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 m3 each), were compared to control-unamended runs. Leaching of approximately 2.1-2.8 and 2.2-3.7 nmol PO4 and 20-26 and 53-55 nmol NOx was measured per each milligram of SD and A, respectively, representing an addition of approximately 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 (N2-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 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.
Highlights
The bioavailability of nutrients and trace metals from aerosols is related to both, the original aerosol chemical and mineralogical composition and the interactions and chemical transformations during atmospheric transport (Baker et al, 2006; Baker and Jickells, 2006; Mackey et al, 2015)
The characteristics of the ambient surface (10 m) water collected during the 8–9 of May north of the Island of Crete and the initial conditions of the experiment (1 day prior to the additions) were typical oligotrophic, representing the Eastern Mediterranean Sea (EMS) offshore waters (Kress et al, 2014; Pitta et al, 2016) as well as other low-nutrient low-chlorophyll (LNLC) systems
We assess the impact of pure Saharan dust vs. mixed aerosols on the surface seawater autotrophic and heterotrophic microbial populations, mimicking the potential effects of an intense Saharan dust storm and a relatively intense mixed aerosol deposition
Summary
In low-nutrient low-chlorophyll (LNLC) marine environments, nutrient and trace metal inputs via atmospheric aerosols are considered important sources of macro and micro nutrients (Duce et al, 1991, 2008; Jickells et al, 2005; Kanakidou et al, 2012), fueling microbial production and influencing the bacterioplankton community structure (Moore et al, 2013; Guieu et al, 2014a; Chien et al, 2016; Rahav et al, 2016a). The Eastern Mediterranean Sea (EMS), located in the so-called dustbelt (Astitha et al, 2012), is considered extremely oligotrophic (reviewed in Siokou-Frangou et al, 2010) and is strongly influenced by natural desert sources that contain some of the highest atmospheric dust (mineral aerosol) concentrations near the Earth’s surface (Klingmüller et al, 2016). It is suggested that the increasing temperatures together with the decreasing relative humidity of the last decade, have promoted soil drying, leading to increased dust emissions in the EMS, a process that is expected to continue in the future due to climate change (Klingmüller et al, 2016) and supply more macro and micro nutrients into its surface oligotrophic water. Recent studies have shown that the exposure to acid processes in the atmosphere can change the phosphorus (P) bioavailability of dust/aerosols (Nenes et al, 2011; Bougiatioti et al, 2016)
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