Introduction to project DUNE, a DUst experiment in a low Nutrient, low chlorophyll Ecosystem

Abstract

International audienceThe main goal of project DUNE was to estimate the impact of atmospheric deposition on an oligotrophic ecosystem based on mesocosm experiments simulating strong atmospheric inputs of eolian mineral dust. Our mesocosm experiments aimed at being representative of real atmospheric deposition events onto the surface of oligotrophic marine waters and were an original attempt to consider the vertical dimension after atmospheric deposition at the sea surface. This introductory paper describes the objectives of DUNE and the implementation plan of a series of mesocosm experiments conducted in the Mediterranean Sea in 2008 and 2010 during which either wet or dry and a succession of two wet deposition fluxes of 10 g m^-2 of Saharan dust have been simulated based on the production of dust analogs from erodible soils of a source region. After the presentation of the main biogeochemical initial conditions of the site at the time of each experiment, a general overview of the papers published in this special issue is presented. From laboratory results on the solubility of trace elements in dust to biogeochemical results from the mesocosm experiments and associated modeling, these papers describe how the strong simulated dust deposition events impacted the marine biogeochemistry. Those multidisciplinary results are bringing new insights into the role of atmospheric deposition on oligotrophic ecosystems and its impact on the carbon budget. The dissolved trace metals with crustal origin - Mn, Al and Fe - showed different behaviors as a function of time after the seeding. The increase in dissolved Mn and Al concentrations was attributed to dissolution processes. The observed decrease in dissolved Fe was due to scavenging on sinking dust particles and aggregates. When a second dust seeding followed, a dissolution of Fe from the dust particles was then observed due to the excess Fe binding ligand concentrations present at that time. Calcium nitrate and sulfate were formed in the dust analog for wet deposition following evapocondensation with acids for simulating cloud processing by polluted air masses under anthropogenic influence. Using a number of particulate tracers that were followed in the water column and in the sediment traps, it was shown that the dust composition evolves after seeding by total dissolution of these salts. This provided a large source of new dissolved inorganic nitrogen (DIN) in the surface waters. In spite of this dissolution, the typical inter-elemental ratios in the particulate matter, such as Ti / Al or Ba / Al, are not affected during the dust settling, confirming their values as proxies of lithogenic fluxes or of productivity in sediment traps. DUNE experiments have clearly shown the potential for Saharan wet deposition to modify the in situ concentrations of dissolved elements of biogeochemical interest such as Fe and also P and N. Indeed, wet deposition yielded a transient increase in dissolved inorganic phosphorus (DIP) followed by a very rapid return to initial condition...

Introduction to the project VAHINE: VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific

“…This sampling strategy was chosen to minimize the effect of zooplankton catches on the plankton abundance and composition in the mesocosms. Guieu et al (2010) and Guieu et al (2014) have performed several mesocosm experiments in the Mediterranean Sea and demonstrated that the type of mesocosms used in the present study is well adapted to conduct replicated process studies on the first levels of the pelagic food web in LNLC environments. In order to evaluate the reproducibility among the three mesocosms deployed during VAHINE, we calculated the coefficient of variation (CV, %) of the main stocks and fluxes measured every day for 23 days for every sampling depth (Table 1; the methods are described in detail in the publications making up this special issue).…”

“…1) is sea-going mesocosms which are entirely transportable and which can be used under low to moderate wind and wave conditions (20-25 kn, 2.5 m wave height). They have been designed in the frame- work of the DUst experiment in a low Nutrient, low chlorophyll Ecosystem (DUNE) project (Guieu et al, 2010;Guieu et al, 2014) and consist of large transparent bags made of two 500 µm thick films of polyethylene (PE) and ethylene-vinyl acetate (EVA, 19 %), with nylon meshing in between to allow maximum resistance and light penetration (produced by HAIKONENE KY, Finland) (Fig. 2).…”

Reviews and syntheses: parameter identification in marine planktonic ecosystem modelling

“…By design these experiments often integrate valuable series of joint and parallel measurements, yielding detailed data from various scientist with different expertise (e.g. Williams and Egge, 1998;Riebesell et al, 2008;Guieu et al, 2014). Drawbacks are uncertainties in initial conditions and also the representativeness of mesocosm data to reflect the real dynamics in the ocean is subject to discussion (e.g.…”

Primary marine aerosol emissions from the Mediterranean Sea during pre-bloom and oligotrophic conditions: correlations to seawater chlorophyll <i>a</i> from a mesocosm study

“…Briefly, the mesocosms used in this study (volume of ∼ 52 m 3 ) were fully described in Guieu et al (2014). The mesocosms consisted of large bags made of two 500 µm thick films of polyethylene mixed with vinyl acetate (EVA, 19 %) with nylon meshing in between to allow maximum resistance and light penetration (Haikonene KY, Finland).…”