events (wet and dry deposition). Here we quantify the modifications of in situ concentrations of dissolved inorganic nitrogen as well as the changes in the primary production and algal biomass (chlorophyll a) after simulated dust deposition events. This study is complementary to the companion paper of Giovanetti et al. (2013) focusing on changes in the structure and composition of the
phytoplanktonic community as well as in the ecophysiological state of the phytoplanktonic cells after two successive wet deposition events simulated during DUNE-2-R.

## 2 Material and methods

### 2.1 Experimental design and dust characterization

Three distinct experimental dust additions into large mesocosms were conducted in June 2008 and June–July 2010 in the northwestern Mediterranean Sea, in the frame of the DUNE (“a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem”) project (http://wwwobs-vlfr.fr/LOV/DUNE/index.html). More precisely, the experiments were realized in the Elbo Bay located in the Natural Preservation Area of Scandola (8.554° E, 42.374° N) which is representative of the LNLC conditions of the open western Mediterranean Sea (Guieu et al., 2010b, 2013a). The mesocosms experiment design and the accuracy of the strategy are fully described in Guieu et al. (2010b). Briefly, six mesocosms (height: 12.5 m, diameter: 2.3 m, surface area: 4.15 m², and volume: 52 m³) entirely designed in plastic were deployed. The bags were made of polyethylene mixed with vinyl acetate and the holding structure of PVC and polyethylene. The anchors were screwed in the sea floor at 25–30 m depth. The mesocosms are closed systems, which prevent lateral advection and allow process studies in the vertical dimension over the time. Sediment traps (a 250 mL HDPE bottle) were screwed to the base of the mesocosms for the collection of the exported material and were changed every day (DUNE-2-R) or every two days (DUNE-1-P, -Q). Three replicate mesocosms (D1, D2 and D3 hereafter referred to as “Dust-meso”) were amended with 41.5 g of mineral dust reproducing a high, but environmentally realistic atmospheric dust deposition of 10 g m⁻² (Guieu et al., 2010b). The dust used in the three seeding experiments resulted from the fine fraction (< 20 µm) of Saharan soils collected in southern Tunisia, which is a Saharan aerosol source area (details in Guieu et al., 2010b; Desboeufs et al., 2013). Three other mesocosms served as controls (C1, C2 and C3 hereafter referred

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to as “Control-meso”). The sampling session took place every morning at the same time over the duration of the experiments. Each day, three different depths (0.1, 5 and 10 m depth) were sampled in the six mesocosms using a trace metal clean system of permanent PVC tubing placed at the center of the bags and connected to a Teflon pump. Additional depths (2.5 and 12.5 m) were sampled in two dust mesocosms during DUNE-2-R for nutrient determination. Every 48 h, seawater was also collected outside the mesocosms (hereafter referred to as “out”) at the same depths in order to test the representativeness of data between the Control-meso and out.

*June 2008 experiments:* Two distinct seeding experiments were conducted with two types of dust. In the first one (10–18 June 2008 hereafter referred to as “DUNE-1-P”), the dust used was previously subjected to physico-chemical transformations through condensation/evaporation cycles that involved HNO₃ and H₂SO₄, mimicking atmospheric transport (Desboeufs et al., 2001; and details in Guieu et al., 2010b). Indeed, Saharan dust collected in the Mediterranean atmosphere is usually mixed with organic and inorganic material, as sulfate and nitrate due to cloud processing during atmospheric transport (e.g. Puteaud et al., 2004). This evapocondensed dust (hereafter referred to as “EC dust”) contained on average 0.045±0.015% of P, 2.31±0.04% of Fe and 1.19±0.05% of N, in weight (Table 1). The amendment of the dust mesocosms in the DUNE-1-P experiment was performed with the EC dust mixed with 2 L of ultrapure water in order to mimic a wet deposition event.

In the second experiment (20–27 June 2008 hereafter referred to as “DUNE-1-Q”), non-processed dust was used (hereafter referred to as “non-EC dust”) and contained on average 0.044±0.009% of P, 2.28±0.19% of Fe and 0.11±0.01% of N (Table 1). The amendment of the dust mesocosms in the DUNE-1-Q experiment was conducted with the non-EC dust mixed with 2 L of unfiltered surface seawater in order to mimic a dry deposition event.

*June–July 2010 experiments:* Two successive seeding experiments (26 June–2 July 2010 hereafter referred to as “DUNE-2-R1” and 3–9 July 2010 hereafter referred to as “DUNE-2-R2”, see details in Guieu et al., 2013a) were performed with the same
amount of EC dust and with the same deposition setup than in DUNE-1-P simulat-
ing a wet deposition event. The dust was characterized by an average content of
0.055 ± 0.003 % P, 2.26 ± 0.03 % Fe and 1.36 ± 0.09 % N (Table 1; see details in Des-
boeufs et al., 2013).

*P, Fe, N contents of the dust:* The particulate P and Fe contents of the EC dust (DUNE-1-P and DUNE-2-R) and non-EC dust (DUNE-1-Q) were similar (p > 0.05, Table 1). Due to the simulation of cloud water processes that involved HNO₃, the N content of EC dust was about 10 fold higher in comparison with the non-EC dust (Table 1; Guieu et al., 2010b, 2013a). Small differences of the N content in EC dust used in 2008 and 2010 experiments (1.19 ± 0.05% and 1.36 ± 0.09%) were observed (details in Guieu et al., 2013a).

## 2.2 Primary production

All materials were previously acid-washed (HCl Suprapur) following trace metal clean procedures. Before sampling, bottles were rinsed three times with the sampled seawater. One sample per depth of unfiltered seawater was collected in the morning at two depths (0.1 and 5 m depth) during DUNE-1-P and -Q and at 5 m depth during DUNE-2-R for determination of primary production (PP). Samples were collected in the six mesocosms and outside the mesocosms before and after dust seeding. During DUNE-1-P and -Q, 2.5 mL of NaH¹³CO₃ (99%, Eurisotop) were added to 4.5 L polycarbonate bottles for ¹³C uptake determination while during DUNE-2-R, 1.5 mL of NaH¹³CO₃ were added to 2.3 L polycarbonate bottles (¹⁵N₂ tracer was also added in order to determine simultaneously N₂ fixation rates using the dual ¹³C/¹⁵N₂ isotopic label technique, see Ridame et al., 2013). Prior to DUNE-2-R, intercomparison of PP rates measured in both 2.3 and 4.5 L incubated volumes showed coefficients of variation (CV) lower than 15% (unpublished data) which was in the same order of magnitude than those found between triplicate mesocosms (CV < 20%). Immediately after sampling, ¹³C tracer was added (Fig. 1a) to obtain a final enrichment of about

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9 atom% excess and each bottle was well shaken. Then, the ¹³C-amended bottles were incubated under in situ conditions on a mooring line, outside the mesocosms for 24 h at the corresponding sampling depths (0.1 m and 5 m depth, Fig. 1b). Incubations were terminated the following morning by filtration onto pre-combusted 25 mm GF/F filters (0.7 µm nominal porosity). Sample filters were stored at -20 °C and dried at 40 °C for 48 h before analysis. Concentrations of carbon and nitrogen in particulate matter and ¹³C-enrichment were quantified with an isotope ratio mass spectrometer (IRMS, Delta plus, ThermoFisher Scientific, Bremen, Germany) coupled with a C/N analyzer (Flash EA, ThermoFisher Scientific) via a type III interface. Standard deviation was 0.009 µmol L⁻¹ and 0.004 µmol L⁻¹ for particulate carbon and nitrogen respectively, and 0.0002 atom% for ¹³C enrichment. From these measurements, the C : N molar ratios in the particulate matter were calculated.

For DUNE-1-P and-Q, PP measured at 0.1 and 5 m depth were integrated over the mesocosm depth (12.5 m). We assume that PP at 5 m depth was similar to that at 12.5 m depth based on the similarity of the chlorophyll *a* concentrations (Chl *a*) measured at 5 and 10 m depth in the Control- and Dust-meso over the experiments (*p* > 0.05). For DUNE-2-R, PP measured at 5 m depth was integrated over the water column by assuming a homogenous distribution. Over DUNE-2-R1, Chl *a* was similar between the three sampling depths (*p* > 0.05). During DUNE-2-R2, Chl *a* at 5 and 10 m depth was similar (*p* > 0.05) while Chl *a* in surface waters was slightly higher (+ ~ 30%, *p* < 0.05) than deeper at 3 sampling times.

## 2.3 Estimation of new production

The increase in PP in the Dust-meso 24 h after seeding can be associated with new production (NP<sub>seeding</sub>) as atmospheric deposition represents a source of external nutrients to the surface layer. We consider that after 24 h, the increase in PP could be partly supported by regenerated nutrients. NP was estimated before seeding and over the course of the experiments in the Control-meso considering that NP represents 15%
of PP during periods of stratification in the western Mediterranean Sea (Marty et al., 2002; Moutin and Raimbaut, 2002; L'Helguen et al., 2002). We estimated the NP in the Dust-meso (NP<sub>dust</sub>) 24 h after seeding using the following equations (from Ridame et al., 2013):

$$5 \quad \mathrm{NP}_{\mathrm{dust}} = \mathrm{NP}_{\mathrm{control}} + \mathrm{NP}_{\mathrm{seeding}}$$

$$\mathrm{NP}_{\mathrm{seeding}} = \mathrm{PP}_{\mathrm{dust}} - \mathrm{PP}_{\mathrm{control}}$$

## 2.4 Chlorophyll a

2 L of seawater from 0.1, 5 and 10 m depth were collected at daily scale and then filtered onto 25 mm GF/F filters. After 24 h extraction in 90% acetone, at 4 °C and in the dark, the fluorescence of Chl a was measured on a Turner Trilogy Laboratory fluorometer (Strickland and Parsons, 1997). The Chl a concentrations measured at the three sampling depths were integrated over the mesocosm depth (12.5 m).

