Project

PEACETIME : ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea

Goal: Processes occurring at the atmosphere-ocean interface are critical to the regulation of the Earth climate and to the delivery of key services provided by marine ecosystems.
The Mediterranean Sea, a hot spot for biodiversity but also for climate change and anthropogenic pressure, is an ideal natural laboratory to study these processes.
PEACETIME will provide the understanding necessary to accurately represent natural and anthropogenic chemical exchanges at the air-sea interface and their impacts on marine ecosystems and services, today and in the future.

In the frame of PEACETIME, an oceanographic cruise onboard the R/V ‘Pourquoi Pas?’ took place in the Western/Central Mediterranean Sea May 10–June 11, 2017. The purpose of this expedition was to study critical processes induced by atmospheric deposition, in particular Saharan dust, occurring at the air-sea interface in the Mediterranean. PEACETIME yields insights into the impact of such processes on the cycle of chemical elements (nutrients, metals) and on the biogeochemical functioning of the pelagic ecosystem. The 40 scientists embarked are experts in atmosphere and ocean domains.

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Project log

Cécile Guieu
added a research item
In the oligotrophic waters of the Mediterranean Sea, during the stratification period, the microbial loop relies on pulsed inputs of nutrients through the atmospheric deposition of aerosols from both natural (e.g., Saharan dust), anthropogenic, or mixed origins. While the influence of dust deposition on microbial processes and community composition is still not fully constrained, the extent to which future environmental conditions will affect dust inputs and the microbial response is not known. The impact of atmospheric wet dust deposition was studied both under present and future environmental conditions (+3 ∘C warming and acidification of −0.3 pH units), through experiments in 300 L climate reactors. In total, three Saharan dust addition experiments were performed with surface seawater collected from the Tyrrhenian Sea, Ionian Sea, and Algerian basin in the western Mediterranean Sea during the PEACETIME (ProcEss studies at the Air–sEa Interface after dust deposition in the MEditerranean sea) cruise in May–June 2017. Top-down controls on bacteria, viral processes, and community, as well as microbial community structure (16S and 18S rDNA amplicon sequencing), were followed over the 3–4 d experiments. Different microbial and viral responses to dust were observed rapidly after addition and were, most of the time, more pronounced when combined with future environmental conditions. The dust input of nutrients and trace metals changed the microbial ecosystem from a bottom-up limited to a top-down controlled bacterial community, likely from grazing and induced lysogeny. The relative abundance of mixotrophic microeukaryotes and phototrophic prokaryotes also increased. Overall, these results suggest that the effect of dust deposition on the microbial loop is dependent on the initial microbial assemblage and metabolic state of the tested water and that predicted warming and acidification will intensify these responses, affecting food web processes and biogeochemical cycles.
Cécile Guieu
added a research item
This study reports the only recent characterization of two contrasted wet deposition events collected during the PEACETIME (ProcEss studies at the Air–sEa Interface after dust deposition in the MEditerranean Sea) cruise in the open Mediterranean Sea (Med Sea) and their impact on trace metal (TM) marine stocks. Rain samples were analysed for Al, 12 TMs (Co, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Ti, V and Zn) and nutrient (N, P, dissolved organic carbon) concentrations. The first rain sample collected in the Ionian Sea (Rain ION) was a typical regional background wet deposition event, whereas the second rain sample collected in the Algerian Basin (Rain FAST) was a Saharan dust wet deposition event. Even in the remote Med Sea, all background TM inputs presented an anthropogenic signature, except for Fe, Mn and Ti. The concentrations of TMs in the two rain samples were significantly lower compared to concentrations in rains collected at coastal sites reported in the literature, due to the decrease in anthropogenic emissions during the preceding decades. The atmospheric TM inputs were mainly dissolved forms, even in dusty Rain FAST. The TM stocks in the mixed layer (ML, 0–20 m) at the FAST station before and after the event showed that the atmospheric inputs were a significant supply of particulate TMs and dissolved Fe and Co for surface seawater. Even if the wet deposition delivers TMs mainly in soluble form, the post-deposition aerosol dissolution could to be a key additional pathway in the supply of dissolved TMs. At the scale of the western and central Mediterranean, the atmospheric inputs were of the same order of magnitude as ML stocks for dissolved Fe, Co and Zn, highlighting the role of the atmosphere in their biogeochemical cycles in the stratified Med Sea. In case of intense dust-rich wet deposition events, the role of atmospheric inputs as an external source was extended to dissolved Co, Fe, Mn, Pb and Zn. Our results suggest that the wet deposition constitutes only a source of some of dissolved TMs for Med Sea surface waters. The contribution of dry deposition to the atmospheric TM inputs needs to be investigated.
Cécile Guieu
added a research item
N2 fixation rates were measured in the 0–1000 m layer at 13 stations located in the open western and central Mediterranean Sea (MS) during the PEACETIME cruise (late spring 2017). While the spatial variability in N2 fixation was not related to Fe, P nor N stocks, the surface composition of the diazotrophic community indicated a strong longitudinal gradient increasing eastward for the relative abundance of non-cyanobacterial diazotrophs (NCDs) (mainly γ-Proteobacteria) and conversely decreasing eastward for photo-heterotrophic group A (UCYN-A) (mainly UCYN-A1 and UCYN-A3), as did N2 fixation rates. UCYN-A4 and UCYN-A3 were identified for the first time in the MS. The westernmost station influenced by Atlantic waters and characterized by highest stocks of N and P displayed a patchy distribution of diazotrophic activity with an exceptionally high rate in the euphotic layer of 72.1 nmolNL-1d-1, which could support up to 19 % of primary production. At this station at 1 % PAR (photosynthetically available radiation) depth, UCYN-A4 represented up to 94 % of the diazotrophic community. These in situ observations of greater relative abundance of UCYN-A at stations with higher nutrient concentrations and dominance of NCDs at more oligotrophic stations suggest that nutrient conditions – even in the nanomolar range – may determine the composition of diazotrophic communities and in turn N2 fixation rates. The impact of Saharan dust deposition on N2 fixation and diazotrophic communities was also investigated, under present and future projected conditions of temperature and pH during short-term (3–4 d) experiments at three stations. New nutrients from simulated dust deposition triggered a significant stimulation of N2 fixation (from 41 % to 565 %). The strongest increase in N2 fixation was observed at the stations dominated by NCDs and did not lead on this short timescale to changes in the diazotrophic community composition. Under projected future conditions, N2 fixation was either increased or unchanged; in that later case this was probably due to a too-low nutrient bioavailability or an increased grazing pressure. The future warming and acidification likely benefited NCDs (Pseudomonas) and UCYN-A2, while disadvantaged UCYN-A3 without knowing which effect (alone or in combination) is the driver, especially since we do not know the temperature optima of these species not yet cultivated as well as the effect of acidification.
Cécile Guieu
added 2 research items
Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built on laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near-surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminum (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. The Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (∼ 100–1000 m), while dFe input from dust was only transient in the surface mixed layer. The rapid transfer of dust to depth, the Fe-binding ligand pool in excess to dFe in subsurface (while nearly saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (
Karine Desboeufs
added a research item
The characterisation of aerosol emissions from volcanoes is a crucial step towards the assessment of their importance for regional air quality and regional-to-global climate. In this paper we present, for the first time, the characterisation of aerosol emissions of the Stromboli volcano, in terms of their optical properties and emission flux rates, carried out during the PEACETIME oceanographic campaign. Using sun-photometric observations realised during a near-ideal full plume crossing, a plume-isolated aerosol optical depth of 0.07–0.08 in the shorter-wavelength visible range, decreasing to about 0.02 in the near infrared range, was found. An Ångström exponent of 1.40 ± 0.40 was also derived. This value may suggest the dominant presence of sulphate aerosols with a minor presence of ash. During the crossing, two separate plume sections were identified, one possibly slightly affected by ash coming from a mild explosion, and the other more likely composed of pure sulphate aerosols. Exploiting the full crossing scan of the plume, an aerosol emission flux rate of 9–13 kg/s was estimated. This value was 50% larger than for typical passively degassing volcanoes, thus pointing to the importance of mild explosions for aerosol emissions in the atmosphere.
