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.

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Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.

Knowing the magnitude and timing of pelagic primary production is important for ecosystem and carbon sequestration studies, in addition to providing basic understanding of phytoplankton functioning. In this study we use data from an ecosystem cruise to Kong Håkon VII Hav, in the Atlantic sector of the Southern Ocean, in March 2019 and more than two decades of satellite-derived ocean color to study phytoplankton bloom phenology. During the cruise we observed phytoplankton blooms in different bloom phases. By correlating bloom phenology indices (i.e., bloom initiation and end) based on satellite remote sensing to the timing of changes in environmental conditions (i.e., sea ice, light, and mixed layer depth) we studied the environmental factors that seemingly drive phytoplankton blooms in the area. Our results show that blooms mainly take place in January and February, consistent with previous studies that include the area. Sea ice retreat controls the bloom initiation in particular along the coast and the western part of the study area, whereas bloom end is not primarily connected to sea ice advance. Light availability in general is not appearing to control the bloom termination, neither is nutrient availability based on the autumn cruise where we observed non-depleted macronutrient reservoirs in the surface. Instead, we surmise that zooplankton grazing plays a potentially large role to end the bloom, and thus controls its duration. The spatial correlation of the highest bloom magnitude with marked topographic features indicate that the interaction of ocean currents with sea floor topography enhances primary productivity in this area, probably by natural fertilization. Based on the bloom timing and magnitude patterns, we identified five different bloom regimes in the area. A more detailed understanding of the region will help to highlight areas with the highest relevance for the carbon cycle, the marine ecosystem and spatial management. With this gained understanding of bloom phenology, it will also be possible to study potential shifts in bloom timing and associated trophic mismatch caused by environmental changes.

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The effects of fish farming on chemical and biological variables of the water column were investigated in three coastal locations along the Mediterranean (Spain, Italy and Greece). Analyses of nutrients, chlorophyll a, particulate organic carbon and nitrogen (POC and PON), heterotrophic bacteria and cyanobacteria (Synechococcus) were carried out on integrated samples (0-30 m depth) taken at various distances from fish farms and the respective reference sites. At one of these sites, water samples were collected by means of Niskin bottles at 09:00, 13:00 and 16:00 h and at three different depths (0 m, 10 m and bottom). Integrated sampling showed no significant changes with distance in any of the variables measured, whereas all variables except PON showed significant changes between the three locations studied. However, changes were found in samples taken by Niskin bottles, with increased concentrations of NH 4 + , POC, PON and decreased cyanobacteria densities at the surface layer in the immediate vicinity of the cages. Samples taken at the cages in the afternoon showed increased PO 43) concentrations in comparison to those taken in the morning. Dilution and grazing are probably both responsible for the lack of detectable signs of eutrophication.

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Plankton biomass and composition in the pelagic zone of oceans is exposed to changes in availability of light and nutrients due to large-scale ocean circulation and water column stratification. We hypothesized that displacement of plankton from surface to deeper darker waters would not only favor heterotrophy over time, as previously suggested, but also first rapidly affect the level of mixotrophy and, consequently, overall microbial grazing in plankton food webs. To test this in an oligotrophic marine system we incubated Eastern Mediterranean water (from 10 m depth north of Crete in September 2010) in 2.8 m 3 mesocosms simulating two different light intensities at the sampling station, surface waters (ca. 10 m; mesocosms L1) and deeper layers (ca. 50-60 m; mesocosms L4). The biomass and abundance of the main planktonic groups were monitored either daily or every second day, depending on the group. Microzooplankton grazing rates and the contribution of mixotrophic feeding were estimated by a combination of dilution experiments and incubations with live fluorescently labeled algae (LFLA). Although no nutrients were added to the mesocosms the chlorophyll a increased during the first 2 days of the experiment in both treatments. This increase resulted from phytoplankton growth in the light L1-mesocosm (autotrophic biomass was ca. doubled in L1 compared to L4), but was mostly due to photoadaptation of the algae in the L4-mesocosm, as indicated by lower carbon to chlorophyll a ratios. By the end of the experiment, the total biomass of protozoan and metazoan grazers in L1 was ca. twofold higher than in L4. The microzooplankton responded within the first 24 h, showing different grazing activity in L1 than in L4. Microzooplankton grazing rates on total Chl a were similar in both treatments; however, phytoplankton instantaneous growth rates were higher in the more illuminated mesocosm. This resulted in a closer coupling between both rates