## 2.5 Nutrient concentrations

20 mL of filtered seawater (< 0.2 µm, Sartobran cartridge filters) from 0.1, 5 and 10 m depth were collected in acid-washed HDPE (high density polyethylene) bottles and frozen until analysis. Nitrate (NO<sub>3</sub><sup>-</sup>) and nitrite (NO<sub>2</sub><sup>-</sup>) were analyzed according to classical methods using the automated colorimetric technique as described in Grasshoff et al. (1999), on a QuAAtro Continuous Flow Analyzer (SEAL Analytical). The precision of measurements was ±30 nM and ±40 nM for NO<sub>3</sub><sup>-</sup> and NO<sub>2</sub><sup>-</sup>, respectively. The limits of detection, defined as three times the standard deviation of the blank were 30 nM and 10 nM for NO<sub>3</sub><sup>-</sup> and NO<sub>2</sub><sup>-</sup> respectively.

Dissolved inorganic phosphorus (DIP) concentrations presented in Pulido-Villena et al. (2010, 2014) were analyzed immediately after collection on 0.2 µm-filtered seawater by spectrophotometry using a long waveguide capillary cell (LWCC); the detection limit was 2 nM (details in Pulido-Villena et al., 2010, 2014). The concentration

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of dissolved Fe (DFe) (< 0.2 µm) was measured by flow injection analysis with online preconcentration and chemiluminescence detection (FIA-CL); the detection limit was 10 p.m. (details in Wagener et al., 2010). These data are presented in Wagener et al. (2010) and Wuttig et al. (2013).

## 2.6 Dissolution experiments under abiotic conditions

Both types of Saharan dust (EC and non-EC dust) used during the DUNE-1-P and -Q experiments were studied to quantify under abiotic conditions the amount of inorganic nitrogen (nitrate + nitrite: NOₓ; ammonium: NH₄⁺) potentially released in the Dust-meso after seeding. This work was complementary to the *in vitro* dissolution experiments performed for the quantification of DFe (Wagener et al., 2010) and DIP (Pulido-Villena et al., 2010) released from dust. All experiments were performed in a clean room under a laminar flow hood using trace metal clean techniques and all materials were previously acid-washed (5% HCl).

Dissolution experiments were conducted in sterile artificial seawater (Chen et al., 1996). For each type of dust, a stock solution of 20 mg dust L⁻¹ was made in artificial seawater and quickly (less than 2 min to minimize the instantaneous dissolution processes) after homogenization, increasing amounts of this stock solution were added to artificial seawater to reach a range of nine concentrations from 0 to 20 mg dust L⁻¹: 0, 0.01, 0.1, 0.5, 1, 3, 5, 10 and 20 mg dust L⁻¹. Each condition was performed in triplicate in 250 mL Nalgene® polycarbonate bottles. After dust inoculation, bottles were placed on an agitation table in the dark for 3 and 24 h. At each time point, 125 mL of each sample were filtered on washed (0.5% HCl and rinsed three times with ultrapure water) Nucleopore® polycarbonate filters (0.2 µm porosity) under a 200 mbar vacuum. The filtration was performed to remove Saharan dust from liquid phase and to stop dissolution processes. The filtrate from each sample was subsampled to measure the concentration of NOₓ and NH₄⁺. Samples for NOₓ were stored in Nalgene® HDPE vials in the dark at 4 °C for 2 days and NH₄⁺ samples were immediately analyzed.

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ysed. Briefly, NOₓ concentration was analysed with an AutoAnalyzer3 Digital Colorimeter (Bran Luebbe) according to the method described by Aminot and Kérouel (2007) (detection limit of 20 nmol L⁻¹). For NH₄⁺ concentration, 40 mL of filtrate were recovered in a Schott flask (Duran) and analysed by the method described in Holmes et al. (1999) with a Turner Trilogy Laboratory fluorometer (detection limit of 11 nmol L⁻¹). Due to salt contamination, the initial concentrations of NOₓ and NH₄⁺ in the artificial seawater were 0.19 ± 0.02 µmol L⁻¹ (n = 12) and 1.46 ± 0.12 µmol L⁻¹ (n = 7), respectively.

## 2.7 Statistical analysis

Means of Chl *a*, PP, and NO₃⁻ concentrations in the Dust- and Control-meso as well as the NOₓ concentrations in the dissolution experiments were compared using a repeated measure ANOVA and a Fisher Least Significant Difference (LSD) means comparison test (α = 0.05). When assumptions for ANOVA were not respected, means were compared using a Kruskal-Wallis test and a post hoc Dunn's test in XLstat software.

# 3 Results

## 3.1 Characteristics of the seawater before seeding and evolution of the environmental conditions during the experiments

Statistical analysis of biological and chemical parameters in Table 2 showed no significant difference between Control-, Dust-meso and out before seeding for all experiments (Guieu et al., 2010b, 2013a). Chl *a* (0.07–0.11 µg L⁻¹) and PP (3.89–5.35 mg C m⁻³ d⁻¹) were initially low, and were slightly higher in DUNE-1-P relative to DUNE-1-Q and DUNE-2-R (*p* < 0.05, Table 2). The picophytoplanktonic biomass (Chl *a* < 3 µm) accounted for about 70% of the total phytoplanktonic biomass before the Q and R seedings (C. Brunet, personal communication, 2013; Giovagnetti et al., 2013) (no data for DUNE-1-P). *Synechococcus* was the most abundant phytoplanktonic organism in the

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< 3 µm size fraction before all the dust additions (unpublished data; Giovagnetti et al., 2013). The initial C : N molar ratios in the particulate fraction, ranging from 7.3 to 7.8 were slightly higher than the Redfield ratio of 6.6 (106/16). The water column was initially DIP depleted (2–5 nM) with concentrations lower in the DUNE-1-Q experiment (2 nM) than in DUNE-1-P and DUNE-2-R (5 nM; *p* < 0.05, Pulido-Villena et al., 2010, 2014). The initial NO₃⁻ and NO₂⁻ concentrations were below detection limit (< 30 nM) in the DUNE-R experiment. By assuming a maximum initial NO₃⁻ concentration of 30 nM in DUNE-2-R, it should lead to a maximum NO₃⁻/DIP ratio of 6. Due to analytical problems, NO₃⁻ concentrations were not available for P and Q experiments but are strongly suspected to be below detection limit before seeding as shown in experiment R and in surface waters of the northwestern Mediterranean Sea during stratification (Marty et al., 2002; Pujo-Pay et al., 2011). The initial DFe concentration was higher for DUNE-2-R (3.3 nM) relative to DUNE-1-P and -Q (~ 2.3 nM, *p* < 0.05, Table 2).

The initial seawater temperatures over the water column were warm (> 19°C, Table 2). During DUNE-1-P, the temperature was lower than during the other ones (Guieu et al., 2013a). Over the duration of this experiment, the temperature in the water column of the mesocosms was homogenous and stable (mean *T* = 19.8 ± 0.5°C, Guieu et al., 2010b) while over the course of DUNE-1-Q, temperature increased rapidly (up to 26.0°C at the surface) leading to a strongly marked thermal stratification typical of summer conditions (mean Δ*T*₀–10m = 3.6°C between 21 and 27 June, Guieu et al., 2013a). Over DUNE-2-R1, changes in temperature were representative of the transition period between spring (low stratification) and summer (strong stratification) conditions. While at the beginning of the second seeding (DUNE-2-R2) stratification was well established, a destratification followed by a restratification was then observed (details in Guieu et al., 2013a). The highest temperature was recorded during DUNE-2-R2 (up to 27.3°C at the surface). Photosynthetically available radiation (PAR) was measured during DUNE-2-R (Guieu et al., 2013a; Giovagnetti et al., 2013). Over the 13 days of the experiment, PAR was high with maximum values of ~ 900 µmol photons m⁻²s⁻¹ at the subsurface. During DUNE-1-P, the percentage of sunshine duration over the whole
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day light was lower than during DUNE-1-Q where the values were close to the maximum (Guieu et al., 2013a). So, it is likely that PAR values increased between P and Q experiments.

After the DUNE-2-R1 and -R2 seedings, strong increases in $NO_3^-$ concentrations at the surface of the Dust-meso were observed relative to the Control-meso where $NO_3^-$ concentrations were lower than 30 nM (Fig. 2a). Ten hours after the first seeding, $NO_3^-$ concentrations in surface waters reached 3.3±0.3 µM and then decreased to 0.5±0.1 µM at the end of R1 (day 6). Five hours after the second seeding, an additional increase in $NO_3^-$ concentrations (+9.8 µM relative to day 6 in the Dust-meso) was detected at the surface leading to $NO_3^-$ concentrations of 10.3±2.4 µM. At the end of R2, $NO_3^-$ concentrations in the Dust-meso decreased up to detection limit. Lower increases in $NO_3^-$ concentrations were recorded at 5 and 10 m depth in the Dust-meso (maximum of ~1 µM) (Fig. S1). $NO_3^-$ and $NO_2^-$ concentrations in the Control-meso were below detection limit at all the three depths over the 13-days duration of the experiment. No change in $NO_2^-$ concentrations was observed in the Dust-meso after both seedings. Depth-integrated $NO_3^-$ concentrations showed that the stock increased to 10.6 mmol $NO_3^-$ m$^{-2}$ few hours after the R1 seeding, and then decreased to value significantly higher relative to the Control-meso at the end of the R1 (6.3 mmol $NO_3^-$ m$^{-2}$, day 6) (Fig. 2b). The second seeding led to an additional increase in the $NO_3^-$ stock of about 10 mmol $NO_3^-$ m$^{-2}$ relative to day 6 in the Dust-meso. The $NO_3^-$ stock was maximum few hours after the R2 seeding (~16 mmol $NO_3^-$ m$^{-2}$) and dropped to undetectable value at the end of the experiment.

