The PEACETIME cruise offered the opportunity to study along the 4300 km travelled both the lower atmosphere and the water column in contrasted biogeochemical regions of the Mediterranean Sea under the influence of different air masses and submitted to contrasted atmospheric deposition. The following section summarizes the main results acquired and the related publications.

Most of the publications from PEACETIME are/will be 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). https://bg.copernicus.org/articles/special_issue1040.html.

A total of thirty articles are/will be published. Details are given in the following sections:  R1 (19 articles published (including in journals other than the Special Issue), R16: 2 under review and 1 submitted. There are still publications in progress at the time of writing this report: heading R20 (7 papers will be submitted to the Special Issue (closing June 2021) and 2 to other journals). 

Atmosphere, surface microlayer and emissions back to the atmosphere.

Continuous atmospheric measurements allowed us to document the composition of the lower troposphere in contrasted areas from the remote Ionian Sea to Balearics Islands through the Tyrrhenian Sea and Messina Strait characterized by a high maritime ship traffic thanks to the PEGASUS installed onboard for the first time (Figure 1 and Desboeufs et al, in prep.).

Figure 1. The PEGASUS platform on board the Pourquoi Pas? during PEACETIME, in a navigation towards the Stromboli. 

Local wind and air masses trajectories analysis showed the influence of distinct air masses (from eastern and western Europe, as well as from Sahara), and various levels of concentrations, enabling to characterize background, polluted or dusty atmospheric conditions. The background level of the most common atmospheric pollutants for Mediterranean open-sea were quantified and discussed in terms of atmospheric reactivity (Desboeufs et al., in prep.,). Both bromine oxide (BrO) and iodine oxides (IO) have been modelled to be present in the Mediterranean marine atmosphere, however their detection has not been demonstrated yet. The first observations of the widespread presence of BrO and IO were reported over the Mediterranean atmosphere. These levels of halogen oxides can cause considerable impacts on the overall tropospheric ozone budget over the Mediterranean and affect pollutant and oxidant levels in Mediterranean coastal cities (Garcia-Nieto et al., in prep.). Particulate and dissolved fractions of two wet deposition events collected on board, - one of which being the dust wet deposition at the FAST station - have been characterized (Desboeufs et al., in prep.). Trace metals fluxes both from dry and wet depositions were significantly lower than those reported in the literature at coastal sites, emphasizing the relevance of offshore measurements to accurately quantify actual atmospheric inputs to the open ocean (Fu et al., in prep.). Anthropogenic signature of aerosols chemical composition, in particular for trace metals, was obvious even in the most remote areas, showing a clear anthropogenic influence in the Mediterranean background (Fu et al., this issue).

Thanks to discrete sampling of the SML away from the R/V, the interface between the atmosphere and the ocean has been shown to accumulate eukaryotic and prokaryotic micro-organisms (Zanker et al., 2021). Even though decreasing abundances of phytoplankton and bacteria within the SML suggest that conditions become more hostile for microbes going from west to east in the Mediterranean Sea, semi-labile organic matter such as microgels and total carbohydrate are still present in the surface. While the neuston diversity might be decreasing, selecting for more oligotrophy-adapted protists, certain species might still be able to thrive, such as the fungi in the surface of the Ionian Sea (Zanker et al., 2021). A rapid increase of gel particles abundance in the SML was observed at station FAST directly after the dust deposition event, indicating a potential influence of atmospheric processes on organic matter dynamics at the air-sea interface (Engel et al. in prep.). SML was also shown to be very important for the exchange of some metals from aerosols which residence time within the surface microlayer were very different among chemical elements (in the order of minutes for iron up to several hours for copper) (Tovar-Sanchez et al., 2020). The highest concentrations of trace metals (and bacteria) in the SML were observed after the dusty rain in the FAST station. The total concentrations of some reactive metals (i.e. Cu, Fe, Pb and Zn) were positively correlated with bacterial abundance, following the same decreasing eastward trend. This likely indicates that bacterioneuston plays a key role in controlling the concentrations and fates of those metals in the SML (Tovar-Sanchez et al., 2020). Interestingly, a strong negative correlation was found between Ni concentration and heterotrophic bacterial abundance in the SML, along the whole transect that could be linked to a toxicity effect.