Cécile Guieu
added a research item
The surface mixed layer (ML) in the Mediterranean Sea is a well-stratified domain characterized by low macronutrients and low chlorophyll content for almost 6 months of the year. In this study we characterize the biogeochemical cycling of nitrogen (N) in the ML by analyzing simultaneous in situ measurements of atmospheric deposition, nutrients in seawater, hydrological conditions, primary production, heterotrophic prokaryotic production, N2 fixation and leucine aminopeptidase activity. Dry deposition was continuously measured across the central and western open Mediterranean Sea, and two wet deposition events were sampled, one in the Ionian Sea and one in the Algerian Basin. Along the transect, N budgets were computed to compare the sources and sinks of N in the mixed layer. In situ leucine aminopeptidase activity made up 14 % to 66 % of the heterotrophic prokaryotic N demand, and the N2 fixation rate represented 1 % to 4.5 % of the phytoplankton N demand. Dry atmospheric deposition of inorganic nitrogen, estimated from dry deposition of nitrate and ammonium in aerosols, was higher than the N2 fixation rates in the ML (on average 4.8-fold). The dry atmospheric input of inorganic N represented a highly variable proportion of biological N demand in the ML among the stations, 10 %–82 % for heterotrophic prokaryotes and 1 %–30 % for phytoplankton. As some sites were visited on several days, the evolution of biogeochemical properties in the ML and within the nutrient-depleted layers could be followed. At the Algerian Basin site, the biogeochemical consequences of a wet dust deposition event were monitored through high-frequency sampling. Notably, just after the rain, nitrate was higher in the ML than in the nutrient-depleted layer below. Estimates of nutrient transfer from the ML into the nutrient-depleted layer could explain up to a third of the nitrate loss from the ML. Phytoplankton did not benefit directly from the atmospheric inputs into the ML, probably due to high competition with heterotrophic prokaryotes, also limited by N and phosphorus (P) availability at the time of this study. Primary producers decreased their production after the rain but recovered their initial state of activity after a 2 d lag in the vicinity of the deep chlorophyll maximum layer.
Cécile Guieu
added a research item
Although atmospheric dust fluxes from arid as well as human-impacted areas represent a significant source of nutrients to surface waters of the Mediterranean Sea, studies focusing on the evolution of the metabolic balance of the plankton community following a dust deposition event are scarce, and none were conducted in the context of projected future levels of temperature and pH. Moreover, most of the experiments took place in coastal areas. In the framework of the PEACETIME project, three dust-addition perturbation experiments were conducted in 300 L tanks filled with surface seawater collected in the Tyrrhenian Sea (TYR), Ionian Sea (ION) and Algerian basin (FAST) on board the R/V Pourquoi Pas? in late spring 2017. For each experiment, six tanks were used to follow the evolution of chemical and biological stocks, biological activity and particle export. The impacts of a dust deposition event simulated at their surface were followed under present environmental conditions and under a realistic climate change scenario for 2100 (ca. +3 ∘C and −0.3 pH units). The tested waters were all typical of stratified oligotrophic conditions encountered in the open Mediterranean Sea at this period of the year, with low rates of primary production and a metabolic balance towards net heterotrophy. The release of nutrients after dust seeding had very contrasting impacts on the metabolism of the communities, depending on the station investigated. At TYR, the release of new nutrients was followed by a negative impact on both particulate and dissolved 14C-based production rates, while heterotrophic bacterial production strongly increased, driving the community to an even more heterotrophic state. At ION and FAST, the efficiency of organic matter export due to mineral/organic aggregation processes was lower than at TYR and likely related to a lower quantity/age of dissolved organic matter present at the time of the seeding and a smaller production of DOM following dust addition. This was also reflected by lower initial concentrations in transparent exopolymer particles (TEPs) and a lower increase in TEP concentrations following the dust addition, as compared to TYR. At ION and FAST, both the autotrophic and heterotrophic community benefited from dust addition, with a stronger relative increase in autotrophic processes observed at FAST. Our study showed that the potential positive impact of dust deposition on primary production depends on the initial composition and metabolic state of the investigated community. This impact is constrained by the quantity of nutrients added in order to sustain both the fast response of heterotrophic prokaryotes and the delayed one of primary producers. Finally, under future environmental conditions, heterotrophic metabolism was overall more impacted than primary production, with the consequence that all integrated net community production rates decreased with no detectable impact on carbon export, therefore reducing the capacity of surface waters to sequester anthropogenic CO2.
Cécile Guieu
added a research item
In low-nutrient low-chlorophyll areas, such as the Mediterranean Sea, atmospheric fluxes represent a considerable external source of nutrients likely supporting primary production, especially during periods of stratification. These areas are expected to expand in the future due to lower nutrient supply from sub-surface waters caused by climate-driven enhanced stratification, likely further increasing the role of atmospheric deposition as a source of new nutrients to surface waters. Whether plankton communities will react differently to dust deposition in a warmer and acidified environment remains; however, an open question. The potential impact of dust deposition both in present and future climate conditions was investigated in three perturbation experiments in the open Mediterranean Sea. Climate reactors (300 L) were filled with surface water collected in the Tyrrhenian Sea, Ionian Sea and in the Algerian basin during a cruise conducted in the frame of the PEACETIME project in May–June 2017. The experiments comprised two unmodified control tanks, two tanks enriched with a Saharan dust analogue and two tanks enriched with the dust analogue and maintained under warmer (+3 ∘C) and acidified (−0.3 pH unit) conditions. Samples for the analysis of an extensive number of biogeochemical parameters and processes were taken over the duration (3–4 d) of the experiments. Dust addition led to a rapid release of nitrate and phosphate, however, nitrate inputs were much higher than phosphate. Our results showed that the impacts of Saharan dust deposition in three different basins of the open northwestern Mediterranean Sea are at least as strong as those observed previously, all performed in coastal waters. The effects of dust deposition on biological stocks were different for the three investigated stations and could not be attributed to differences in their degree of oligotrophy but rather to the initial metabolic state of the community. Ocean acidification and warming did not drastically modify the composition of the autotrophic assemblage, with all groups positively impacted by warming and acidification. Although autotrophic biomass was more positively impacted than heterotrophic biomass under future environmental conditions, a stronger impact of warming and acidification on mineralization processes suggests a decreased capacity of Mediterranean surface plankton communities to sequester atmospheric CO2 following the deposition of atmospheric particles.
Cécile Guieu
added 2 research items
Lithogenic elements such as aluminum (Al), iron (Fe), rare earth elements (REEs), thorium (232Th and 230Th, given as Th) and protactinium (Pa) are often assumed to be insoluble. In this study, their dissolution from Saharan dust reaching Mediterranean seawater was studied through tank experiments over 3 to 4 d under controlled conditions including controls without dust addition as well as dust seeding under present and future climate conditions (+3 ∘C and −0.3 pH). Unfiltered surface seawater from three oligotrophic regions (Tyrrhenian Sea, Ionian Sea and Algerian Basin) were used. The maximum dissolution was low for all seeding experiments: less than 0.3 % for Fe, 1 % for 232Th and Al, about 2 %–5 % for REEs and less than 6 % for Pa. Different behaviors were observed: dissolved Al increased until the end of the experiments, Fe did not dissolve significantly, and Th and light REEs were scavenged back on particles after a fast initial release. The constant 230Th/232Th ratio during the scavenging phase suggests that there is little or no further dissolution after the initial Th release. Quite unexpectedly, comparison of present and future conditions indicates that changes in temperature and/or pH influence the release of Th and REEs in seawater, leading to lower Th release and a higher light REE release under increased greenhouse conditions.
Cécile Guieu
added a research item
Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built onto laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminium (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (~100–1000 m), while dFe input from dust was only transient in the surface mixed-layer. The rapid transfer of dust to depth (up to ~180 m d−1), the Fe-binding ligand pool in excess to dFe in subsurface (while nearly-saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (
Cécile Guieu
added a research item
Although atmospheric dust fluxes from arid as well as human-impacted areas represent a significant source of nutrients to surface waters of the Mediterranean Sea, studies focusing on the evolution of the metabolic balance of the plankton community following a dust deposition event are scarce and none were conducted in the context of projected future levels of temperature and pH. Moreover, most of the experiments took place in coastal areas. In the framework of the PEACETIME project, three dust-addition perturbation experiments were conducted in 300-L tanks filled with surface seawater collected in the Tyrrhenian Sea (TYR), Ionian Sea (ION) and in the Algerian basin (FAST) onboard the R/V “Pourquoi Pas?” in late spring 2017. For each experiment, six tanks were used to follow the evolution of chemical and biological stocks, biological activity and particle export. The impacts of a dust deposition event simulated at their surface were followed under present environmental conditions and under a realistic climate change scenario for 2100 (ca. +3 °C and −0.3 pH units). The tested waters were all typical of stratified oligotrophic conditions encountered in the open Mediterranean Sea at this period of the year, with low rates of primary production and a metabolic balance towards net heterotrophy. The release of nutrients after dust seeding had very contrasting impacts on the metabolism of the communities, depending on the station investigated. At TYR, the release of new nutrients was followed by a negative impact on both particulate and dissolved 14C-based production rates, while heterotrophic bacterial production strongly increased, driving the community to an even more heterotrophic state. At ION and FAST, the efficiency of organic matter export due to mineral/organic aggregation processes was lower than at TYR likely related to a lower quantity/age of dissolved organic matter present at the time of the seeding. At these stations, both the autotrophic and heterotrophic community benefited from dust addition, with a stronger relative increase in autotrophic processes observed at FAST. Our study showed that the potential positive impact of dust deposition on primary production depends on the initial composition and metabolic state of the investigated community. This potential is constrained by the quantity of nutrients added in order to sustain both the fast response of heterotrophic prokaryotes and the delayed one of primary producers. Finally, under future environmental conditions, heterotrophic metabolism was overall more impacted than primary production, with the consequence that all integrated net community production rates decreased with no detectable impact on carbon export, therefore reducing the capacity of surface waters to sequester anthropogenic CO2.
Cécile Guieu
added a research item
Ice-nucleating particles (INPs) have a large impact on the climate-relevant properties of clouds over the oceans. Studies have shown that sea spray aerosols (SSAs), produced upon bursting of bubbles at the ocean surface, can be an important source of marine INPs, particularly during periods of enhanced biological productivity. Recent mesocosm experiments using natural seawater spiked with nutrients have revealed that marine INPs are derived from two separate classes of organic matter in SSAs. Despite this finding, existing parameterizations for marine INP abundance are based solely on single variables such as SSA organic carbon (OC) or SSA surface area, which may mask specific trends in the separate classes of INP. The goal of this paper is to improve the understanding of the connection between ocean biology and marine INP abundance by reporting results from a field study and proposing a new parameterization of marine INPs that accounts for the two associated classes of organic matter. The PEACETIME cruise took place from 10 May to 10 June 2017 in the Mediterranean Sea. Throughout the cruise, INP concentrations in the surface microlayer (INPSML) and in SSAs (INPSSA) produced using a plunging aquarium apparatus were continuously monitored while surface seawater (SSW) and SML biological properties were measured in parallel. The organic content of artificially generated SSAs was also evaluated. INPSML concentrations were found to be lower than those reported in the literature, presumably due to the oligotrophic nature of the Mediterranean Sea. A dust wet deposition event that occurred during the cruise increased the INP concentrations measured in the SML by an order of magnitude, in line with increases in iron in the SML and bacterial abundances. Increases in INPSSA were not observed until after a delay of 3 days compared to increases in the SML and are likely a result of a strong influence of bulk SSW INPs for the temperatures investigated (T=-18 ∘C for SSAs, T=-15 ∘C for SSW). Results confirmed that INPSSA are divided into two classes depending on their associated organic matter. Here we find that warm (T≥-22 ∘C) INPSSA concentrations are correlated with water-soluble organic matter (WSOC) in the SSAs, but also with SSW parameters (particulate organic carbon, POCSSW and INPSSW,-16C) while cold INPSSA (T
Cécile Guieu
added a research item
One pathway by which the oceans influence climate is via the emission of sea spray that may subsequently influence cloud properties. Sea spray emissions are known to be dependent on atmospheric and oceanic physicochemical parameters, but the potential role of ocean biology on sea spray fluxes remains poorly characterized. Here we show a consistent significant relationship between seawater nanophytoplankton cell abundances and sea-spray derived Cloud Condensation Nuclei (CCN) number fluxes, generated using water from three different oceanic regions. This sensitivity of CCN number fluxes to ocean biology is currently unaccounted for in climate models yet our measurements indicate that it influences fluxes by more than one order of magnitude over the range of phytoplankton investigated.
Cécile Guieu
added a research item
The dissolved iron supply controls half of the oceans’ primary productivity. Resupply by the remineralization of sinking particles, and subsequent vertical mixing, largely sustains this productivity. However, our understanding of the drivers of dissolved iron resupply, and their influence on its vertical distribution across the oceans, is still limited due to sparse observations. There is a lack of empirical evidence as to what controls the subsurface iron remineralization due to difficulties in studying mesopelagic biogeochemistry. Here we present estimates of particulate transformations to dissolved iron, concurrent oxygen consumption and iron-binding ligand replenishment based on in situ mesopelagic experiments. Dissolved iron regeneration efficiencies (that is, replenishment over oxygen consumption) were 10- to 100-fold higher in low-dust subantarctic waters relative to higher-dust Mediterranean sites. Regeneration efficiencies are heavily influenced by particle composition. Their make-up dictates ligand release, controls scavenging, modulates ballasting and may lead to the differential remineralization of biogenic versus lithogenic iron. At high-dust sites, these processes together increase the iron remineralization length scale. Modelling reveals that in oceanic regions near deserts, enhanced lithogenic fluxes deepen the ferricline, which alter the vertical patterns of dissolved iron replenishment, and set its redistribution at the global scale. Such wide-ranging regeneration efficiencies drive different vertical patterns in dissolved iron replenishment across oceanic provinces.
Cécile Guieu
added a research item
In spring, the Mediterranean Sea, a well-stratified low-nutrient–low-chlorophyll region, receives atmospheric deposition by both desert dust from the Sahara and airborne particles from anthropogenic sources. Such deposition translates into a supply of new nutrients and trace metals for the surface waters that likely impact biogeochemical cycles. However, the relative impacts of the processes involved are still far from being assessed in situ. After summarizing the knowledge on dust deposition and its impact on the Mediterranean Sea biogeochemistry, we present in this context the objectives and strategy of the PEACETIME project and cruise. Atmospheric and marine in situ observations and process studies have been conducted in contrasted areas encountering different atmospheric deposition context, including a dust deposition event that our dedicated “fast-action” strategy allowed us to catch. Process studies also include artificial dust seeding experiments conducted on board in large tanks in three ecoregions of the open waters of the Mediterranean Sea for the first time. This paper summarizes the work performed at sea and the type of data acquired in the atmosphere, at the air–sea interface and in the water column. An overview of the results presented in papers of this special issue (and in some others published elsewhere) is presented.
Cécile Guieu
added an update
Structure and functioning of epipelagic mesozooplankton and response to dust events during the spring PEACETIME cruisein the Mediterranean Sea
Guillermo Feliú, Marc Pagano, Pamela Hidalgo, and François Carlotti
Biogeosciences Discuss., https://doi.org/10.5194/bg-2020-126, 2020
Accepted for BIOGEOSCIENCES
 