After the simulated wet deposition in DUNE-1-P and DUNE-2-R1, Pulido-Villena et al. (2010, 2014) showed transient increases in DIP concentrations few hours after seeding (up to +12 nM at the surface) leading to net increases in the DIP stock (from +21 to +24 µmol DIPm$^{-2}$) in the Dust-meso relative to the Control-meso. 24 h after these seedings, DIP stock dropped to initial concentrations and remained constant until the end of the experiments. Over DUNE-2-R2, the second dust addition resulted
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in a higher increase in the DIP stock (+54 µmol DIP m⁻² in the Dust-meso relative to the Control-meso) which remained stable and significantly higher relative to the controls until the end of the experiment (Pulido-Villena et al., 2014). No change in the DIP concentration was recorded in the Dust-meso after the Q seeding simulating a dry deposition event (Pulido-Villena, personal communication, 2013).

## 3.2 Response of the phytoplanktonic community to dust seeding

Depth-integrated Chl *a* and PP showed no significant differences between OUT and the Control-meso during the whole P and Q experiments (Figs. 3 and 4). From day 4 in the DUNE-2-R experiment, integrated Chl *a* and PP increased in OUT relative to the Control-meso (Fig. 5) as bacterial abundance did (Pulido-Villena et al., 2014). Over the duration of the three experiments, integrated Chl *a* and PP were homogeneous and remained stable in the Control-meso as shown by the low coefficients of variation (CV < 20%). The mean C : N molar ratio in the particulate matter did not change significantly (*p* > 0.05) in the Dust-meso after all the seedings (P, Q, R) relative to the Control-meso (Fig. S2). The average C : N ratios were 7.5 ± 0.4, 7.6 ± 0.6 and 7.8 ± 0.6 respectively over the duration of the P, Q and R experiments.

*Response of the phytoplanktonic community to a wet deposition event.* In the DUNE-1-P experiment, the EC dust addition led to a 1.6 fold increase (*p* < 0.05) in the integrated Chl *a* relative to the Control-meso 24 h after the seeding and to an average 1.9 fold increase over the duration of the experiment (Fig. 3). The increase in Chl *a* was observed during the entire duration of the experiment as shown by the 1.9 fold increase (*p* < 0.05) detected 7 days after the seeding. The increase in Chl *a* was observed at all the sampling depths (Fig. S3) with similar relative changes (*p* > 0.05). Like Chl *a*, integrated PP strongly increased after the seeding during 7 days (1.8 fold on average) compared to the unamended controls (*p* < 0.05, Fig. 3). 24 h after seeding, PP in the Dust-meso was about twice higher in comparison with the Control-meso. Relative increases in PP at the surface and at 5 m depth were similar (*p* > 0.05, Fig. S4).
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*Response of the phytoplanktonic community to a dry deposition event.* Over the duration of the DUNE-1-Q experiment, integrated Chl *a* and PP did not significantly increase after the addition of non-EC dust throughout the water column (*p* > 0.05, Figs. 4, S5, S6). A slight stimulation of PP (1.2 fold, *p* < 0.05) was detected in the Dust-meso at 5 m depth, 48 h after dust addition (Fig. S6).

*Response of the phytoplanktonic community to 2 successive wet deposition events.* In the DUNE-2-R experiment, the first and second seedings led to averaged 1.9 and 2.4 fold increases in integrated Chl *a* relative to the Control-meso over the duration of the experiments (Fig. 5). Six days after both seedings, Chl *a* in the Dust-meso was still twice higher (*p* < 0.05) than in the Control-meso. The increase in Chl *a* was observed all over the three depths (Fig. S7). Integrated PP in the Dust-meso was enhanced 2.3 fold on average after the first seeding from day 1 (2.5 fold) to day 6 (2.5 fold) (Figs. 5, S8). Relative to the PP in the Dust-meso at the end of R1 (day 6), the second seeding permitted an additional 1.8 fold stimulation of PP 24 h after R2 (day 8). At the end of the experiment (day 13), PP in the Dust-meso was still twice higher than that in the unamended controls.

*Comparison of the relative changes between P, R1 and R2 experiments-* The mean relative changes (RC, Dust/Control) in PP were similar over the duration of R1 and R2 (RC = 2.3 and 2.4, respectively, *p* > 0.05, Fig. 6) while the RC in Chl *a* was higher over R2 (RC = 2.4, *p* < 0.05) than over R1 (RC = 1.9) (Fig. 6). After a first wet deposition event simulated in the P and R1 experiments, the respective RC in Chl *a* were similar (RC = 1.9, *p* > 0.05) while the RC in PP was higher over DUNE-2-R1 (RC = 2.3) than over P (RC = 1.8) (*p* < 0.05). The RC in both PP and Chl *a* were significantly higher after the R2 seeding than after the P seeding. The relative increase in Chl *a* was similar (*p* > 0.05) to that in PP in the P and R2 experiments. In the R1 experiment, the relative increase in PP (RC = 2.3) was slightly higher than that in Chl *a* (RC = 1.9).
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## 3.3 Nitrogen solubility associated with dust under abiotic conditions

Additions of both EC and non-EC dust did not significantly change ($p > 0.05$) the $NH_4^+$ concentrations compared to the unamended artificial seawater (data not shown). Significant increases ($p < 0.05$) in the $NO_x$ concentrations were observed concomitantly with increasing EC dust concentrations for both contact times (Fig. 7a). No significant differences in the $NO_x$ concentrations ($p > 0.05$) released from EC dust have been found between 3 and 24 h contact times. $NO_x$ concentrations increased linearly ($r^2 = 0.99$) with the increasing EC dust concentration up to 17 µM for 20 mg dust $L^{-1}$. The percentages of dissolution ($\Delta NO_x \times 100/N_{tot}$) reached 100% from EC dust concentrations higher than 0.5 mg $L^{-1}$, at both time points (Fig. 7b). Additions of non-EC dust did not significantly increase ($p > 0.05$) $NO_x$ concentrations relative to the unamended artificial seawater (Fig. 7a).

## 4 Discussion

The initial characteristics of seawater in all experiments were typical of LNLC environments as depicted by low nutrient concentrations ($DIP, NO_3^-$), low primary production and low phytoplanktonic biomass. These values were consistent with previous measurements in the surface waters of the open western Mediterranean Sea during stratification periods (Moutin and Raimbault, 2002; Bosc et al., 2004; Lopez-Sandoval et al., 2011; Pujo-Pay et al., 2011) and at the DYFAMED (DYnamique des Flux Atmosphériques en MEDiterranée) times series station http://wwwobs-vlfr.fr/sodyf/; Marty and Chiaverini, 2002: Marty et al., 2002, 2008; Pulido-Villena et al., 2010) located in the northwestern Mediterranean Sea (43°25′ N 07°52′ E).
## 4.1 Contrasted responses of the phytoplanktonic community to dust deposition events

Wet and dry dust deposition events induced contrasted responses of the phytoplanktonic community. The dust seeding experiments mimicking a wet deposition event (P, R1 and R2) induced a significant stimulation of both Chl *a* and PP from 1.8 to 2.4 fold (Fig. 6) while no changes were observed after the seeding simulating a dry deposition event (Q). The response of the phytoplankton to a wet deposition event was (1) fast as shown by significant increases in PP and algal biomass 24 h after dust addition, (2) long as depicted by increases recorded during at least a week after seeding and (3) homogeneous over the water column as illustrated by the stimulation of Chl *a* throughout the mesocosms. Despite the strong increases in biomass and PP following the wet deposition events, the values remained typical of oligotrophic systems (maximum of 0.23 µg Chl *a* L⁻¹ and 13.1 mgC m⁻³ d⁻¹ in the Dust-meso). PP and Chl *a* in the Dust-meso were higher over DUNE-2-R2 than over R1. The second dust addition (R2) led to a higher Chl *a* increase (R2, RC = 2.4) relative to the first one (R1, RC = 1.9). During the first dust addition, 65 % of the increase in total phytoplanktonic biomass was indeed due to picophytoplankton (< 3 µm) while during the second one, the nano and microphytoplankton (> 3 µm) contributed to about 50 % of the Chl *a* increase (Giovagnetti et al., 2013). Surprisingly, no increase in Chl *a* was observed after the Q seeding while the flux of POC export was higher in the Dust-meso (8 ± 3 mg m⁻² d⁻¹) compared to the controls (2 ± 1 mg m⁻² d⁻¹) 24 h after addition (Desboeufs et al., 2013; Guieu et al., 2013b). This could be explained by potential top-down effects through grazing pressure which could have regulated the phytoplanktonic biomass. Slight increases in PP (+20 % at 5 m depth; *p* < 0.05, Fig. S6) and in *Synechococcus* abundance (+30 % at 5 m depth, *p* < 0.05, unpublished data) were recorded after the Q amendment. The abundance of copepods representing the main contributor to zooplankton increased also in the Dust-meso at the end of the Q experiment (+45 %) relative to the Control-meso (L. Stemmann, unpublished data, 2013). The increase in POC export flux without

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an associated increase in the biogenic stock could be also explained by the lithogenic ballast effect through aggregation and/or DOC sorption processes onto the settling particles (Desboeufs et al., 2013; Bressac et al., 2013).

## 4.2 Atmospheric inputs of new nutrients and pathways of deposition

Differences in the atmospheric supply of bioavailable new nutrients depending on the pathway of deposition (wet or dry) and on the type of dust (EC and non-EC) may explain the variability in the response of the phytoplanktonic community (Table 3). In the P and R experiments, the amendment of the mesocosms was performed with the EC dust mixed with ultrapure water in order to mimic a wet deposition event (and thus cloud process including the mixing of particles with compounds from anthropogenic activities such as $HNO_3$, Desboeufs et al., 2013) whereas in DUNE-Q, non EC dust mixed with seawater was used to simulate a dry deposition event.