The emission of aerosols from the ocean to the atmosphere was also a focus of the PEACETIME project. During the cruise, Freney et al. (2021) have shown that the composition of sea spray reflected the oligotrophic conditions encountered along the transect, with an average 22±6 % organic matter content. Although this low content had little variations throughout the transect, it was mostly correlated with the particulate organic carbon seawater concentration and could be predicted using this variable. Moreover, the organic content of sea spray aerosols could be discriminated into several organic classes, each of them having specific climate-relevant properties. For instance, the Mixed Organic Aerosol (MOA) class, especially linked to the nanophytoplankton cell abundance, has an impact on the seawater surface tension, that in turn determines the number of sea spray particles and cloud condensation nuclei emitted to the atmosphere (Sellegri et al. 2021). Neither the chemical composition, nor the number emissions of sea spray and cloud condensation nuclei were significantly influenced by the occurrence of the dust rain event during the FAST action. Neither was the ice nucleating ability of sea spray, although the ice nucleating ability of the SML was significantly enhanced (Trueblood et al., 2021), in relation with the enrichment of iron and bacteria cell number in the SML during this event.

Biogeochemical and physical features from instruments

A high resolution quantification of particulate matter distribution by LOPC-Laser Optical Particle Counter, revealed potential long distance transport in the central Ionian Sea, bringing together particulate matter originating from the Northern Ionian and Modified Atlantic water masses (Berline et al., in rev.). The role played by vertical diffusion in the nutrient enrichment of the Levantine Intermediate Waters was assessed (Taillandier et al., 2020), this process being particularly relevant inside thermohaline staircases. Thanks to a high profiling frequency over a four-year period, BGC-Argo float observations revealed the temporal continuity of the layering patterns encountered during the cruise PEACETIME, and their impact on vertical and lateral transfers of nitrate between the deep reservoir and the surface productive zone (Taillandier et al., 2020). From the diel cycles of optical properties measured by BGC-Argo profiling floats in the entire water column, it was possible to derive estimates of biological production (Barbieux et al., in prep, this issue), revealing that the subsurface chlorophyll maximum (SCM) contributed substantially (~ 40%) to the biological production of the whole productive layer during the summer period in the Ligurian Sea. In the Ionian Sea, the SCM contributed less importantly to the summer time production than in the Ligurian Sea, but still in a non-negligible way (~ 20%). Hence, Barbieux et al. work suggests that because the SCM layer may contribute substantially to the total production of the water column in Mediterranean oligotrophic waters, it may be missed by estimates derived from ocean color satellite surface chlorophyll a data.

Biogeochemistry from discrete sampling during PEACETIME

The vertical distribution of chlorophyll, phytoplankton carbon biomass and primary production, together with bacterial production, with a focus on the deep chlorophyll maximum (DCM) was established along the cruise track (Marañón et al., 2020). The DCM was also a primary production maximum, with an enhanced contribution by diatoms, which sustained similar biomass turnover rates to those found in well-lit surface waters. Bacterial production tended to peak at the surface and at or above the DCM, and bacterial carbon demand exceeded dissolved primary production. Among primary producers, a focus has been placed on diazotrophs (Ridame et al., in prep.). The depth-integrated (0-100 m) di-nitrogen fixation rates encountered a low spatial variability between the different Mediterranean basins, except at station 10, in the Algerian basin where the fluxes were unusually high for the Mediterranean Sea (up to 72 nmol N. L^-1. d^-1 at 60 m). At that station, the diazotrophic community was dominated by UCYN-A that represented more than 90% of the nifH population (Ridame et al., in prep.). Specific phylotypes partitioned with depth layers with UCYN-A1 being the most abundant in surface water (<10 m) and with the relative abundance of UCYN-A4 increasing with depth (from 60-90 m), while a mix of the different phylotypes was observed at 200m. Ectoenzymatic activities (alkaline phosphatase, aminopeptidase and b-glucosidase) were determined for high and low affinity enzymatic systems at four selected layers (surface, deep chlorophyll maximum, LIW core and upper mesopelagic layer) (van Wambeke et al., 2021). The in situ aminopeptidase hydrolysis rates of dissolved N-proteins represented about 48% of the heterotrophic bacterial N demand in epipelagic layers (van Wambeke et al., 2021). Within the surface mixed layer, the contribution of this N source to N demand of heterotrophic bacteria was on average 1.4 times higher than total (organic + inorganic) dry atmospheric deposition determined at the same time (van Wambeke et al., in rev.). Cross-basin simultaneous measurements of nanomolar phosphate concentrations, atmospheric deposition and alkaline phosphatase activity are presented in Pulido-Villena et al., sub.. For the first time, the relative contribution of diapycnal fluxes and atmospheric inputs to phosphate supply to surface waters is assessed and compared to phosphate supplied by hydrolysis of organic phosphorus to satisfy P requirements. Diapycnal flux of phosphate to the mixed layer was particularly weak at FAST and was 2 order of magnitude lower than atmospheric soluble flux (Pulido-Villena et al., sub.). Vertical diffusion fluxes from the interior into the depleted layer (across nutriclines) were much higher (Taillandier et al., 2020, Pulido-Villena et al., sub.); however, those nutrients were not injected up to the shallower mixed layer that was rather directly impacted by atmospheric deposition as depicted at ION and FAST after wet deposition (van Wambeke et al., in rev.). Thanks to a high frequency sampling and measurements of nutrients at nanomolar level, the rapid and short impact of those rains on the dynamics of N and P biogeochemical cycles and biological activity were investigated (van Wambeke et al., in rev. and Figure 2).

