The Potential Impact of Saharan Dust and Polluted Aerosols on Microbial Populations in the East Mediterranean Sea, an Overview of a Mesocosm Experimental Approach

Abstract

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.

Bacterial Growth and Mortality after Deposition of Saharan Dust and Mixed Aerosols in the Eastern Mediterranean Sea: A Mesocosm Experiment

“…Thanks are also due to George Piperakis for his technical assistance throughout the experiment, Nafsika Papageorgiou for measuring chlorophyll a concentration, Manolis Tsapakis for measuring dissolved nutrient concentration, Christos Panagiotopoulos and Kalliopi Violaki for measuring dissolved organic carbon concentration, and Elisabet Laia Sá Lago for her valuable recommendations regarding the viral reduction approach. Nutrient leaching experiments were conducted in the Israel Oceanographic and Limnological Research (Haifa) and at the University of Leeds, and results are published in Herut et al (2016). Finally, three reviewers are gratefully acknowledged for their valuable comments that largely improved the quality of the manuscript.…”

Response of the Calanoid Copepod Clausocalanus furcatus, to Atmospheric Deposition Events: Outcomes from a Mesocosm Study

“…Our data showed that, although increased food availability was not true, the feeding performance of the dominant copepod C. furcatus did have a response to the atmospheric deposition, through the changes mediated in the microbial food web (Tsagaraki et al, this issue;Herut et al, 2016). This is of great importance for the coupling between lower and higher trophic levels, through a more efficient transfer of food toward top consumers.…”

“…A more detailed description of the mesocosm set up and experimental design is provided by Tsagaraki et al (this issue) and Herut et al (2016). For the copepod feeding and production experiments, we used all three treatments (C, SD, and A).…”

Planktonic Lipidome Responses to Aeolian Dust Input in Low-Biomass Oligotrophic Marine Mesocosms

“…To further investigate this phenomenon, the MESOAQUA experiment (May 2012; Heraklion, Crete) simulated intense dry atmospheric deposition in the ultraoligotrophic EMS by spiking surface seawater mesocosms (Cretan Sea) with two different natural aerosols collected in the Levantine Basin: Saharan dust (SD) or mixed aerosols (A; Herut et al, 2016). Biogeochemical responses to the addition of SD or A, including plankton community composition and production, pigment concentration, and cycling of macro-nutrients and trace elements, were compared in triplicate to unamended control mesocosms over a period of 8 days (Guo et al, 2016;Herut et al, 2016;Rahav et al, 2016;Tsiola et al, 2016;Tsagaraki et al, under revision, this SI).…”

“…Biogeochemical responses to the addition of SD or A, including plankton community composition and production, pigment concentration, and cycling of macro-nutrients and trace elements, were compared in triplicate to unamended control mesocosms over a period of 8 days (Guo et al, 2016;Herut et al, 2016;Rahav et al, 2016;Tsiola et al, 2016;Tsagaraki et al, under revision, this SI). In addition to these bulk biogeochemical measures of food web dynamics, the diversity of algal and bacterial cell membrane lipids provides a moderate level of taxonomic and physiological information (e.g., Sato, 1992;Guschina and Harwood, 2006;Sohlenkamp and Geiger, 2015) that can be applied to assess organic matter reactivity (Harvey and Macko, 1997;Wakeham et al, 1997), plankton communities in the ocean (e.g., Van Mooy and Fredricks, 2010), and carbon and nutrient fluxes in marine food webs (Dalsgaard et al, 2003;Sebastián et al, 2016).…”