Contrasted Saharan dust events in LNLC environments: impact on nutrient dynamics and primary production

International audienceThe response of the phytoplanktonic community (primary production and algal biomass) to contrasted Sa-haran dust events (wet and dry deposition) was studied in the framework of the DUNE ("a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem") project. We simu-lated realistic dust deposition events (10 g m −2) into large mesocosms (52 m 3). Three distinct dust addition experiments were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition; DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dis-solved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO − 3) (net in-creases up to +9.8 µM NO − 3 at 0.1 m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO 3 . The dry deposi-tion event was shown to be a negligible source of NO − 3 . By transiently increasing DIP and NO − 3 concentrations in N–P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phy-toplanktonic activity. Due to the higher input of NO − 3 relative to DIP, and taking into account the stimulation of the bio-logical activity, a wet deposition event resulted in a strong increase in the NO − 3 /DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggest-ing a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP strongly increased after wet dust de-position events, from initially 15 % to 60–70 % 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE ex-periments show that wet and dry dust deposition events in-duce contrasting responses of the phytoplanktonic commu-nity due to differences in the atmospheric supply of bioavail-able new nutrients. Our results from original mesocosm ex-periments demonstrate that atmospheric dust wet deposition Published by Copernicus Publications on behalf of the European Geosciences Union. 4784 C. Ridame et al.: Phytoplanktonic response to Saharan dust events greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO − 3 /DIP ratio, leading to a switch in the nu-trient limitation of the phytoplanktonic activity

Saharan input of phosphate to the oligotrophic water of the open western Mediterranea Sea

International audienceA Saharan soil, considered as a proxy for Saharan aerosols, was used to perform a series of dissolution experiments: various amounts of Saharan soil were exposed to ultra pure water and seawater for varying lengths of time. The concentration of phosphate released was proportional to the amount of dust introduced. In the case of Saharan events associated with a significant amount of rain, the main dissolution of phosphorus will occur in the air column; for Saharan events associated with only few drops of rainwater, the main dissolution will occur in the surface seawater. The Saharan dust represent a source of phosphate to the surface water and may play a role in biological activity especially during the oligotrophic period. In the western Mediterranean in oligotrophic conditions, biological production is P-limited and the atmosphere becomes the main pathway of nutrients to the surface mixed layer. At the scale of the oligotrophic season, the input of "Saharan DIP" are negligible compared to the new production integrated over the productive layer. At the event time scale, the production induced by "Saharan DIP" can represent up to 15% of the integrated new production, and up to 14% of the total primary production in the mixed surface layer. These inputs of atmospheric DIP would promote fixation of atmospheric N2 that, in return, may enhance the new production in the surface layer

Impact of Atmospheric Deposition on Marine Chemistry and Biogeochemistry

International audienceThis chapter presents the current knowledge on the impact of atmospheric deposition from natural sources, such as Saharan dust, and from anthropogenic activities, on marine chemistry and biogeochemistry of the open Mediterranean Sea. Results from process studies and observations at sea that have been conducted over the past decade are summarized along with recent findings from a numerical biogeochemical model of the ocean that accounts for atmospheric deposition

Impact of Atmospheric Deposition on Marine Chemistry and Biogeochemistry

International audienceThis chapter presents the current knowledge on the impact of atmospheric deposition from natural sources, such as Saharan dust, and from anthropogenic activities, on marine chemistry and biogeochemistry of the open Mediterranean Sea. Results from process studies and observations at sea that have been conducted over the past decade are summarized along with recent findings from a numerical biogeochemical model of the ocean that accounts for atmospheric deposition

Impact of Atmospheric Deposition on Marine Chemistry and Biogeochemistry

International audienc

Does phosphate adsorption onto Saharan dust explain the unusual N/P ratio in the Mediterranean Sea ?

A Saharan soil, considered as a proxy for Saharan aerosols, was used to perform radio-labelled phosphate adsorption experiments using 33PO43-: leached particles were exposed to poisoned western Mediterranean seawater for varying lengths of time. The measured adsorption capacity of Saharan dust for phosphate was 0.13 µmol.g-1. Considering this value and an annual Saharan dust deposition of 12.5 t.km-2.yr-1, we show that Saharan particles do not represent a significant sink for seawater phosphate in the western Mediterranean Sea. This result is in agreement with that determined from a similar approach conducted in the eastern basin. As a consequence, the unusual N/P ratio measured in the whole Mediterranean Sea (up to 29) can not be explained by the adsorption process of seawater phosphate onto Saharan dust

Strong stimulation of N₂ fixation in oligotrophic Mediterranean Sea: results from dust addition in large in situ mesocosms