**DIP** – The total content of P was similar in EC and non-EC dust (Table 1). Through *in vitro* dissolution experiments, both types of dust have been evidenced as a source of DIP (Pulido-Villena unpublished data; Pulido-Villena et al., 2010; Losno et al., 2013). Nevertheless, the dissolution of P associated with Saharan dust has been shown to be about 2 to 5 fold higher, for an equivalent dust concentration, in ultrapure water (wet deposition) than in seawater (dry deposition) likely due to a lower pH in ultrapure water (Ridame and Guieu, 2002). Indeed, in situ measurements showed significant increases in the DIP stock after simulated wet deposition events and no changes after the simulated dry deposition event (Pulido-Villena, personal communication, 2013) while abiotic dissolution experiments in seawater have shown a release of DIP from non-EC dust (+3 nM for a particulate dust concentration of $1 mgL^{-1}$ after a contact time of 6 h, Pulido-Villena, unpublished data). The low amount of DIP released after the dry deposition event could have been quickly used by the biological activity explaining why no increases in DIP concentrations were recorded after the Q seeding.

**$NO_3^-$** – The N content of EC dust was about 10 fold higher compared to the non-EC dust, resulting in significant changes in the atmospheric supply of dissolved inorganic
nitrogen (DIN) depending on the pathway of deposition. Abiotic dissolution experiments showed that EC dust — mimicking Saharan dust wet deposition — was a significant source of NOₓ whereas non-EC dust — mimicking Saharan dust dry deposition (Q) — was a negligible source of NOₓ. Due to the simulation of cloud water processes that involved HNO₃, dissolution of EC dust released NOₓ as NO₃⁻. The release of NOₓ by EC dust under *in vitro* conditions was confirmed by in situ increases in NO₃⁻ concentrations throughout the Dust-meso after the DUNE-2-R1 and -R2 wet deposition events (Fig. 2, Table 3). Assuming a homogeneous distribution of the EC dust within the Dust-meso, it would lead to a particulate dust concentration of about 1 mgL⁻¹ leading to a potential input of 0.9 µM NO₃⁻ (Fig. 7a). The vertical profiles of NO₃⁻ in the mesocosms indicate a maximum net increase of NO₃⁻ of +9.8 µM at the surface soon after the R2 seeding (Fig. 2a), and reveal a non-homogeneous vertical distribution of the mineral particles in the Dust-meso. This was confirmed by optical measurements by Bressac et al. (2012, 2013) and by vertical distribution of crustal elements as aluminum (Desboeufs et al., 2013). Based on the dissolution results (Fig. 7), a complete dissolution of the NO₃⁻ associated with the EC dust (P, R1 and R2 seedings) is expected within three hours after additions, resulting in theoretical inputs of atmospheric new N of 8.5, 9.7 and 9.7 mmol NO₃⁻ m⁻², respectively in the Dust-meso for DUNE-1-P and DUNE-2-R1 and -R2. Over DUNE-2-R, the successive net increases in the NO₃⁻ stock just after seeding (R1: +10.6 mmol NO₃⁻ m⁻² and R2: +10 mmol NO₃⁻ m⁻², Fig. 2b) are in the same order of magnitude than the input of atmospheric NO₃⁻ based on a total dissolution of N in EC dust (9.7 mmol NO₃⁻ m⁻²). In addition to the atmospheric input of new DIN, N₂ fixation could be a significant source of new N as strong stimulations of N₂ fixation (up to 5.3 fold, Table 3) were detected after all the DUNE seedings (Ridame et al., 2013). The increases in N₂ fixation rates led to net inputs of new N of +39, +15 and +22 µmol N m⁻² respectively after P, R1 and R2 seedings over the duration of the experiments (Ridame et al., 2013) which are negligible compared to the estimated supply of new N from the wet deposition events (from 8.5 to 9.7 mmol NO₃⁻ m⁻²). Thus, the increase

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in NO₃⁻ concentrations observed after simulated wet deposition events could not be explained by the increase in N₂ fixation activity. Over DUNE-1-Q, N₂ fixation could be the main source of external N as dry deposition of non-EC dust was shown to be a negligible source of NO₃⁻. The slight stimulation of PP 48 h after the Q seeding at 5 m depth (+20 %) could not be explained by the strong stimulation of N₂ fixation recorded 24 h (5.2 fold increase) and 48 h (3.3 fold increase) after addition. The increase in PP converted to N with a C : N ratio of 7.6 (measured during DUNE-1-Q) required an input of ~9 nmol N L⁻¹ d⁻¹ which was much more higher than the input from increased N₂ fixation (~1 nmol N L⁻¹ d⁻¹).

DFe – EC and non-EC dust have a similar Fe content (Table 1) and represent a source of DFe to surface waters (Wagener et al., 2010; Losno et al., 2013). Results of dissolution experiments of both types of dust in artificial rainwater did not show any significant differences in DFe concentrations released for equivalent dust concentration and contact time (Losno et al., 2013). Despite these similarities, unexpected changes in the DFe concentrations were observed after the DUNE seedings by Wagener et al. (2010) and Wuttig et al. (2013) (Table 3). DFe concentrations dropped to about 1.5–2 nM after the DUNE-1-P, -Q and DUNE-2-R1 seedings due to DFe scavenging on settling dust particles and aggregates in the Dust-meso (T. Wagener, personal communication, 2013; Wagener et al., 2010; Wuttig et al., 2013) while a transient increase in the DFe concentrations in surface waters of about 2 nM was observed after the DUNE-2-R2 seeding (Wuttig et al., 2013).

The simulated wet deposition events induced significant increases in the DIP and NO₃⁻ concentrations in the mesocosms as well as significant stimulations of PP and Chl *a* (Table 3). On the opposite, after the simulated dry deposition event, no significant changes in DIP (and likely in NO₃⁻ concentrations) as in Chl *a* and PP were detected in the mesocosms (Table 3). As a consequence, the pathway of deposition and the type of dust clearly control the response of the phytoplanktonic community (biomass and productivity). As wet deposition is the main pathway of Saharan dust deposition over the western Mediterranean Sea (e.g., Lo}*ye*-*Pilot and Martin, 1996), atmospheric
deposition is probably the main source of new nitrogen ($NO_3^-$) (Markaki et al., 2010) and new phosphorus (DIP) to the open surface waters during stratification periods.

## 4.3 Nutrient factors controlling primary production and biogeochemical consequences

**Iron** – Before seeding, DFe concentrations ranged from 2.3 to 3.3 nM in the tested waters (Table 2). Despite the decreases in DFe concentrations after the P and R1 seedings, phytoplanktonic biomass and productivity increased (Table 3). This indicates that during the DUNE experiments, the bioavailability of Fe was not a controlling factor of the growth and $CO_2$ fixing activity of the phytoplanktonic community which is in good agreement with previous results of Bonnet et al. (2005) during summer in the northwestern Mediterranean Sea.

**Nitrogen and Phosphorus** – The non-stimulation of PP after the dry deposition event suggest that the phytoplanktonic community may be limited by N, P or NP co-limited. In the DUNE-2-R experiment, the estimated $NO_3^-$/DIP ratio before seeding (< 6) was lower than the Redfield ratio (16/1) suggesting a N limitation of the phytoplankton activity. As the DIP concentration was also initially extremely low (5 nM, Table 2), PP could be likely co-limited by both N and P as previously shown during summer in the northwestern Mediterranean Sea (Tanaka et al., 2011). By increasing DIP and $NO_3^-$ concentrations in P- and N-depleted surface waters, simulated wet deposition events relieve the potential N or NP colimitation of the phytoplanktonic activity. The wet deposition events induced significant changes in the biogeochemical conditions of the tested waters by modifying the $NO_3^-$ and DIP stock and by altering the ambient $NO_3^-$/DIP ratio (Fig. 8). Due to the higher input of $NO_3^-$ relative to DIP, wet deposition events (P, R1, R2) are characterized by a $NO_3^-$/DIP ratio much more higher than the Redfield ratio resulting in increases in the $NO_3^-$/DIP ratio in the Dust-meso just after seeding. Studies have previously shown that the atmospheric input to the Mediterranean Sea displays

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a high N/P ratio for dissolved inorganic forms (Sandroni et al., 2007; Markaki et al., 2010).

DUNE-1-P and DUNE-2-R1 – Both experiments showed similar patterns in the response of the phytoplanktonic community to a wet event as well as in the evolution of the DIP concentration. As the $NO_3^-$ stock in the Dust-meso was higher than in the controls at the end of R1 and as the DIP stock was similar to the controls, the $NO_3^-$/DIP ratio strongly increased from < 6 (initially) to > 150 at the end of the R1 experiment (Fig. 8) suggesting a switch from an initial N limitation or NP colimitation of the phytoplanktonic activity towards a severe P limitation until the end of R1. Net increase in integrated PP (PP$_{dust}$ – PP$_{control}$) 24 h after seeding was converted to P using a C/P molar ratio of 245/1 determined in the particulate organic matter in surface waters of the northwestern Mediterranean Sea during stratification (Tanaka et al., 2011) and to N using a C : N ratio measured during DUNE. The phytoplanktonic P requirement (+25 and +18 µmol DIP m$^{-2}$ for DUNE-1-P and DUNE-2-R1 at day 1) was in the same order of magnitude than the net increase in the DIP stock in the Dust-meso (+21 and +24 µmol DIP m$^{-2}$ for DUNE-1-P and DUNE-2-R1) while the phytoplanktonic N requirement was much lower than the increase in the $NO_3^-$ stock. The stimulation of PP 24 h after seeding can explain the rapid DIP depletion. Bacterial respiration was also stimulated after these seedings (Pulido-Villena, 2010, 2014) suggesting a potential competition for the DIP uptake between heterotrophic bacteria and autotrophic phytoplankton as already reported during microcosm experiments in the tropical Atlantic and Mediterranean Sea (Thingstad et al., 2005; Maranon et al., 2010). As the ~ 2 fold increase in PP and Chl a in the Dust-meso lasted during a week, it suggests a rapid remineralization of DIP by heterotrophic bacteria and/or the uptake of dissolved organic phosphorus (DOP) as a source of P through the alkaline phosphatase enzyme (Beardall et al., 2001). The PP integrated over the duration of the experiments and the estimated nutrient requirements showed that the atmospheric input of $NO_3^-$ largely exceeds the phytoplanktonic N demand which may explain the $NO_3^-$ accumulation at the end of the experiment.

