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| Titel |
Introduction to project DUNE, a DUst experiment in a low Nutrient, low chlorophyll Ecosystem |
| VerfasserIn |
C. Guieu, F. Dulac, C. Ridame, P. Pondaven |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 11, no. 2 ; Nr. 11, no. 2 (2014-01-29), S.425-442 |
| Datensatznummer |
250117151
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| Publikation (Nr.) |
 copernicus.org/bg-11-425-2014.pdf |
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| Zusammenfassung |
The 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 conditions or no return to initial conditions when a second dust
seeding followed. By transiently increasing DIP and DIN concentrations in
P- and N-starved surface waters of the Mediterranean Sea, wet deposition of
Saharan dust can likely relieve the potential P and/or N limitation of
biological activity; this has been directly quantified in terms of
biological response. Wet deposition of dust strongly stimulated primary
production and phytoplanktonic biomass during several days. Small
phytoplankton (< 3 μm) was more stimulated after the first dust
addition, whereas the larger size class (> 3 μm) significantly
increased after the second one, indicating that larger-sized cells need
further nutrient supply in order to be able to adjust their physiology and compete
for resource acquisition and biomass increase. Among the microorganisms
responding to the atmospheric inputs, diazotrophs were stimulated by both wet
and dry atmospheric deposition, although N2 fixation was shown to be
only responsible for a few percent of the induced new production. Dust
deposition modified the bacterial community structure by selectively stimulating
and inhibiting certain members of the bacterial community. The microbial food
web dynamics were strongly impacted by dust deposition. The carbon budget
indicates that the net heterotrophic character (i.e., ratio of net primary production to
bacteria respiration < 1) of the tested waters remained (or was even
increased) after simulated wet or dry deposition despite the significant
stimulation of autotrophs after wet events. This indicates that the oligotrophic tested
waters submitted to dust deposition are a net CO2 source. Nonetheless, the
system was able to export organic material, half of it being associated with
lithogenic particles through aggregation processes between lithogenic
particles and organic matter. These observations support the "ballast" hypothesis and
suggest that this "lithogenic carbon pump" could represent a major
contribution of the global carbon export to deep waters in areas receiving
high rates of atmospheric deposition. Furthermore, a theoretical microbial
food web model showed that, all other things being equal, carbon, nitrogen
and phosphorus stoichiometric mismatch along the food chain can have a
substantial impact on the ecosystem response to nutrient inputs from dusts,
with changes in the biomass of all biological compartments by a factor of
~ 2–4, and shifts from net autotrophy to net heterotrophy. Although the model was kept
simple, it highlights the importance of stoichiometric constrains on the
dynamics of microbial food webs. |
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