Cécile Guieu
added a research item
The release of lithogenic elements (which are often assumed to be insoluble) such as Aluminum (Al), Iron (Fe), Rare Earth Elements (REE), Thorium (Th) and Protactinium (Pa) by Saharan dust reaching Mediterranean seawater was studied through tank experiments over 3 to 4 days under controlled conditions including control without dust addition and dust seeding under present and future climate conditions (+3 °C and −0.3 pH unit). Unfiltered surface seawater from 3 oligotrophic regions (Tyrrhenian Sea, Ionian Sea and Algerian Basin) were used. The maximum dissolution fractions were low for all seeding experiments: less than 0.3 % for Fe, 1 % for 232Th and Al, about 2–5 % for REE and less than 6 % for Pa. Different behaviors were observed: dissolved Al increased until the end of the experiments, Fe did not dissolve significantly and Th and light REE were scavenged back on the particles after a fast initial release. The constant 230Th/232Th ratio during the scavenging phase suggests that there is little or no further dissolution after the initial Th release. Quite unexpectedly, comparison of present and future conditions indicates that changes in temperature and/or pH influence the release of thorium and REE in seawater, leading to a lower Th release and a higher light REE release under increased greenhouse conditions.
Cécile Guieu
added an update
New paper by Vincent Taillandier et al. just published in the Special issue | Atmospheric deposition in the low-nutrient-low-chlorophyll (LNLC) ocean: effects on marine life today and in the future (BG/ACP inter-journal SI)
 