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DUNE-2-R2 – Although the two seedings performed during DUNE-2-R were identical, the evolution of the $NO_3^-$ and DIP stock after the second seeding was different relative to that observed after the first one (Fig. 8). The $NO_3^-$ stock in the dust-meso decreased to undetectable values at the end of R2 while the DIP stock remained higher than in the controls. This leads to a $NO_3^-$/DIP ratio much lower than 16/1 indicating a potential N limitation of the phytoplanktonic activity at the end of the experiment. Over R2, the change in size structure of the phytoplanktonic community towards a larger dominance of big cells is consistent with a higher consumption of $NO_3^-$ (Giovagnetti et al., 2013) as large cells have higher half-saturation constants (Aksnes and Egge, 1991; Hein et al., 1995) and higher minimum cellular metabolic requirements than small cells (Grover, 1991).

The DIP stock remained surprisingly stable and significantly higher relative to the controls until the end of R2 despite twice higher rates of PP in the Dust-meso than in the Control-meso. This does not imply that the input of new DIP was not used by phytoplankton but rather that the rate of DIP consumption through PP was compensated by an equivalent rate of DIP production and thus maintaining a quite constant DIP stock. It is unlikely that the increase in PP was sustained by DOP as the use of DOP by phytoplankton is more energy consuming through the synthesis of alkaline phosphatase enzyme. We thus hypothesize that the atmospheric DIP was assimilated by phytoplankton to sustain increases in biomass and PP, and that heterotrophic remineralization of organic phosphorus kept DIP stock quite stable through high rates of P recycling. This would imply that during R2, heterotrophic bacteria did not out-compete phytoplankton for P resource as is suspected for R1. As suggested by Pulido-Villena et al. (2014), even if carbon appeared not to be the limiting nutrient, bacterial respiration could have been stimulated by labile phytoplankton-derived dissolved organic matter (DOM). Besides carbon, phytoplankton DOM also provides P, offering an alternate source of the limiting element to bacteria. This shift in the P source could explain the non-utilization of DIP by heterotrophic bacteria during the second seeding.
*Consequences of dust deposition on new production* – During the summer oligotrophic conditions in the western Mediterranean Sea, PP is mainly dominated by regenerated production as about 85% of PP is sustained by internal recycling of organic matter within the euphotic zone through remineralization processes (Marty et al., 2002; Moutin and Raimbaut, 2002; L'Helguen et al., 2002). As atmospheric deposition constitutes a source of external nutrients to the surface mixed layer, it induces by definition new production (NP). Thus, the increase in PP in the Dust-meso 24 h after seeding observed in the DUNE-1-P and DUNE-2-R experiments can be associated with NP. We consider that after 24 h, the increase in PP could be mainly supported by regenerated nutrients. The contribution of NP to PP increased in the Dust-meso from initially 15% to 60–70% 24 h after seeding. Wet simulated dust deposition event induced a switch from a regenerated-production based system (NP/PP = 0.15) to a new-production based system (NP/PP = 0.60–0.70).

## 5 Summary and conclusions

Our results from original mesocosm experiments demonstrate that atmospheric dust deposition does greatly influence primary productivity and algal biomass in LNLC environments. The response of the phytoplanktonic community to dust events was quantified: primary production and biomass increased ~2 fold and the stimulation can be observed for at least one week. Differences in the atmospheric supply of bioavailable new nutrients depending on the pathway (wet or dry) and on the type of dust led to contrasted responses of the phytoplanktonic community. Primary production increased significantly only after wet deposition. In addition to being a source of DIP, wet deposition due to cloud processes and mixing with anthropogenic species such as HNO₃, represents also a significant source of NO₃⁻ inducing significant increase in the ambient NO₃⁻/DIP ratio. As dry deposition was not a significant source of NO₃⁻, primary production was likely N limited or co-limited by both N and P. By transiently increasing DIP and DIN concentrations in P-N starved surface waters of the Mediterranean Sea,

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wet deposition of Saharan dust relieves the potential N or NP co-limitation of the phytoplanktonic activity. This study underlines the importance of Saharan dust deposition on the phytoplankton dynamics in the Mediterranean Sea and potentially in all LNLC areas impacted by dust deposition such as the tropical Atlantic and Pacific Oceans. Despite the significant stimulation of the phytoplanktonic production and the increase in the POC export after wet events (Desboeufs et al., 2013; Bressac et al., 2013), Guieu et al. (2013b) have shown that simulated wet deposition did not result in a simple fertilization effect as the oligotrophic ecosystem keeps or reinforces its net heterotrophic character thanks to the high organic carbon remineralization due to microbial food web processes.

Modeling studies suggest that changes in climate and land use practices over recent decades may have altered dust fluxes and thus, aeolian Fe, P and N inputs to the oceans (e.g., Mahowald and Luo, 2003). In addition, in the future, a warming atmosphere and surface waters could potentially increase the stratification of the surface waters in the Mediterranean Sea (Somot et al., 2008) and other oceanic areas such as subtropical gyres (Bopp et al., 2001). In response to this, the biogeochemical impact of the Saharan deposition events on the primary production could be more pronounced in particular during stratification periods.

**Supplementary material related to this article is available online at**
http://www.biogeosciences-discuss.net/11/753/2014/bgd-11-753-2014-supplement.pdf.

*Acknowledgements.* This work is a contribution of the DUNE experiment (a DUst experiment in a low Nutrient, low chlorophyll Ecosystem, http://wwwobs-vlfr.fr/LOV/DUNE/index.html) and was funded by the LEFE, UPMC and ANR-DUNE under the contract "ANR-07-BLAN-0126-01". We would like to sincerely thank the "Réserve naturelle de Scandola, Parc naturel régional de Corse", in particular J. M. Dominici, Jo Albertini and collaborators, for professionalism and cooperation in the implementation of the field work in the bay of Elbo. Lars Stemmann and

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the RadeZoo service from the “Observatoire Océanographique de Villefranche-sur-mer” are acknowledged for providing zooplankton data. E. Pulido-Villena, T. Wagener, K. Desboeufs, F. Louis, J. M. Grisoni, D. Luquet, M. Deschatres, J. Caparros, N. Leblond, F. Dulac, C. Brunet, K. Wuttig, M. Bressac, I. Biegala, S. Blain, C. Rouvières, V. Giovagnetti and L. Gilleta are greatly acknowledged for their involvement during the DUNE experiments. DUNE was endorsed by the international SOLAS (Surface Ocean – Lower Atmosphere) program in February 2009, (http://www.solas-int.org/activities/project-endorsement.html).

References

Aminot, A. and Kerouel, R.: Dosage automatique des nutriments dans les eaux marines in Ifremer, 188 p., 2007.

Aksnes, D. L. and Egge, J. K.: A theoretical model for nutrient uptake in phytoplankton, Mar. Ecol.-Prog. Ser., 70, 65–72, 1991.

Baker, A. R., Kelly, S. D., Biswas, K. F., Witt, M., and Jickells, T. D.: Atmospheric deposition of nutrients to the Atlantic Ocean, Geophys. Res. Lett., 30, 2296, doi:10.1029/2003GL018518, 2003.

Beardall, J., Young, E., and Roberts, S.: Approaches for determining phytoplankton nutrient limitation, Aquat. Sci., 63, 44–69, 2001.

Bishop, J. K. B., Davis, R. E., and Sherman, J. T.: Robotic observations of dust storm enhancement of carbon biomass in the North Pacific, Science, 298, 817–821, 2002.

Blain, S., Queguiner, B., Armand, L., Belviso, S., Bombled, B., Bopp, L., Bowie, A., Brunet, C., Brussaard, C., Carlotti, F., Christaki, U., Corbiere, A., Durand, I., Ebersbach, F., Fuda, J. L., Garcia, N., Gerringa, L., Griffiths, B., Guigue, C., Guillerm, C., Jacquet, S., Jeandel, C., Laan, P., Lefèvre, D., Lo Monaco, C., Malits, A., Mosseri, J., Obernosterer, I., Park, Y.-H., Picheral, M., Pondaven, P., Remenyi, T., Sandroni, V., Sarthou, G., Savoye, N., Scouarnec, L., Souhaut, M., Thuiller, D., Timmermans, K., Trull, T., Uitz, J., Van Beek, P., Veldhuis, M., Vincent, D., Viollier, E., Vong, L., and Wagener, T.: Effect of natural iron fertilization on carbon sequestration in the Southern Ocean, Nature, 446, 1070–1074, doi:10.1038/nature05700, 2007.

Bonnet, S. and Guieu, C.: Atmospheric forcing on the annual in the Mediterranean Sea. A one year survey, J. Geophys. Res., 111, C09010, doi:10.1029/2005JC003213, 2006.

5

10

15

20

25

30
<figure><img src="image_115.png" /></figure>

<figure><img src="image_116.png" /></figure>

<figure><img src="image_117.png" /></figure>

<figure><img src="image_118.png" /></figure>

<figure><img src="image_119.png" /></figure>

Bonnet, S., Guieu, C., Chiaverini, J., Ras, J., and Stock, A.: Effect of atmospheric nutrients on the autotrophic communities in a low nutrient, low chlorophyll system, Limnol. Oceanogr., 50, 1810–1819, 2005.

Bopp, L., Monfray, P., Aumont, O., Dufresne, J. P., Le Treut, H., Madec, G., Terray, L., and Orr, J. C.: Potential impact of climate change on marine export production, Global Biogeochem. Cy., 15, 81–99, 2001.

