Saharan Dust Deposition Effects on the Microbial Food Web in the Eastern Mediterranean: A Study Based on a Mesocosm Experiment

Abstract

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.

Phytoplankton and Bacterial Response to Desert Dust Deposition in the Coastal Waters of the Southeastern Mediterranean Sea: A Four-Year In Situ Survey

“…It was assessed that during dust events, about 5 µmol PO 4 3+ and 300 µmol NO 3 − may be released per gram of deposited dust [2]. These nutrient additions can induce a fertilization effect and result in increased bacterial/phytoplankton biomass and activity, as observed in bioassay microcosm experiments using reagent-based nutrient additions [23] and/or dust/aerosol additions [13,18,61].…”

Aerosols in atmospheric chemistry and biogeochemical cycles of nutrients

“…Arneth et al 2010), especially via atmospheric deposition of nutrients carried by aerosols (e.g. Mahowald et al 2008, Aumont and Bopp 2006, Aumont et al 2008 and the carbon sequestration sink (e.g Guieu et al 2014a, 2014b, Pitta et al 2017, Christodoulaki et al 2016, Duce et al 2008. Atmospheric O 2 and biomass production, both required to sustain life on Earth, are controlled by the photosynthetic activity of terrestrial and marine ecosystems (Pepper et al 2015), which in turn is limited (Moore et al 2013) by the availability of trace elements needed for synthesis of amino acids, nucleic acids and proteins, the building blocks of life.…”

Model Simulations of a Mesocosm Experiment Investigating the Response of a Low Nutrient Low Chlorophyll (LNLC) Marine Ecosystem to Atmospheric Deposition Events

“…Atmospheric deposition of nitrogen and phosphorus represents an important source of nutrients, enhancing the marine productivity in oligotrophic areas, such as the Mediterranean. Within ADAMANT project, a mesocosm experiment was carried out (Lagaria et al, 2017;Pitta et al, 2017), adding dissolved inorganic nitrogen and phosphorus by means of atmospheric dust (single addition and repetitive addition in three successive doses) in controlled tanks with Cretan Sea water and compared with control (blank) tanks. Observations on almost all components of the pelagic ecosystem on a 10-day period allowed investigating the effect of atmospheric deposition and the pathways of the added nutrients in the marine ecosystem.…”

“…Details of water collection and transfer, as well as of mesocosm's filling can be found in Pitta et al (2017). The mesocosms were submerged in a 150 m 3 concrete tank, with running sea surface water that kept the mesocosms at 20.2 ± 0.3 • C throughout the experiment.…”

“…Water sampling from the mesocosms was made in order to determine: (a) inorganic nutrients, dissolved organic nitrogen, particulate and total organic carbon, size fractionated Chl-a, viruses, bacteria, Prochlorococcus, Synechococcus, picoeukaryotes (autotrophic and heterotrophic), bacterial production, primary production (approximately daily), (b) nanoflagellates (autotrophic and heterotrophic), dinoflagellates (autotrophic and mixotrophic), ciliates (heterotrophic and mixotrophic), diatoms, coccolithophores (approximately every other day), and (c) metazoans (at the start and end of the experiment). Details on sampling and analysis protocols of these parameters, as well as the literature carbon conversion factors used to convert raw biological data to carbon can be found in Pitta et al (2017) and Lagaria et al (2017). Most of the measured plankton variables mentioned above (except bacteria and diatoms) did not have a direct correspondence with the model's state variables (see Figure 1).…”