Cécile Guieu
added an update
A Two-Component Parameterization of Marine Ice Nucleating Particles Based on Seawater Biology and Sea Spray Aerosol Measurements in the Mediterranean Sea by Trueblood et al; https://www.atmos-chem-phys-discuss.net/acp-2020-487/
Mediterranean nascent sea spray organic aerosol and relationships with seawater biogeochemistry by Freney et al.,
 
Cécile Guieu
added a research item
The Sea Surface Microlayer (SML) is known to be enriched by trace metals relative to the underlying water and harbor diverse microbial communities (i.e., neuston). However, the processes linking metals and biota in the SML are not yet fully understood. The metal (Cd, Co, Cu, Fe, Ni, Mo, V, Zn and Pb) concentrations in aerosol samples in the SML (dissolved and total fractions) and in subsurface waters (SSWs; dissolved fraction at ∼1 m depth) from the western Mediterranean Sea were analyzed in this study during a cruise in May–June 2017. The composition and abundance of the bacterial community in the SML and SSW, the primary production, and Chl a in the SSW were measured simultaneously at all stations during the cruise. Residence times in the SML of metals derived from aerosol depositions were highly variable and ranged from minutes for Fe (3.6±6.0 min) to a few hours for Cu (5.8±6.2 h). Concentrations of most of the dissolved metals in both the SML and SSW were positively correlated with the salinity gradient and showed the characteristic eastward increase in the surface waters of the Mediterranean Sea (MS). In contrast, the total fraction of some reactive metals in the SML (i.e., Cu, Fe, Pb and Zn) showed a negative correlation with salinity and a positive correlation with microbial abundance, which might be associated with microbial uptake. Our results show a strong negative correlation between the dissolved and total Ni concentration and heterotrophic bacterial abundance in the SML and SSW, but we cannot ascertain whether this correlation reflects a toxicity effect or is the result of some other process.
Cécile Guieu
added a research item
Plain Language Summary Microscopic plants and animals in seawater require nutrients to survive. One of these key nutrients is iron, dissolved in seawater at very low concentrations. The growth of around half of the microscopic life in the upper ocean is dependent on the availability of this dissolved iron. These organisms form the bottom of the food chain, and their growth is linked to marine productivity and the drawdown of carbon into the deep ocean, in turn influencing climate change. Because iron tends to not dissolve easily in seawater, it must bind with compounds known as ligands, which help keep iron dissolved. However, processes controlling the composition of this ligand pool are poorly understood. As material sinks through the water column, it is broken down by marine microbes, releasing iron and ligands. Here we have studied the release of iron, ligands, and a specific type of ligand known as humic substances, during the microbial degradation of sinking particles. By doing this, we have identified a large fraction of the released ligand pool. This furthers our understanding of the processes controlling dissolved iron concentrations and distributions in ocean waters, providing key information for biogeochemical modeling and for calculating carbon sequestration in seawater.
Cécile Guieu
added a research item
In spring, the Mediterranean Sea, a well-stratified low nutrient low chlorophyll region, receives atmospheric deposition both desert dust from the Sahara and airborne particles from anthropogenic sources. Such deposition translates into a supply of new nutrients and trace metals for the surface waters that likely impact biogeochemical cycles. However, the quantification of the impacts and the processes involved are still far from being assessed in situ. In this paper, we provide a state of the art regarding dust deposition and its impact on the Mediterranean Sea biogeochemistry and we describe in this context the objectives and strategy of the PEACETIME project and cruise, entirely dedicated to filling this knowledge gap. Our strategy to go a step forward than in previous approaches in understanding these impacts by catching a real deposition event at sea is detailed. The PEACETIME oceanographic campaign took place in May–June 2017 and we describe how we were able to successfully adapt the planned transect in order to sample a Saharan dust deposition event, thanks to a dedicated strategy, so-called Fast Action. That was successful, providing, for the first time in our knowledge, a coupled atmospheric and oceanographic sampling before, during and after an atmospheric deposition event. Atmospheric and marine in situ observations and process studies have been conducted in contrasted area and we summarize the work performed at sea, the type of data acquired and their valorization in the papers published in the special issue.
Cécile Guieu
added an update
Poster presented at the 14th International Conference on Mercury as a Global Pollutant (ICMGP 2019) based on data collected during PEACETIME cruise (GEOTRACES process study).
ICMGP 2019, 8-13 September 2019, Krakow, Poland
 