Bosc, E., Bricaud, A., and Antoine, D.: Seasonal and interannual variability in algal biomass and primary production in the Mediterranean Sea, as derived from 4 yr of SeaWiFS observations, Global Biogeochem. Cy., 18, GB1005, doi:10.1029/2003GB002034, 2004.

Boyd, P. W., Watson, A. J., Law, C. S., Abraham, E. R., Trull, T., Murdoch, R., Bakker, D. C., Bowie, A. R., Buesseler, K. O., Chang, H., Charette, M., Croot, P., Downing, K., Frew, R., Gall, M., Hadfield, M., Hall, J., Harvey, M., Jameson, G., LaRoche, J., Liddicoat, M., Ling, R., Maldonado, M. T., McKay, R. M., Nodder, S., Pickmere, S., Pridmore, R., Rintoul, S., Safi, K., Sutton, P., Strzepek, R., Tanneberger, K., Turner, S., Waite, A., and Zeldis, J.: A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization, Nature, 407, 695–702, 2000.

Boyd, P. W., Jickells, T., Law, C. S., Blain, S., Boyle, E. A., Buesseler, K. O., Coale, K. H., Cullen, J. J., de Barr, H. J. W., Follow, M., Harvey, M., Lancelot, C., Levasseur, M., Owens, N. P. J., Pollard, R., Rivkin, R. B., Sarmiento, J., Schoemann, V., Smetacek, V., Takeda, S., Tsuda, A., Turner, S., and Watson, A. J.: Mesoscale iron enrichment experiments 1993–2005: synthesis and future directions, Science, 315, 5812, 612–617, doi:10.1126/science.1131669, 2007.

Bressac, M., Guieu, C., Doxaran, D., Bourrin, F., Obolensky, G., and Grisoni, J. M.: A mesoscale experiment coupled with optical measurements to observe the fate and sinking of atmospheric particles in clear oligotrophic waters, Geo-Mar. Lett., 32, 153–164, 2012.

Bressac, M., Guieu, C., Doxaran, D., Bourrin, F., Desboeufs, K., Leblond, N., and Ridame, C.: Quantification of the lithogenic carbon pump following a dust deposition event, Biogeoosciences Discuss., 10, 13639–13677, doi:10.5194/bgd-10-13639-2013, 2013.

Chen, Y. B., Zehr, J. P., and Mellon, M.: Growth and nitrogen fixation of the diazotrophic filamentous nonheterocystous cyanobacterium *Trichodesmium* sp. IMS 101 in defined media: evidence for a circadian rhythm, J. Phycol., 32, 916–923, 1996.

de Baar, H. J. W., Boyd, P. W., Coale, K. H., Landry, M. R., Tsuda, A., Assmy, P., Bakker, D. C. E., Bozec, Y., Barber, R. T., Brzezinski, M. A., Buesseler, K. O., Boyé, M., Croot, P.,

5

10

15

20

25

30
<figure><img src="image_120.png" /></figure>

<figure><img src="image_121.png" /></figure>

<figure><img src="image_122.png" /></figure>

<figure><img src="image_123.png" /></figure>

<figure><img src="image_124.png" /></figure>

Gervais, F., Gorbunov, M. Y., Harrison, P. J., Hiscock, W. T., Laan, P., Lancelot, C., Law, C. S., Levasseur, M., Marchetti, A., Millero, F. J., Nishioka, J., Nojiri, Y., van Oijen, T., Riebesell, U., Rijkenberg, M. J. A., Saito, H., Takeda, S., Timmermans, K. R., Veldhuis, M. J. W., Waite, A. M., and Wong, C.-S.: Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment, J. Geophys. Res., 110, C09S16, doi:10.1029/2004JC002601, 2005.

Desboeufs, K. V., Losno, R., and Colin, J. L.: Factors influencing aerosol solubility during cloud process, Atmos. Environ., 35, 3529–3537, 2001.

Desboeufs, K. V., Leblond, N., Wagener, T., Bon Nguyen, E., and Guieu, C.: Chemical fate and settling of dust after atmospheric deposition: study case of dust seedings in large mesoscosms, Biogeosciences, in preparation, 2013.

Duarte, C. M., Dachs, J., Llabrés, M., Alonso-Laita, P., Gasol, J. M., Tovar-Sánchez, A., Wilhemy, S. and Agustí, S.: Aerosol inputs enhance new production in the subtropical northeast Atlantic, J. Geophys. Res., 111, G04006, doi:10.1029/2005JG000140, 2006.

Emerson, S., Quay, P., Karl, D., Winn, C., Tupas, L., and Landry, M.: Experimental determination of the organic carbon flux from open-ocean surface waters, Nature, 389, 951–954, 1997.

Falkowski, P. G.: Evolution of the nitrogen cycle and its influence on the biological sequestration of CO₂ in the ocean, Nature, 387, 272–275, 1997.

Franchy, G., Ojeda, A., López-Cancio, J., and Hernández-León, S.: Plankton community response to Saharan dust fertilization in subtropical waters off the Canary Islands, Biogeosciences Discuss., 10, 17275–17307, doi:10.5194/bgd-10-17275-2013, 2013.

Giovagnetti, V., Brunet, C., Conversano, F., Tramontano, F., Obernosterer, I., Ridame, C., and Guieu, C.: Assessing the role of dust deposition on phytoplankton ecophysiology and succession in a low-nutrient low-chlorophyll ecosystem: a mesocosm experiment in the Mediterranean Sea, Biogeosciences, 10, 2973–2991, doi:10.5194/bg-10-2973-2013, 2013.

Grasshoff, K., Kremling, K., and Ehrhardt, M.: Methods of Seawater Analysis, 3 edn., Wiley-VCH, 1999.

Grover, J. P.: Resource competition in a variable environment: phytoplankton growing according to the variable-internal-stores model, Am. Nat., 138, 811–835, 1991.

Guerzoni, S., Chester, R., Dulac, F., Herut, B., Loye-Pilot, M.-D., Measures, C., Migon, C., Molinaroli, E., Moulin, C., Rossini, P., Saydam, C., Soudine, A., and Ziveri, P.: The role of atmospheric deposition in the biogeochemistry of the Mediterranean Sea, Prog. Oceanogr., 44, 147–190, 1999.

5

10

15

20

25

30
<figure><img src="image_125.png" /></figure>

<figure><img src="image_126.png" /></figure>

<figure><img src="image_127.png" /></figure>

<figure><img src="image_128.png" /></figure>

<figure><img src="image_129.png" /></figure>

Guieu, C., Loye-Pilot, M.-D., Benyaya, L., and Dufour, A.: Spatial variability of atmospheric fluxes of metals (Al, Fe, Cd, Zn and Pb) and phosphorus over the whole Mediterranean from a one year monitoring experiment: biogeochemical implications, Mar. Chem., 120, 164–178, 2010a.

Guieu, C., Dulac, F., Desboeufs, K., Wagener, T., Pulido-Villena, E., Grisoni, J.-M., Louis, F., Ridame, C., Blain, S., Brunet, C., Bon Nguyen, E., Tran, S., Labiadh, M., and Dominici, J.-M.: Large clean mesocosms and simulated dust deposition: a new methodology to investigate responses of marine oligotrophic ecosystems to atmospheric inputs, Biogeosciences, 7, 2765–2784, doi:10.5194/bg-7-2765-2010, 2010b.

Guieu, C., Dulac, F., Ridame, C., and Pondaven, P.: Introduction to the project DUNE, a DUst experiment in a low Nutrient, low chlorophyll Ecosystem, Biogeosciences Discuss., 10, 12491–12527, doi:10.5194/bgd-10-12491-2013, 2013a.

Guieu, C., Ridame, C., Pulido-Villena, E., Blain, S., Bressac, M., Desboeufs, K., and Dulac, F.: Dust deposition over oligotrophic marine environment: impact on carbon budget, Biogeo-SCIENCES, in preparation, 2013b.

Hein, M., Pedersen, M. F., and Sand-Jensen, K.: Size-dependent nitrogen uptake in micro- and macroalgae, Mar. Ecol.-Prog. Ser., 118, 247–253, 1995.

Herut, B., Krom, M. D., Pan, G., and Mortimer, R.: Atmospheric input of nitrogen and phosphorus to the Southeast Mediterranean: sources, fluxes and possible impact, Limnol. Oceanogr., 44, 1683–1692, 1999.

Holmes, R. M., Aminot, A., Kerouel, R., Hooker, B. A., Peterson, B. J.: A simple and precise method for measuring ammonium in marine and freshwater ecosystems, Can. J. Fish. Aquat. Sci., 56, 1801–1808, 1999.

Jickells, T. D., An, Z. S., Andersen, K. K., Baker, A. R., Bergametti, G., Brooks, N., Cao, J. J., Boyd, P. W., Duce, R. A., Hunter, K. A., Kawahata, H., Kubilay, N., La Roche, J., Liss, P. S., Mahowald, N., Prospero, J. M., Ridgwell, A. J., Tegen, I., and Torres, R.: Global iron connections between desert dust, ocean biogeochemistry, and climate, Science, 308, 67–71, 2005.

Johnson, K. S., Elrod, V. A., Fitzwater, S. E., Plant, J. N., Chavez, F. P., Tanner, S. J., Gordon, R. M., Westphal, D. L., Perry, K. D., Wu, J., and Karl, D.: Surface ocean-lower atmosphere interactions in the Northeast Pacific Ocean Gyre: aerosols, iron, and the ecosystem response, Global Biogeochem. Cy., 17, 1063, doi:10.1029/2002GB002004, 2003.

5

10

15

20

25

30
<figure><img src="image_130.png" /></figure>

<figure><img src="image_131.png" /></figure>

<figure><img src="image_132.png" /></figure>

<figure><img src="image_133.png" /></figure>

L'Helguen, S., Le Corre, P., Madec, C., and Morin, P.: New and regenerated production in the Almeria-Oran front area, eastern Alboran Sea, Deep-Sea Res. Pt. I, 49, 83–99, 2002.