International audienceThe response of N2 (dinitrogen) fixation to contrasted (wet and dry) Saharan dust deposition was studied in the framework of the DUNE project (a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem) during which realistic simulations of dust deposition (10 g m-2) into large mesocosms (52 m3) were performed. Three distinct experimental dust additions were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition, DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R: simulation of 2 successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. Here we show that wet and dry dust deposition induced a rapid (24 h or 48 h after dust additions), strong (from 2- to 5.3-fold) and long (at least 4-6 days duration) increase in N2 fixation, indicating that both wet and dry Saharan dust deposition was able to relieve efficiently the nutrient limitation(s) of N2 fixation. This means in particular that N2 fixation activity was not inhibited by the significant input of nitrate associated with the simulated wet deposition (~ 9 mmol NO3- m-2). The input of new nitrogen associated with N2 fixation was negligible relative to the atmospheric NO3- input associated with the dust. The contribution of N2 fixation to primary production was negligible (≤ 1%) before and after dust addition in all experiments, indicating that N2 fixation was a poor contributor to the nitrogen demand for primary production. Despite the stimulation of N2 fixation by dust addition, the rates remained low, and did not significantly change the contribution of N2 fixation to new production since only a maximum contribution of 10% was observed. The response of N2 fixation by diazotrophs and CO2 fixation by the whole phytoplankton community suggests that these metabolic processes were limited or co-limited by different nutrients. With this novel approach, which allows us to study processes as a function of time while atmospheric particles are sinking, we show that new atmospheric nutrients associated with Saharan dust pulses do significantly stimulate N2 fixation in the Mediterranean Sea and that N2 fixation is not a key process in the carbon cycle in such oligotrophic environments

Impact of iron limitation on primary production (dissolved and particulate) and secondary production in cultured Trichodesmium sp.

International audienceDiazotrophic cyanobacteria play an important role in biogeochemical cycles of carbon and nitrogen and, hence, in oceanic productivity in the tropical and subtropical regions of the ocean. Although many studies have examined the impact of iron (Fe) limitation on particulate primary production and dinitrogen (N2) fixation in the colonial cyanobacterium Trichodesmium, none have looked at the impact of Fe limitation on the percentage extracellular release (PER) and secondary production (SP) in Fe-limited cultures of this cyanobacterium. Here, we present the results of a series of culture experiments during which we examined the impact of 3 concentrations of dissolved iron (DFe) on total primary production (TPP = dissolved + particulate primary production, i.e. DPP + PPP), PER and on SP. Under severe Fe limitation (5 nM DFe), biomass, growth rates, TPP and N2 fixation were strongly reduced, while PER increased relative to the rates ob served at the highest Fe concentration. Moreover, reducing Fe concentration induced an increase in the percentage of photosynthetically fixed C used for algal growth, while the percentage of C used to support algal respiration decreased. Reduced Fe concentrations also induced a decrease in SP and in the SP:DPP ratio, indicating that the efficiency of transfer of fixed carbon from autotrophic to heterotrophic processes is reduced. This suggests that Fe, either directly through influencing cellular processes or indirectly through influencing organic matter structure or nitrogen availability, is controlling SP and, thus, microbial carbon utilization. These results suggest that the amount of carbon entering into the microbial loop may be reduced under Fe limitation, thus leading to an accumulation of dissolved organic carbon with potentially important impacts on microbial carbon cycling and, ultimately, on the biological carbon pum

Iron limitation and DMSP in diatoms and cyanobacteria

International audienc

C, N and P stoichiometric mismatch between resources and consumers influence the dynamics of a marine microbial food web model and its response to atmospheric N and P inputs

International audienceResults from the DUNE experiments reported in this issue have shown that nutrient input from dust deposition in large mesocosms deployed in the western Mediterranean induced a response of the microbial food web, with an increase of primary production rates (PP), bacterial respiration rates (BR), as well as autotrophic and heterotrophic biomasses. Additionally, it was found that nutrient inputs strengthened the net heterotrophy of the system, with NPP : BR ratios < 1. In this study we used a simple microbial food web model, inspired from previous modelling studies, to explore how C, N and P stoichiometric mismatch between producers and consumers along the food chain can influence the dynamics and the trophic status of the ecosystem. Attention was paid to the mechanisms involved in the balance between net autotrophy vs. net heterotrophy. Although the model was kept simple, predicted changes in biomass and PP were qualitatively consistent with observations from DUNE experiments. Additionally, the model shed light on how ecological stoichiometric mismatch between producers and consumers can control food web dynamics and drive the system toward net heterotrophy. In the model, net heterotrophy was notably driven by the parameterisation of the production and excretion of extra DOC from phytoplankton under nutrient-limited conditions. This mechanism yielded to high C : P and C : N ratios of the DOM pool, and subsequent postabsorptive respiration of C by bacteria. The model also predicted that nutrient inputs from dust strengthened the net heterotrophy of the system; a pattern also observed during two of the three DUNE experiments (P and Q). However, the model was not able to account for the low NPP : BR ratios (down to 0.1) recorded during the DUNE experiments. Possible mechanisms involved in this discrepancy were discussed