Cécile Guieu
added a research item
The Sea Surface Microlayer (SML) is known to be enriched in trace metals relative to the underlaying water and to harbor diverse microbial communities (i.e. neuston). However, the processes linking metals and biota in the SML are not yet fully understood. In this study, we analyzed the metal (Cd, Co, Cu, Fe, Ni, Mo, V, Zn and Pb) concentrations in aerosol samples, SML (dissolved and total fractions) and in subsurface waters (SSW; dissolved fraction at ~ 1 m depth) from the Western Mediterranean Sea during a cruise in May–June 2017. The bacterial community composition and abundance in the SML and SSW, and the primary production and Chl-a in the SSW were measured simultaneously at all stations during the cruise. Residence times of particulate metals derived from aerosols deposition ranged from a couple of minutes for Co (2.7 ± 0.9 min; more affected by wind conditions) to a few hours for Cu (3.0 ± 1.9 h). Concentration of most dissolved metals in both, the SML and SSW, were well correlated with the salinity gradient and showed the characteristic eastward increase in surface waters of the Mediterranean Sea (MS). Contrarily, the total fraction of some reactive metals in the SML (i.e. Cu, Fe, Pb and Zn) showed negative trends with salinity, these trends of concentrations seem to be associate to microbial uptake. Our results suggest a toxic effect of Ni on neuston and microbiology community’s abundance of the top meter of the surface waters of the Western Mediterranean Sea.
Cécile Guieu
added an update
The drifters released during the Peacetime oceanographic cruise contributed to the MedSVP (Mediterranean Surface Velocity Program, http://nettuno.ogs.trieste.it/sire/medsvp/index.php). The analysis of this precious dataset has provided new insights about the circulation in the Sicily Channel and Southern Tyrrhenian Sea (Menna et al, 2019, https://doi.org/10.3390/w11071355).
 
Cécile Guieu
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This is the updated list of current outputs (paper, presentation at conferences, general public conferences...) of the PEACETIME project
 
Cécile Guieu
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The post-cruise meeting of the PEACETIME project took place at Laboratoire d'Océanographie de Villefranche (LOV) 30-31 May 2018. The meeting lasted two full days and was attended by 40 scientists on site and also few people by visioconference.
One year after the PEACETIME cruise, these 2 days were the occasion to present the data acquired both for the atmosphere and the ocean. The presentations reflected well the large effort that was made before the meeting by the participants to share and discuss the interactions between data and approaches. Today more that 50% of the data are already stored in the LEFE CYBER data base http://www.obs-vlfr.fr/proof/php/PEACETIME/peacetime.php.
After the main presentations (see the full list below), scientists met in small groups to discuss the results’ exploitation strategy and a provisional list of publications has started. The meeting was attended by many students and post-docs. In short, a fruitfull meeting with an always enthusiastic PEACETIME team!
The list of the presentations:
· Guieu C. & Desboeufs K. : Introduction
· Barillon S., Doglioli A., Petrenko A., Rousselet L. : Surface circulation at the PEACETIME LD Stations
· Doglioli A., Petrenko A., Barrillon S., de Verneil A., Rousselet L., Barre L., Cardo C., Berline L., Simon-Bot F., Feliu G., Carlotti F., Bhairy N., Rougier G., Fauvin O., Monfret M. : Mesoscale hydrodynamic features and particles distribution along the PEACETIME transect using a Moving Vessel Profiler
· Prieur L., V. Taillandier, F. D’Ortenzio : Internal waves and drifts at station FAST using ADCP and CTD data
· K. Desboeufs, J-F. Doussin, P. Formenti, Y. Fu, C. Giorio, V. Riffault, G. Siour, S. Triquet : Atmospheric overview and specific events during Peacetime cruise
· Fu Y., K. Desboeufs, S. Triquet , JF Doussin, C. Giorio, F. Dulac, Maisonneuve, Zapf, Feron, A. Tovar-Sánchez, M. Bressac, C. Guieu : Atmospheric deposition of nutrients and trace metals during PEACETIME cruise
· Tovar-Sanchez A. et al. : Trace Metals in the Surface microlayer (SML) and SML addition experiments
· Tovar-Sanchez A., M. Bressac, T. Wagener, C. Ridame, S. Jacquet et al. : Dissolved Trace Metals in the Water Column
· Bressac M. et al. : Mesopelagic iron remineralization patterns in the Subantarctic and Mediterranean Sea
· Gaillard S., E. Pulido-Villena, K. Djaoudi, F. Van Wambeke : First overview of biogeochemical cycling of dissolved phosphate during the PEACETIME cruise
· Guieu C., E. Pulido-Villena, F. van Wambeke, Y. Fu, et al. : N, P and N/P ratio: link with deposition ?
· Uitz J., N. Haëntjens, A. Bricaud, J. Ras, C. Dimier, M. Thyssen, G. Grégori, V. Taillandier, F. D’Ortenzio, E. Boss : Bio-optics and phytoplankton biomass and diversity
· Engel A. & B. Zäncker : Organic Matter in the Surface Microlayer during the PEACETIME campaign
· van Wambeke F., J. Dinasquet, E. Maranon, M. Perez-Lorenzo, I. Obernosterer, J. Caparros, B. Marie, P. Catala, F. Gazeau : Carbon dynamics and plankton distribution during Peacetime cruise
· Sellegri et al., : Marine primary emissions from the underway seawater flow: links with the seawater biogeochemistry?
· Carlotti F.; Feliu G. : Structure of zooplankton community during peacetime. Result of size spectrum and diversity
· Jacquet S., A. Dufour, P. Layoun (M2), C. Comby, O. Galtier, A. Huet, S. Barry (L3), Tamburini C., Garel M., F. van Wambeke, Lefèvre D., Wagener T., Heimbürger L-E : Ba-based mesopelagic POC remineralization during PEACETIME Cruise
· Garel M., Lefevre D., F. Van Wambeke, F. Gazeau, A. Engel, T. Blasco, V. Taillandier, N. Bhairy, S. Guasco, B. Zaenker, C. Tamburini : Remineralization into the mesopelagic waters: prokaryotic heterotrophic production and estimation of prokaryotic respiration
· Paul Layoun et al. : MeHgas a novel tracer for organic carbon remineralization rates
· Gazeau et al. (all the minicosms team) : Present and future impact of dust: Results from the on board Climate Reactors experiments
· Irisson JO, et al. : (Zoo)plankton in Climate Reactors
· Baudoux AC, Bigeard E. et al. : Viral dynamics during the minicosmexperiments
· Mallet M. et al. : ALADIN-Climate model configuration
 