Longhurst, A., Sathyendranath, S., Platt, T., and Caverhill, C.: An estimate of global primary production in the ocean from satellite radiometer data, J. Plankton Res., 17, 1245–1271, 1995.

López-Sandoval, D. C., Fernández, A., and Marañón, E.: Dissolved and particulate primary production along a longitudinal gradient in the Mediterranean Sea, Biogeosciences, 8, 815–825, doi:10.5194/bg-8-815-2011, 2011.

Losno, R., Aghnatios, C., Desboeufs, K., Velay, J. M., Abdelhouab, M., Bon-Nguyen, E., Dutilac, F., and Guieu, C.: Solubility variation of elements in some wind erodible Saharan soil, Biogeosciences, in preparation, 2013.

Loçe-Pilot, M.-D., and Martin, J. M: Saharan dust input to the Western Mediterranean: an eleven years record in Corsica, in: The Impact of Desert Dust across the Mediterranean, edited by: Guerzoni, S., and Chester, R., Kluwer A.P., Dordrecht, 191–199, 1996.

Mahowald, N. M. and Luo, C.: A less dusty future?, Geophys. Res. Lett., 30, 1903, doi:10.1029/2003GL017880, 2003.

Mahowald, N., Muhs, D., Levis, S., Rasch, P., Yoshioka, M., and Zender, C.: Change in atmospheric mineral aerosols in response to climate: last glacial period, preindustrial, modern and doubled-carbon dioxide climates, J. Geophys. Res., 111, D10202, doi:10.1029/2005JD006653, 2006.

Mahowald, N., Jickells, T. D., Baker, A. R., Artaxo, P., Benitez-Nelson, C. R., Bergametti, G., Bond, T. C., Chen, Y., Cohen, D. D., Herut, B., Kubilay, N., Losno, R., Luo, C., Maenhaut, W., McGee, K. A., Okin, G. S., Siefert, R. L., and Tsukuda, S.: Global distribution of atmospheric phosphorus sources, concentrations and deposition rates, and anthropogenic impacts, Global Biogeochem. Cy., 22, GB4026, doi:10.1029/2008GB003240, 2008.

Maranon, E., Fernandez, A., Mourino-Carballido, B., Martinez-Garcia, S., Teira, E., Cermeno, P., Choucino, P., Huete-Ortega, M., Fernandez, E., Calvo-Diaz, A., Anxelu, X., Moran, G., Bode, A., Moreno-Ostos, E., Varela, M. M., Patey, M. D., and Achterberg, E. P.: Degree of oligotrophy controls the response of microbial plankton to Saharan dust, Limnol. Oceanogr., 55, 2339–2352, 2010.

Markaki, Z., Loçe-Pilot, M. D., Violaki, K., Benyahya, L., and Mihalopoulos, N.: Variability of atmospheric deposition of dissolved nitrogen and phosphorus in the Mediterranean and possible link to the anomalous seawater N/P ratio, Mar. Chem., 120, 187–194, 2010.
<figure><img src="image_134.png" /></figure>

<figure><img src="image_135.png" /></figure>

<figure><img src="image_136.png" /></figure>

<figure><img src="image_137.png" /></figure>

<figure><img src="image_138.png" /></figure>

Martin, J. H.: Glacial-interglacial CO₂ change: the iron hypothesis, Paleoceanography, 5, 1–13, 1990.

Marty, J.-C. and Chiaverini, J.: Seasonal and interannual variations in phytoplankton production at DYFAMED time-series station, northwestern Mediterranean Sea, Deep-Sea Res. Pt. II, 49, 2017–2030, 2002.

Marty, J.-C., Chiaverini, J., Pizay, M.-D., and Avril, B.: Seasonal and interannual dynamics of nutrients and phytoplankton pigments in the western Mediterranean Sea at the DYFAMED time-series station (1991–1999), Deep-Sea Res. Pt. II, 49, 1965–1985, 2002.

Marty, J.-C., Garcia, N., and Raimbault, P.: Phytoplankton dynamics and primary production under late summer conditions in the NW Mediterranean Sea, Deep-Sea Res. Pt. I, 55, 1131–1149, 2008.

Moutin, T. and Raimbault, P.: Primary production, carbon export and nutrients availability in western and eastern Mediterranean Sea in early summer 1996 (MINOS cruise), J. Marine Syst., 33–34, 273–288, 2002.

Pujo-Pay, M., Conan, P., Oriol, L., Cornet-Barthaux, V., Falco, C., Ghiglione, J.-F., Goyet, C., Moutin, T., and Prieur, L.: Integrated survey of elemental stoichiometry (C, N, P) from the western to eastern Mediterranean Sea, Biogeosciences, 8, 883–899, doi:10.5194/bg-8-883-2011, 2011.

Pulido-Villena, E., Rérolle, V., and Guieu, C.: Transient fertilizing effect of dust in P-deficient LNLC surface ocean, Geophys. Res. Lett., 37, L01603, doi:10.1029/2009GL041415, 2010.

Pulido-Villena, E., Baudoux, A.-C., Obernosterer, I., Landa, M., Caparros, J., Catala, P., Georges, C., Harmand, J., and Guieu, C.: Microbial food web dynamics in response to a Saharan dust event: results from a mesocosm study in the oligotrophic Mediterranean Sea, Biogeosciences Discuss., 11, 337–371, doi:10.5194/bgd-11-337-2014, 2014.

Putaud, J.-P., Van Dingenen, R., Dell'Acqua, A., Raes, F., Matta, E., Decesari, S., Facchini, M. C., and Fuzzi, S.: Size-segregated aerosol mass closure and chemical composition in Monte Cimone (I) during MINATROC, Atmos. Chem. Phys., 4, 889–902, doi:10.5194/acp-4-889-2004, 2004.

Ridame, C. and Guieu, C.: Saharan input of phosphate to the oligotrophic water of the open western Mediterranean Sea, Limnol. Oceanogr., 47, 857–869, 2002.

Ridame, C., Guieu, C., and L'Helguen, S.: Strong stimulation of N₂ fixation in oligotrophic Mediterranean Sea: results from dust addition in large in situ mesocosms, Biogeosciences, 10, 7333–7346, doi:10.5194/bg-10-7333-2013, 2013.

5

10

15

20

25

30
<figure><img src="image_139.png" /></figure>

<figure><img src="image_140.png" /></figure>

<figure><img src="image_141.png" /></figure>

<figure><img src="image_142.png" /></figure>

Ridgwell, A. J. and Watson, A. J.: Feedback between aeolian dust, climate and atmospheric CO₂ in glacial time, Paleooceanography, 17, 1059, doi:10.1029/2001PA000729, 2002.

Sandroni, V., Raimbault, P., Migon, C., Garcia, N., and Gouze, E.: Dry atmospheric deposition and diazotrophy as sources of new nitrogen to northwestern Mediterranean oligotrophic surface waters, Deep-Sea Res. Pt. I., 54, 1859–1870, 2007.

Somot, S., Sevault, F., Deque, M., and Crepon, M.: 21st century climate change scenario for the Mediterranean using a coupled atmosphere–ocean regional climate model, Global Planet. Change, 63, 112–126, 2008.

Strickland, J. D. and Parsons, T. R.: A Practical Handbook of Seawater Analysis, 2nd edn., Bull. Fish. Res. Bd. Can., 167, 1997.

Tanaka, T., Thingstad, T. F., Christaki, U., Colombet, J., Cornet-Barthaux, V., Courties, C., Grattepanche, J.-D., Lagaria, A., Nedoma, J., Oriol, L., Psarra, S., Pujo-Pay, M., and Van Wambeke, F.: Lack of P-limitation of phytoplankton and heterotrophic prokaryotes in surface waters of three anticyclonic eddies in the stratified Mediterranean Sea, Biogeosciences, 8, 525–538, doi:10.5194/bg-8-525-2011, 2011.

Ternon, E., Guieu, C., Ridame, C., L'Helguen, S., and Catala, P.: Longitudinal variability of the biogeochemical role of Mediterranean aerosols in the Mediterranean Sea, Biogeosciences, 8, 1067–1080, doi:10.5194/bg-8-1067-2011, 2011.

Thingstad, T. F., Krom, M. D., Mantoura, R. F. C., Flaten, G. A. F., Groom, S., Herut, B., Kress, N., Law, C. S., Pasternak, A., Pitta, P., Psarra, S., Rassoulzadegan, F., Tanaka, T., Tselepides, A., Wassmann, P., Woodward, E. M. S., Wexels Riser, C., Zodiatis, G., and Zohary, T.: Nature of phosphorus limitation in the ultraoligotrophic Eastern Mediterranean, Science, 309, 1068–1071, 2005.

Wagener, T., Guieu, C., and Leblond, N.: Effects of dust deposition on iron cycle in the surface Mediterranean Sea: results from a mesocosm seeding experiment, Biogeosciences, 7, 3769–3781, doi:10.5194/bg-7-3769-2010, 2010.

Wuttig, K., Wagener, T., Bressac, M., Dammshäuser, A., Streu, P., Guieu, C., and Croot, P. L.: Impacts of dust deposition on dissolved trace metal concentrations (Mn, Al and Fe) during a mesocosm experiment, Biogeosciences, 10, 2583–2600, doi:10.5194/bg-10-2583-2013, 2013.

