Beschreibung: Based on an unprecedented dissolved barium (D_Ba) data set collected in the Mediterranean Sea during a zonal transect between the Lebanon coast and Gibraltar (M84/3 cruise, April 2011), we decompose the D_Ba distribution to isolate the contribution of biogeochemical processes from the impact of the oceanic circulation. We have built a simple parametric water mass analysis (Parametric Optimum Multiparameter analysis) to reconstruct the contribution of the different Mediterranean water masses to the thermohaline structure. These water mass fractions have then been used to successfully reconstruct the background vertical gradient of D_Ba reflecting the balance between the large-scale oceanic circulation and the biological activity over long time scales. Superimposed on the background field, several D_Ba anomalies have been identified. Positive anomalies are associated with topographic obstacles and may be explained by the dissolution of particulate biogenic barium (P_Ba barite) of material resuspended by the local currents. The derived dissolution rates range from 0.06 to 0.21 μmol m−2 d−1. Negative anomalies are present in the mesopelagic region of the western and eastern basins (except in the easternmost Levantine basin) as well as in the abyssal western basin. This represents the first quantification of the nonconservative component of the D_Ba signal. These mesopelagic anomalies could reflect the subtraction of D_Ba during P_Ba barite formation occurring during organic carbon remineralization. The deep anomalies may potentially reflect the transport of material toward the deep sea during winter deep convection and the subsequent remineralization. The D_Ba subtraction fluxes range from −0.07 to −1.28 μmol m−2 d−1. D_Ba-derived fluxes of P_Ba barite (up to 0.21 μmol m−2 d−1) and organic carbon (13 to 29 mmol C m−2 d−1) are in good agreement with other independent measurements suggesting that D_Ba can help constrain remineralization horizons. This study highlights the importance of quantifying the impact of the large-scale oceanic circulation in order to better understand the biogeochemical cycling of elements and to build reliable geochemical proxies.

Beschreibung: Highlights: • D_Ba is investigated along a high resolution and quasi-zonal transect in the MedSea. • The D_Ba content ranges from 38 to 85 nmol kg− 1 with local deep D_Ba maxima reaching up to 172 nmol kg− 1. • The water column is largely undersaturated with respect to barite (0.2 〈 SI 〈 0.6). • The D_Ba distribution is impacted by the large-scale Mediterranean circulation and biogeochemical processes. • Local changes in the D_Ba patterns may be the key to better constrain the C dynamics in the MedSea. Abstract: The dissolved barium (D_Ba) data set for the Mediterranean Sea is here expanded with data from a large-scale transect sampled in April 2011 (M84/3 cruise) at high resolution. A total of 833 seawater samples have been analyzed for D_Ba. Over the basin the D_Ba content ranges from 38 to 85 nmol kg− 1 with local deep D_Ba maxima reaching up to 172 nmol kg− 1. Deep D_Ba maxima are associated with near bottom waters influenced by benthic processes and brine waters. The water column is largely undersaturated with respect to barite (BaSO4, the main phase of particulate biogenic barium P_Ba), with water column barite saturation state ranging between 0.2 and 0.6 over the basin. This new D_Ba dataset shows that the general zonal distribution of D_Ba is impacted by the large-scale Mediterranean circulation, as evidenced by the Levantine Intermediate Water zonal and meridional progression as well as by the eastward flow of surface Atlantic Water. However biogeochemical processes are also at play, as suggested by an elevated D_Ba content of deep waters and by local lower D_Ba contents in intermediate waters. These features could be attributed to active cycling between the particulate and dissolved Ba phases. Since P_Ba barite has been recognized in previous studies as a proxy for particulate organic carbon remineralization at intermediate depths, the significance of local changes in the water column D_Ba patterns may be the key to better constrain the Ba and carbon dynamics in the Mediterranean Sea.

Beschreibung: The export of organic carbon to the deep ocean is mediated by sinking of large particles, such as marine snow, the formation of which is enhanced in the presence of transparent exopolymer particles (TEP) . TEP form from dissolved and colloidal polysaccharides by aggregation processes. Especially when running into nutrient limitation phytoplankton organisms are a source of TEP in pelagic ecosystems as the cells release a significant amount of the assimilated carbon in the form of polysaccharides. Because CO_2 concentration influences carbon assimilation rates, we hypothesized that polysaccharide exudation and aggregation into TEP is related to CO_2 concentration under nutrient limiting conditions. We tested this hypothesis in several lab and outdoor experiments with natural populations and cultures of phytoplankton exposed to various levels of CO_2 concentrations. Our results indicate that TEP production increases with CO_2 concentration and provides an enhanced sink for carbon during phytoplankton blooms.

Beschreibung: One of the most prominent consequences of human activities is the progressive rise in atmospheric pCO_2. This will also cause changes in seawater carbonate chemistry. However, to what extent anthropogenic perturbations in marine growth conditions will affect biological processes like species composition, ecosystem functioning, carbon production and vertical carbon export in the sea, as well as possible feedback mechanisms, is still under debate. In the present study, for the first time CO_2 effects were tested on a natural marine plankton community. In a series of floating mesocosms in a Norwegian fjord a phytoplankton bloom dominated by the coccolithophore Emiliania huxleyi was induced. By covering the enclosures by gas-tight domes, functioning as greenhouses, we were able to maintain CO_2 concentrations in the overlaying atmosphere ranging from pre-industrial to projected year 2100 levels over a three-week period. Here we present an overview of the experiment and report on the development of the bloom under different pCO_2 levels.

Beschreibung: Cyanophages are important components of aquatic ecosystems, but their genetic diversity has been little investigated in freshwaters. A yearlong survey was conducted in surface waters of the two largest natural perialpine lakes in France (Lake Annecy and Lake Bourget) to investigate part of this cyanophage diversity through the analysis of both structural (e.g., g20 ) and functional (e.g., psbA ) genes. We found that these cyanophage signature genes were prevalent throughout the year but that the community compositions of g20 cyanomyoviruses were significantly different between the two lakes. In contrast, psbA -containing cyanophages seemed to be more similar between the two ecosystems. We also found that a large proportion of g20 sequences grouped with cyanomyophage isolates. psbA sequences, belonging to phages of Synechococcus spp., were characterized by distinct triplet motifs (with a novel viral triplet motif, EFE). Thus, our results show that cyanophages (i) are a diverse viral community in alpine lakes and (ii) are clearly distinct from some other freshwater and marine environments, suggesting the influence of unique biogeographic factors.

Beschreibung: We studied the direct effects of CO2 Engel et al. in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (190 parts per million by volume (ppmV) CO2 organisms in a mesocosm experiment. In nine outdoor enclosures (11 m3 present (410 ppmV CO2 and high (710 ppmV CO2 pCO2 and related changes in seawater carbonate chemistry on marine planktonic each), the partial pressure of CO2 (pCO2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO2 within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO2 This broad influence of CO2 -related effects. Normalization of production rates . on the E. huxleyi bloom suggests that changes in CO2 physiology with likely effects on the marine biogeochemistry.