Cécile Guieu
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Ifremer Annual Report 2017: Interview about PEACETIME cruise (english and french version)
 
Cécile Guieu
added 2 research items
The bioavailability of iron (Fe) may limit primary production in half of the global ocean. However, most research has focused on sources of new Fe, and little attention has been directed toward controls on the release of dissolved iron (DFe) during particle remineralization at depth. Within the mesopelagic zone, heterotrophic bacterial activity results in a dramatic decrease in the particulate organic carbon exported from surface waters, and concurrent release of DFe-a potential source of bioavailable Fe when resupplied to surface waters. Despite their importance, bacterial remineralization and the fate of particulate iron (PFe) in the mesopelagic are poorly understood due to technical issues involved in studying this stratum. Using an innovative approach, the concurrent in situ measurement of bacterial remineralization of sinking particles (based on time-series of oxygen consumption) and associated DFe release rates within the mesopelagic have been performed within two contrasting regions. Study sites are characterized by different bacterial remineralization rates, Fe supply mechanisms and inventories, and Fe scavenging removal by sinking particles. During GEOTRACES process studies in the Subantarctic (SOTS) and Mediterranean (PEACETIME), trace-metal clean versions (TM-RESPIRE) of the surface-tethered free-drifting RESPIRE (REspiration of Sinking Particles In the subsuRface ocEan, Boyd et al. 2015; DOI:10.1002/lom3.10043) particle interceptor were deployed within the upper mesopelagic zone to determine whether the fate of PFe (and other trace elements) at depth is primarily driven by particle properties that are imprinted within the surface ocean (i.e., biotic vs. lithogenic) or mesopelagic particle transformations. This key question was addressed by comparing rates of particle export, bacterial remineralization, and DFe release, in the context of other measurements (e.g. PFe concentrations, bacterial activity), and proxies (e.g. thorium disequilibria, barium).
Cécile Guieu
added a research item
Processes occurring at the atmosphere-ocean interface are critical to the regulation of the Earth climate and to the delivery of key services provided by marine ecosystems. The Mediterranean Sea, a hot spot for biodiversity but also for climate change and anthropogenic pressure, is an ideal natural laboratory to study these processes. The goal of the PEACETIME project is to provide the understanding necessary to accurately represent natural and anthropogenic chemical exchanges at the air-sea interface and their impacts on marine ecosystems and services, today and in the future. In the frame of the PEACETIME project (http://peacetime-project.org/), an oceanographic cruise onboard the R/V 'Pourquoi Pas?' took place in the Western/Central Mediterranean Sea May 10-June 11, 2017. The purpose of this expedition was to study the processes induced by atmospheric deposition, in particular Saharan dust, occurring at the air-sea interface in the Mediterranean Sea, a region of the world where atmospheric input plays a key role as a nutrient source for the marine biosphere. Combining in situ observations in the atmosphere and ocean with process studies in the water column as well as Climate Reactors incubation experiments, we characterized the chemical, biological and physical properties of the atmosphere, the marine surface micro-layer, and the deeper layers of the Mediterranean. Incubation experiments were set to reproduce different water temperatures and pCO 2 conditions so that scientists could assess the atmospheric impacts in both present and future climate conditions. Moreover, the PEACETIME strategy included an "in-situ, real-time" approach: catching a real event of atmospheric deposition in Mediterranean waters, and documenting the ensemble of interactions induced on the surface ocean ecosystem. A fine-tuned team of people (on and off-board) worked together to examine quasi-real time dust transport forecasts and satellite observations, adjust the cruise track, and position the ship in an area where deposi-tion events were forecasted. This unique coordinated effort succeeded, and the scientists were able to sample and measure the "real-time" effects of a dust deposition event on the marine surface waters. PEACETIME yields insights into the impact of atmospheric deposition on the cycle of chemical elements (nutri-ents, metals), on the biogeochemical functioning of the pelagic ecosystem and on the retroaction to the atmosphere. A first set of results concerning physical, chemical, and biological measurements on both the marine stations, the atmospheric and marine underway and the incubation experiments will be presented to highlight the major findings so far.
Cécile Guieu
added an update
Over 40 scientists met in Marseille on the 6th and 7th November to share and discuss the results obtained during the recent PEACETIME oceanographic cruise (May-June 2017). The meeting, held at the Mediterranean Institute of Oceanography (MIO), gathered both the scientists that were onboard the R/V and those who were on land, and allowed the different groups involved to check on the status and the advancement of the analyses.
One of the main aims of the PEACETIME expedition was the characterization of the biogeochemical processes induced by atmospheric inputs over the Mediterranean Sea, and notably the response of the system to Saharan dust inputs.
To that purpose, the cruise combined in-situ observations of both the atmosphere and the ocean, as well as Climate Reactors incubation experiments. Incubation experiments were set to reproduce different water temperatures and pCO2 conditions so that scientists could assess the atmospheric impacts in both present and future climate conditions.
Moreover, the PEACETIME strategy included an “in-situ, real-time” approach: catching a real event of atmospheric deposition in Mediterranean waters, and documenting the ensemble of interactions induced on the surface ocean ecosystem. A fine-tuned team of people (on and off-board) worked together to examine quasi-real time dust transport forecasts and satellite observations, adjust the cruise track, and position the ship in an area where deposition events were forecasted.
This unique coordinated effort succeeded, and the scientists were able to sample and measure the “real-time” effects of a dust deposition event on the marine surface waters.
A first set of results concerning physical, chemical, and biological measurements on both the marine stations, the atmospheric and marine underway and the incubation experiments were presented during the meeting, and provided an interesting overview on the major findings so far.
All participants agreed on the importance of using common criteria to define the environmental framework (meteorological and physical conditions, but also the working scale resolution), and on the necessity of using homogeneous databases, with common templates that fit large databases such as the French MISTRALS, LEFE-CYBER and the GEOTRACES formats, to ease the exchange of information.
The next meeting is set on 30-31 May 2018 at LOV in Villefranche.
For further information, please visit the PEACETIME project website, http://peacetime-project.org/
Contact:
Cécile Guieu: guieu@obs-vlfr.fr
(report prepared by Estela Romero for MERMEX https://mermex.mio.univ-amu.fr/)
 
