Abstract. The effect of ocean acidification and changing water conditions on primary (and secondary) marine aerosol emissions is not well understood on a regional or a global scale. To investigate this effect as well as the indirect effect on aerosol that changing biogeochemical parameters can have, ∼ 52 m 3 pelagic mesocosms were deployed for several weeks in the Mediterranean Sea during both winter pre-bloom and summer oligotrophic conditions and were subjected to various levels of CO 2 to simulate the conditions foreseen in this region for the coming decades. After seawater sampling, primary bubble-bursting aerosol experiments were performed using a plunging water jet system to test both chemical and physical aerosol parameters (10-400 nm). Comparing results obtained during pre-bloom and oligotrophic conditions, we find the same four lognormal modal diameters (18.5± 0.6, 37.5± 1.4, 91.5± 2.0, 260± 3.2 nm) describing the aerosol size distribution during both campaigns, yet pre-bloom conditions significantly increased the number fraction of the second (Aitken) mode, with an amplitude correlated to virus-like particles, heterotrophic prokaryotes, TEPs (transparent exopolymeric particles), chlorophyll a and other pigments. Organic fractions determined from kappa closure calculations for the diameter, D p ∼ 50 nm, were much larger during the pre-bloom period (64 %) than during the oligotrophic period (38 %), and the organic fraction decreased as the particle size increased. Combining data from both campaigns together, strong positive correlations were found between the organic fraction of the aerosol and chlorophyll a concentrations, heterotrophic and autotrophic bacteria abundance, and dissolved organic carbon (DOC) concentrations. As a consequence of the changes in the organic fraction and the size distributions between pre-bloom and oligotrophic periods, we find that the ratio of cloud condensation nuclei (CCN) to condensation nuclei (CN) slightly decreased during the pre-bloom period. The enrichment of the seawater samples with microlayer samples did not have any effect on the size distribution, organic content or the CCN activity of the generated primary aerosol.Published by Copernicus Publications on behalf of the European Geosciences Union. Partial pressure of CO 2 , pCO 2 , perturbations had little effect on the physical or chemical parameters of the aerosol emissions, with larger effects observed due to the differences between a pre-bloom and oligotrophic environment.

Recent estimates of nutrient budgets for the Eastern Mediterranean Sea (EMS) indicate that atmospheric aerosols play a significant role as suppliers of macro-and micro-nutrients to its Low Nutrient Low Chlorophyll water. Here we present the first mesocosm experimental study that examines the overall response of the oligotrophic EMS surface mixed layer (Cretan Sea, May 2012) to two different types of natural aerosol additions, "pure" Saharan dust (SD, 1.6 mg l −1 ) and mixed aerosols (A-polluted and desert origin, 1 mg l −1 ). We describe the rationale, the experimental set-up, the chemical characteristics of the ambient water and aerosols and the relative maximal biological impacts that resulted from the added aerosols. The two treatments, run in triplicates (3 m 3 each), were compared to control-unamended runs. Leaching of ∼2.1-2.8 and 2.2-3.7 nmol PO 4 and 20-26 and 53-55 nmol NO x was measured per each milligram of SD and A, respectively, representing an addition of ∼30% of the ambient phosphate concentrations. The nitrate/phosphate ratios added in the A treatment were twice than those added in the SD treatment. Both types of dry aerosols triggered a positive change (25-600% normalized per 1 mg l −1 addition) in most of the rate and state variables that were measured: bacterial abundance (BA), bacterial production (BP), Synechococcus (Syn) abundance, chlorophyll-a (chl-a), primary production (PP), and dinitrogen fixation (N 2 -fix), with relative changes among them following the sequence BP>PP≈N2-fix>chl-a≈BA≈Syn. Our results show that the "polluted" aerosols triggered a relatively larger biological Herut et al.Impact of Aerosols on LNLC Seawater change compared to the SD amendments (per a similar amount of mass addition), especially regarding BP and PP. We speculate that despite the co-limitation of P and N in the EMS, the additional N released by the A treatment may have triggered the relatively larger response in most of the rate and state variables as compared to SD. An implication of our study is that a warmer atmosphere in the future may increase dust emissions and influence the intensity and length of the already well stratified water column in the EMS and hence the impact of the aerosols as a significant external source of new nutrients.

Abstract. The effect of ocean acidification and changing water conditions on primary marine aerosol emissions is not well understood on a regional or a global scale. To investigate this effect as well as the indirect effect on aerosol that changing biogeochemical parameters can have, ~52 m3 pelagic mesocosms were deployed for several weeks in the Mediterranean Sea during both winter pre-bloom and summer oligotrophic conditions and were subjected to various levels of CO2 to simulate the conditions foreseen in this region for the coming decades. After seawater sampling, primary bubble-bursting aerosol experiments were performed using a plunging water jet system to test both chemical and physical aerosol parameters. Comparing results obtained during pre-bloom and oligotrophic conditions, we find the same four log-normal modal diameters (18.5, 37.5, 91.5, 260 nm) describing the aerosol size distribution during both campaigns, yet pre-bloom conditions significantly increased the number fraction of the second (Aitken) mode, with an amplitude correlated to virus-like particles, heterotrophic prokaryotes, TEPs, chlorophyll a and other pigments. Organic fractions determined from κ closure calculations for Dp ~50 nm were much larger during the pre-bloom period (64%) than during the oligotrophic period (38%), and the organic fraction increased as the particle size decreased. Combining data from both campaigns together, strong positive correlations were found between the organic fraction of the aerosol and chlorophyll a concentrations, heterotrophic and autotrophic bacteria abundance, and dissolved organic carbon (DOC) concentrations. As a consequence of the changes in the organic fraction and the size distributions between pre-bloom and oligotrophic periods, we find that the ratio of cloud condensation nuclei (CCN) to condensation nuclei (CN) slightly decreased during the pre-bloom period. The enrichment of the seawater samples with microlayer samples did not have any effect on the size distribution, organic content or the CCN activity of the generated primary aerosol. pCO2 perturbations had little effect on the physical or chemical parameters of the aerosol emissions, with larger effects observed due to the differences between a pre-bloom and oligotrophic environment.

composition than did that of SD. Our findings show that episodic atmospheric deposition events might affect the microbial community with regards to their abundance, activity and composition over a short period of time, and thus regulate the function of the microbial community and carbon cycling in oligotrophic waters.

The effect of episodicity of Saharan dust deposition on the pelagic microbial food web was studied in the oligotrophic Eastern Mediterranean by means of a mesocosm experiment in May 2014. Two different treatments in triplicates (addition of natural Saharan dust in a single-strong pulse or in three smaller consecutive doses of the same total quantity), and three unamended controls were employed; chemical and biological parameters were measured during a 10-day experiment. Temporal changes in primary (PP) and bacterial (BP) production, chlorophyll a (Chl a) concentration and heterotrophic bacteria, Synechococcus and mesozooplankton abundance were studied. The results suggested that the auto-and hetero-trophic components of the food web (at least the prokaryotes) were enhanced by the dust addition (and by the nitrogen and phosphorus added through dust). Furthermore, a 1-day delay was observed for PP, BP, and Chl a increases when dust was added in three daily doses; however, the maximal values attained were similar in the two treatments. Although, the effect was evident in the first osmotrophic level (phytoplankton and bacteria), it was lost further up the food web, masked under the impact of grazing exerted by predators such as heterotrophic flagellates, ciliates and dinoflagellates. This was partly proved by two dilution experiments. This study demonstrates the important role of atmospheric deposition and protist grazing when evaluating the effect on oligotrophic systems characterized by increased numbers of trophic levels.