5

10

15

20

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<table><caption>**Table 1.** Particulate P, Fe and N (%, in weight) in the EC and non-EC dust used during DUNE (from Ridame et al., 2013). Means that were not significantly different for a given chemical element between the different experiments (*p* > 0.05) are labeled with the same letter (in parenthesis).</caption><thead><tr><th></th><th>DUNE-1-P<br>10–18 Jun 2008</th><th>DUNE-1-Q<br>20–27 Jun 2008</th><th>DUNE-2-R (R1, R2)<br>26 Jun–9 Jul 2010</th></tr></thead><tbody><tr><td>Dust treatment</td><td>evapocondensed</td><td>non-processed</td><td>evapocondensed</td></tr><tr><td>Simulated deposition</td><td>wet</td><td>dry</td><td>wet</td></tr><tr><td>P (%)</td><td>0.045 ± 0.015<sup>a</sup> (A)</td><td>0.044 ± 0.009<sup>a</sup>(A)</td><td>0.055 ± 0.003<sup>b</sup>(A)</td></tr><tr><td>Fe (%)</td><td>2.31 ± 0.04<sup>a</sup> (B)</td><td>2.28 ± 0.19<sup>a</sup>(B)</td><td>2.26 ± 0.03<sup>b</sup>(B)</td></tr><tr><td>N (%)</td><td>1.19 ± 0.05<sup>a</sup> (C)</td><td>0.11 ± 0.01<sup>a</sup>(D)</td><td>1.36 ± 0.09<sup>b</sup>(E)</td></tr></tbody></table>

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<footnote><sup>a</sup> Guieu et al. (2010b).</footnote>

<footnote><sup>b</sup> Desboeufs et al. (2013).</footnote>
<table><caption>**Table 2.** Initial temperature and biological and chemical properties of seawater before seeding in experiments DUNE-1-P, -Q and DUNE-2-R (average in the Dust-meso, Control-meso and Out). DIP: dissolved inorganic phosphorus; dl: detection limit (30 nM for $NO_3^-$); nd: no data. Data for temperature, chlorophyll *a*, DIP, $NO_3^-$ and DFe are the means at 0.1, 5 and 10 m depth. Data for primary production and C : N ratio are the means at 0.1 and 5 m depth for P and Q experiments and at 5 m depth for R experiment. Means that were not significantly different for a given parameter between the different experiments (*p* > 0.05) are labeled with the same letter (in parenthesis).</caption><thead><tr><th></th><th>DUNE-1-P</th><th>DUNE-1-Q</th><th>DUNE-2-R</th></tr></thead><tbody><tr><td>Temperature (°C)</td><td>19.6 ± 0.7 (A)</td><td>20.3 ± 0.5 (B)</td><td>21.2 ± 0.4 (C)</td></tr><tr><td>Chlorophyll <i>a</i> (µg L<sup>-1</sup>)</td><td>0.11 ± 0.03 (A)</td><td>0.08 ± 0.02 (B)</td><td>0.07 ± 0.02 (B)</td></tr><tr><td>Primary production, mg C m<sup>-3</sup> d<sup>-1</sup></td><td>5.35 ± 1.11 (A)</td><td>4.16 ± 0.38 (B)</td><td>3.89 ± 0.46 (B)</td></tr><tr><td>C : N (mol/mol)</td><td>7.8 ± 0.5 (A)</td><td>7.3 ± 0.5 (B)</td><td>7.5 ± 0.6 (A, B)</td></tr><tr><td>DIP, nM</td><td>5 ± 2<sup>a</sup> (A)</td><td>2 ± 0<sup>b</sup> (B)</td><td>5 ± 3<sup>c</sup> (A)</td></tr><tr><td>$NO_3^-$, nM</td><td>nd</td><td>nd</td><td>&lt; dl</td></tr><tr><td>DFe, nM</td><td>2.4 ± 0.3<sup>d</sup> (A)</td><td>2.3 ± 0.3<sup>e</sup> (A)</td><td>3.3 ± 0.8<sup>f</sup> (B)</td></tr></tbody></table>

<sup>a</sup> Pulido-Villena et al. (2010).

<sup>b</sup> E. Pulido-Villena, personal communication (2013).

<sup>c</sup> Pulido-Villena et al. (2014).

<sup>d</sup> Wagener et al. (2010).

<sup>e</sup> T. Wagener, personal communication, 2013.

<sup>f</sup> Wuttig et al. (2013).

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<table><caption>**Table 3.** Summary of the biological and chemical changes observed after the DUNE seedings in the Dust-meso relative to the Control-meso. Results from this study; Ridame et al. (2013); Pulido-Villena et al. (2010, 2014); Pulido-Villena, personal communication (2013); Wagener et al. (2010); T. Wagener, personal communication, 2013; Wuttig et al. (2013)</caption><thead><tr><th>DUNE-1-P</th><th>DUNE-1-Q</th><th>DUNE-2-R1</th><th>DUNE-2-R2</th></tr><tr><th>wet</th><th>dry</th><th>wet</th><th>wet</th></tr></thead><tbody><tr><td>PP ↑</td><td>PP ↔</td><td>PP ↑</td><td>PP ↑</td></tr><tr><td>Chl a ↑</td><td>Chl a ↔</td><td>Chl a ↑</td><td>Chl a ↑</td></tr><tr><td>N<sub>2</sub> Fixation ↑</td><td>N<sub>2</sub> Fixation ↑</td><td>N<sub>2</sub> Fixation ↑</td><td>N<sub>2</sub> Fixation ↑</td></tr><tr><td>Bacterial respiration ↑</td><td>Bacterial respiration↑</td><td>Bacterial respiration ↑</td><td>Bacterial respiration ↑</td></tr><tr><td>DIP ↑</td><td>DIP ↔</td><td>DIP ↑</td><td>DIP ↑</td></tr><tr><td>NO<sub>x</sub> ↑<sup>*</sup></td><td>NO<sub>x</sub> ↔<sup>*</sup></td><td>NO<sub>x</sub> ↑</td><td>NO<sub>x</sub> ↑</td></tr><tr><td>DFe ↓</td><td>DFe ↓</td><td>DFe ↓</td><td>DFe ↑</td></tr></tbody></table>

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<figure><img src="image_151.png" /></figure>

<footnote>* From abiotic dissolution experiments, this study.</footnote>
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<figure><img src="image_153.png" /><figcaption>**Fig. 1. (A)** Addition of ¹³C tracer immediately after seawater sampling, (B) incubations of the ¹³C-amended bottles under in situ conditions at the corresponding sampling depths.</figcaption></figure>

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<figure><img src="image_158.png" /><figcaption>**Fig. 2. (A)** Mean NO₃⁻ concentration (µmol L⁻¹) during the DUNE-2-R experiment in the Dust-meso at the surface (0.1 m depth). The dotted lines represent the time of the dust seedings. Data represent the average and standard deviation of the three replicate mesocosms. NO₃⁻ concentrations were under detection limit (< 30 nM) in the Control-meso and outside over the duration of the experiment as well as in the Dust-meso before seeding (day 0) and at the end of the experiment (day 13), (B) integrated NO₃⁻ (mmol N m⁻²) over the depth of the mesocosm (12.5 m) in the Dust-meso during the DUNE-2-R experiment. Assuming a maximum NO₃⁻ concentration of 30 nM in the Control-meso, the integrated NO₃⁻ should be lower than 0.4 mmol N m⁻² (red line).</figcaption></figure>

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<figure><img src="image_167.png" /><figcaption>**Fig. 3.** Mean integrated chlorophyll *a* (Chl *a*) in mg m⁻² (top panel) and mean integrated primary production (PP) in mg C m⁻² d⁻¹ (bottom panel) over mesocosms during the DUNE-1-P experiment in the Control-meso (black dot), Dust-meso (orange dot) and out (grey dot). The dotted line represents the time of the dust seeding. Data in the Control- and Dust-meso represent the average and standard deviation of the three replicate mesocosms. Means in the Dust-meso that were significantly different from the Control-meso (*p* < 0.05) are labeled with the * symbol.</figcaption></figure>
<figure><img src="image_168.png" /><figcaption>**Fig. 4.** Mean integrated chlorophyll *a* (Chl *a*) in mg m⁻² (top panel) and mean integrated primary production (PP) in mg C m⁻² d⁻¹ (bottom panel) over mesocosms during the DUNE-1-Q experiment in the Control-meso (black dot), Dust-meso (orange dot) and out (grey dot). The dotted line represents the time of the dust seeding. Data in the Control- and Dust-meso represent the average and standard deviation of the three replicate mesocosms.</figcaption></figure>

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<figure><img src="image_177.png" /><figcaption>**Fig. 5.** Mean integrated chlorophyll *a* (Chl *a*) in mg m⁻² (top panel) and mean integrated primary production (PP) in mg C m⁻² d⁻¹ (bottom panel) over mesocosms during the DUNE-2-R1 and -R2 experiments in the Control-meso (black dot), Dust-meso (orange dot) and out (grey dot). The dotted line represents the time of the dust seeding. Data in the Control- and Dust-meso represent the average and standard deviation of the three replicate mesocosms. Means in the Dust-meso that were significantly different from the Control-meso (*p* < 0.05) are labeled with the * symbol.</figcaption></figure>
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<figure><img src="image_182.png" /><figcaption>**Fig. 6.** Box plot of the relative changes in integrated Chl *a* (Chl *a*<sub>dust</sub>/Chl *a*<sub>control</sub>) and PP (PP<sub>dust</sub>/PP<sub>control</sub>) over the duration of the DUNE experiments. Means that were not significantly different between the different parameters and experiments (*p* > 0.05) are labeled with the same letter.</figcaption></figure>
<figure><img src="image_183.png" /><figcaption>**Fig. 7.** Results of abiotic dissolution experiments: (A) ΔNOₓ ([NOₓ]after the introduction of dust - [NOₓ]initial) in µM relative to the amount of dust (EC and non EC dust) introduced in seawater for contact times of 3 and 24 h, (B) percentage of NOₓ released from EC dust (NOₓ % = ΔNOₓ × 100/Ntotal dust) as a function of the dust amount introduced in seawater for contact times of 3 and 24 h. Data represent the average and standard deviation of the three replicates.</figcaption></figure>

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<figure><img src="image_192.png" /><figcaption>**Fig. 8.** Conceptual representation of the temporal changes in the variation (Dust – Control) of NO₃⁻ (red line) and DIP stock (blue line) (A) after the DUNE-2-R1 seeding and (B) after the DUNE-2-R2 seeding.</figcaption></figure>