Cécile Guieu
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Less than 5 months after the PEACETIME cruise ended, embarking and non-embarking scientists involved in the project will meet to discuss the many results achieved during the cruise and since. This 2 days meeting will take place at M I O on 6 and 7 of November. 40 scientists from 14 labs will attend.
The goal of the meeting is to show, share and discuss all the data in a very informal way, in plenary then in small working groups and start elaborating a common strategy to valorize our work. Projects of papers will then be presented in spring during the final PEACETIME meeting.
PEACETIME : Process studies at the air-sea interface after dust deposition in the Mediterranean.
For further info on the PEACETIME project, please visit the website: http://peacetime-project.org/
Contact: Cécile Guieu and Karine Desboeufs
 
Cécile Guieu
added an update
PEACETIME oceanographic cruise: MISSION ACCOMPLISHED
The 40 scientists who embarked the PEACETIME cruise all did an amazing work during that month at sea. The campaign departed on May 10 and ended on June 11 at the La Seyne harbor. All the work envisaged upstream has been achieved. In terms of operation at sea:
- 90 profiles with the classical CTD-rosette loaded with optical instruments were done for sample collection for the measurement of stocks and biological and chemical fluxes (including Hyperbaric conditions) and Physical characterization of the water column
- samples were collected in 'ultra trace' condition thanks to 27 profiles with the "clean" rosette: measurements of metals could be made on board (Aluminum and dissolved iron) indicating the regions impacted by atmospheric Saharan deposits (note that this work is done within the framework of the international program GEOTRACES as PEACETIME is endorsed as a process-study for this program).
- The zooplankton biodiversity will be studied through 27 nets tows along the transect.
- The micro-layer, which is the area of ​​the interface between the atmosphere and the ocean, could be sampled 17 times from a rubber boat and will be characterized from a biological and chemical point of view.
- An innovative system of continuous "clean" pumping at 5 m under the boat thanks to a large peristaltic pump allowed to automatically characterize chemically, biologically and physically (including optical measurements) the surface seawater throughout the whole transect. The water flushed in a dedicated laboratory has also been studied continuously with regard to its particle and gas emission properties in order to study the feedbacks from the ocean to the atmosphere.
- Air sampling was carried out throughout the whole campaign using a dedicated container to monitor continuous air composition, parameters of atmospheric dynamics such as boundary layer, and radiative parameters (incident radiation, Optical thickness, optical properties of the particles). In addition to these continuous sampling, 3 rains were collected and analyzed during the campaign, including the expected rainfall in FAST ACTION station.
Several float-profilers and lagrangian drifters have been retrieved / dropped during the journey: they will allow us to continue to study the system that we have characterized at a given moment. Three drifting moorings were deployed during the 2 long stations and the FAST station: the line was heavily loaded with different types of sediment traps, several types of instruments measuring in situ respiration and physical instrumentation: this will allow us to better understand the fate of matter between the surface and the deep waters, in particular the link between the deposits of Saharan dust and the export of carbon.
Another specific feature of PEACETIME was to embark 8 "climate reactors": those experimental devices reproduce on a small scale the air-sea exchanges under current and future environmental conditions (acidification and increase of the temperature of the sea water). Three experiments involving a large number of scientists on board (duration min = 5 days) were successfully conducted in 3 regions presenting distinct in situ characteristics.
Finally, thanks to our FAST ACTION strategy, which was thought well upstream of the campaign, we were able to observe in situ the deposit of Saharan dust and monitor the effects in the ocean and the feedbacks to the atmosphere for 6 days. We are expecting particularly original results from this FAST ACTION.
Daily news of the campaign were available on our Twitter account (114 tweet were posted during the campaign, still visible here: https://twitter.com/peacetimecruise)
We would like to thank again the captain and crew of the Pourquoi Pas? as it has been a real pleasure to work together on board of this great R/V.
Website of the project: http://peacetime-project.org/
 
Cécile Guieu
added a project goal
Processes occurring at the atmosphere-ocean interface are critical to the regulation of the Earth climate and to the delivery of key services provided by marine ecosystems.
The Mediterranean Sea, a hot spot for biodiversity but also for climate change and anthropogenic pressure, is an ideal natural laboratory to study these processes.
PEACETIME will provide the understanding necessary to accurately represent natural and anthropogenic chemical exchanges at the air-sea interface and their impacts on marine ecosystems and services, today and in the future.
In the frame of PEACETIME, an oceanographic cruise onboard the R/V ‘Pourquoi Pas?’ took place in the Western/Central Mediterranean Sea May 10–June 11, 2017. The purpose of this expedition was to study critical processes induced by atmospheric deposition, in particular Saharan dust, occurring at the air-sea interface in the Mediterranean. PEACETIME yields insights into the impact of such processes on the cycle of chemical elements (nutrients, metals) and on the biogeochemical functioning of the pelagic ecosystem. The 40 scientists embarked are experts in atmosphere and ocean domains.