Figure 2. Synthetic view of biogeochemical processes and exchanges between the surface mixed layer and the rest of the nutrient depleted layer (NDLb) before the rain and evolution after the rain. A complex and timely-controlled trophic cascade within the microbial food web has been observed thanks to simultaneous observation in the atmosphere and the water column, before, during and after a wet deposition event. (after van Wambeke et al., in rev.)

By studying the zooplankton taxa assemblages in the Tyrrhenian Basin few days after a dust event and at FAST before and after a dust event, PEACETIME offered the first in situ observation of mesozooplankton responses to natural Saharan dust depositions (Feliu et al., 2020). The changes in mesozooplankton taxonomic structure appear to be a relevant indicator of that response, with an initial phase with no real dominance of taxa, then a disturbed state of the community with strong dominance of certain herbivorous taxa and the appearance of carnivorous species, and finally a recovery state towards a more stable system with diversification of the community (Feliu et al., 2020).

Mesopelagic processes

Simultaneous measurements of dissolved and (suspended) particulate concentrations, along with the sinking fraction (PPS5 sediment traps), were used to estimate the residence time of Aluminium and Iron at the three long stations submitted to contrasted atmospheric deposition conditions (Figure 3 and Bressac et al., sub.). This study confirms the important dust deposition that affected a large area few days before our arrival in the Tyrrhenian Sea. Results are discussed and related to the origin (Saharan vs. anthropogenic), magnitude (2 to 3 orders of magnitude difference), and timing (few days before vs. during the occupation) of the atmospheric deposition events. At the FAST long station, the sampling performed at a high temporal/vertical resolution during a wet dust deposition event allowed estimating in situ the Fe and Al dissolution rates and the magnitude of post-depositional processes (e.g. scavenging) (Bressac et al., sub.).

Figure 3. Longitudinal distribution of Dissolved iron during PEACETIME. The surface layer are enriched in DFe, depicting the atmospheric flux and the accumulation in the surface mixed layer during the stratification period. (from Bressac et a., submitted).

Bressac et al. (2019) compared concurrent oxygen consumption, DFe and Fe-binding ligand replenishment rates in the Mediterranean Sea (PEACETIME) and Southern Ocean (EDDY, SOTS), two contrasting biogeochemical provinces characterized by differing contributions from biogenic and lithogenic sinking material. Mesopelagic iron regeneration efficiencies were heavily influenced by particle composition with 10- to 100-fold higher values in low-dust subantarctic waters relative to high-dust Mediterranean sites. Such wide-ranging regeneration efficiencies drive different vertical patterns in DFe replenishment across oceanic provinces. Further analyses by Whitby et al. (2020) demonstrated the importance of eHS in supporting DFe supplied by particle degradation, and highlighted how microbial removal of eHS ligands with increasing POC consumption could influence the amount of DFe resupplied by bacterial remineralization.

Particulate biogenic barium (Baxs) in the mesopelagic layer (100-1000 m depth) formed during organic matter degradation by heterotrophic prokaryotes, was used as a proxy for particulate organic carbon (POC) remineralization (Jacquet et al., in rev.). Important spatial variability in POC remineralisation was measured, being for example more important and deeper in Algeria and the Ionian Basin compared to the Tyrrhenian Basin. Higher lithogenic impact on Ba signal (>20%) was found at Stations 4, 5 and TYR (250-450m) (based on Al correction) and could be related to the strong dust event that occurred few days before our arrival.

Present and future impact of dust deposition: results from on-board experiments.

The impact of dust deposition both in present and future climate conditions was assessed through perturbation experiments performed for the first time in the open Mediterranean Sea during the PEACETIME cruise. Experiments were conducted in large Climate reactors (300 L) (Figure 4) at the long stations to test the impact of dust deposition on biogeochemistry stocks and processes and compare those impacts in contrasted open waters. Both temperature and pH were considered in order to test the effects both under present and future climate conditions.

Figure 4. Climate reactors inside a container on board during PEACETIME. 3 dust addition experiments were performed for the first time in open Mediterranean Sea during the campaign.

The experimental protocol comprised two unmodified control tanks, two tanks enriched with a Saharan dust analog and two tanks enriched with the dust analog and maintained under warmer (+3 ºC) and acidified (-0.3 pH unit) conditions. Gazeau et al. 2021, present the general setup of the experiments and the impacts of dust seeding and/or future climate change scenario on nutrients and biological stocks. The effects of dust deposition on biological stocks were highly different between the three stations and could not be attributed to differences in their degree of oligotrophy but rather to the initial metabolic state of the community (Figure 5). Furthermore, ocean acidification and warming did not drastically modify the composition of the autotrophic assemblage with all groups benefiting from warmer and acidified conditions, suggesting an exacerbation of effects from atmospheric dust deposition in the future (Figure 5). Similar to the stocks, the effects on biological rates were highly different between the three stations (Gazeau et al., sub.). Heterotrophic prokaryote production rates and enzymatic activities were significantly enhanced at all three stations following simulated dust deposition, especially under future climate conditions. Overall, bacterial mortality (through grazing and viral lysis) was enhanced after dust addition under future climate conditions (Dinasquet et al., in prep.). Microbial (prokaryotes and eukaryotes) community composition shifted overtime in response to the perturbations, compared to the control communities that remained relatively stable. No visible effect of dust deposition on primary production rates could be observed at station TYR while clear enhancements were observed at the two other stations (Gazeau et al., sub. And Figure xx). Finally, as for bacteria, pH and temperature exacerbated these effects at those stations (ION and FAST). Dust addition induced a strong increase in N₂ fixation rates that was similar both under present and future climate conditions at TYR and ION (Ridame et al., in prep.). These experiments also permitted to study the fate of some lithogenic elements associated with dust, allowing a first direct assessment of the low percentage of dissolution for thorium (~ 1%) and protactinium ( < 6 %) after Saharan dust deposition in seawater (Roy-Barman et al., 2021). Unforeseen effects of temperature and/or pH on the release of thorium and rare earth elements in seawater led to a lower Th release and a higher light REE release under increased greenhouse conditions. Contrasted responses were observed: aluminum kept dissolving over the course of the experiment whereas thorium and light rare earth elements initially released in seawater were scavenged back on the particles.

Figure 5. Results from dust addition experiments performed in open Mediterranean Sea. (only 4 parameters over the 20 measured are presented). Max relative change were most of the time higher compared to a compilation of results from experiments performed with coastal waters. The main result is that dust addition exacerbates the initial metabolic state of the tested waters. In green: results for the future conditions of temperature and pH: while the release of nutrients from the dust was found to be similar as for present conditions, the very large increase in stocks and fluxes indicate a clear exacerbation of dust impacts in the future climate. (see Gazeau et al., accepted and Gazeau et al., sub.)

Conclusion

The PEACETIME oceanographic expedition conducted in spring 2017 cruised over a 20° longitudinal gradient across the western and central Mediterranean Sea during the season characterized by strong stratification, low productivity and high probability of wet dust deposition. Those conditions were required in order to fulfill the objectives of the project aiming at quantifying the biogeochemical processes at play after atmospheric deposition and its impact on ecosystem functioning. Different atmospheric situations were encountered during the cruise. In particular, luckily, three contrasted situations in term of atmospheric deposition characterized the long duration stations, allowing the acquisition of a large dataset under different dynamical and biogeochemical in situ conditions to explore the chemical and ecosystem response to deposition. Thanks to an adaptive strategy based on a large panel of atmosphere and ocean real time observations and forecast models, the track of the cruise was optimized from day to day. In particular, it was possible to reach on time an area located more than 800 km away where an event was forecasted and actually observed and sampled. In situ observations could be completed in a very relevant way by three dust addition experiments considering present and future climate conditions. All the studies performed on board closely involved scientists from different disciplines making these outputs truly interdisciplinary.

The PEACETIME process study is an important contribution to IMBER and SOLAS international programs. It also provides input function at the ocean surface for GEOTRACES key tracers when Saharan dust is deposited over the ocean and taking into account the biological activity and the impact of climate change. It will contribute to significantly improve the tracer-based mapping of lithogenic dust deposition over the ocean.