Using isotopes to understand landscape‐scale connectivity in a groundwater‐dominated, lowland catchment under drought conditions

ACKNOWLEDGEMENTS The authors are grateful to all colleagues involved in the sample collection and infrastructure installation in the DMC (in particular H. Dämpfling, J. Freymüller, H. Wang, S. Jordan, A. Douinot, A. Wieland, N. Weiß, L. Kuhlemann, C. Marx, L. Lachmann and W. Lehmann). We thank D. Dubbert for support with the extensive isotope analysis, as well as Department 6 of the IGB (in particular T. Rossoll) for help with the sampling, measurement equipment and insights to the long‐term catchment infrastructure and background. We are thankful for trustful collaboration with B. Bösel and technical support by the WLV furthermore access to their well (Wasser und Landschaftspflegeverband Untere Spree)Peer reviewedPublisher PD

Unraveling the Importance of Polyphenols for Microbial Carbon Mineralization in Rewetted Riparian Peatlands

There have been widespread attempts to rewet peatlands in Europe and elsewhere in the world to restore their unique biodiversity as well as their important function as nutrient and carbon sinks. However, changes in hydrological regime and therefore oxygen availability likely alter the abundance of enzyme-inhibiting polyphenolic compounds, which have been suggested as a “latch” preventing large amounts of carbon from being released into the atmosphere by microbial mineralization. In recent years, a variety of factors have been identified that appear to weaken that latch including not only oxygen, but also pH. In minerotrophic fens, it is unknown if long-term peat mineralization during decades of drainage and intense agricultural use causes an enrichment or a decline of enzyme-inhibiting polyphenols. To address this, we collected peat samples and fresh roots of dominating plants (i.e., the peat parent material) from the upper 20 cm peat layer in 5 rewetted and 6 natural fens and quantified total phenolic content as well as hydrolysable and condensed tannins. Polyphenols from less decomposed peat and living roots served partly as an internal standard for polyphenol analysis and to run enzyme inhibition tests. As hypothesized, we found the polyphenol content in highly decomposed peat to be eight times lower than in less decomposed peat, while condensed tannin content was 50 times lower in highly degraded peat. In addition, plant tissue polyphenol contents differed strongly between peat-forming plant species, with the highest amount found in roots of Carex appropinquata at 450 mg g−1 dry mass, and lowest in Sphagnum spp. at 39 mg g−1 dry mass: a 10-fold difference. Despite large and clear differences in peat and porewater chemistry between natural and rewetted sites, enzyme activities determined with Fluorescein diacetate (FDA) hydrolysis and peat degradation were not significantly correlated, indicating no simple linear relationship between polyphenol content and microbial activity. Still, samples with low contents of polyphenols and condensed tannins showed the highest microbial activities as measured with FDA

Extensive nitrogen loss from permeable sediments off North-West Africa

The upwelling area off North-West Africa is characterized by high export production, high nitrate and low oxygen concentration in bottom waters. The underlying sediment consists of sands that cover most of the continental shelf. Due to their permeability sands allow for fast advective pore water transport and can exhibit high rates of nitrogen (N) loss via denitrification as reported for anthropogenically eutrophied regions. However, N loss from sands underlying naturally eutrophied waters is not well studied, and in particular, N loss from the North-West African shelf is poorly constrained. During two research cruises in April/May 2010/2011, sediment was sampled along the North-West African shelf and volumetric denitrification rates were measured in sediment layers down to 8 cm depth using slurry incubations with 15N-labeled nitrate. Areal N loss was calculated by integrating volumetric rates down to the nitrate penetration depth derived from pore water profiles. Areal N loss was neither correlated with water depth nor with bottom water concentrations of nitrate and oxygen but was strongly dependent on sediment grain size and permeability. The derived empirical relation between benthic N loss and grains size suggests that pore water advection is an important regulating parameter for benthic denitrification in sands and further allowed extrapolating rates to an area of 53,000 km2 using detailed sediment maps. Denitrification from this region amounts to 995 kt yr-1 (average 3.6 mmol m-2 d-1) which is 4 times higher than previous estimates based on diffusive pore water transport. Sandy sediments cover 50-60% of the continental shelf and thus may contribute significantly to the global benthic N loss

Diurnal Changes in Hypoxia Shape Predator-Prey Interaction in a Bird-Fish System

Animals often face changing environments, and behavioral flexibility allows them to rapidly and adaptively respond to abiotic factors that vary more or less regularly. However, abiotic factors that affect prey species do not necessarily affect their predators. Still, the prey’s response might affect the predator indirectly, yet evidence from the wild for such a classical bottom-up effect of abiotic factors shaping several trophic levels remains sparse. In many aquatic environments, daily changes in oxygen concentrations occur frequently. When oxygen levels drop to hypoxic levels, many fishes respond with aquatic surface respiration (ASR), during which they obtain oxygen by skimming the upper, oxygenated surface layer. By increasing time at the surface, fish become more vulnerable to fish-eating birds. We explored these cascading effects in a sulfidic spring system that harbors the endemic sulphur molly (Poecilia sulphuraria) as prey species and several fish-eating bird species. Sulfide-rich springs pose harsh conditions as hydrogen sulfide (H2S) is lethal to most metazoans and reduces dissolved oxygen (DO). Field sampling during three daytimes indicated that water temperatures rose from morning to (after)noon, resulting in the already low DO levels to decrease further, while H2S levels showed no diurnal changes. The drop in DO levels was associated with a decrease in time spent diving in sulphur mollies, which corresponded with an increase in ASR. Interestingly, the laboratory-estimated threshold at which the majority of sulphur mollies initiate ASR (ASR50: <1.7 mg/L DO) was independent of temperature and this value was exceeded daily when hypoxic stress became more severe toward noon. As fish performed ASR, large aggregations built up at the water surface over the course of the day. As a possible consequence of fish spending more time at the surface, we found high activity levels of fish-eating birds at noon and in the afternoon. Our study reveals that daily fluctuations in water’s oxygen levels have the potential to alter predator-prey interactions profoundly and thus highlights the joined actions of abiotic and biotic factors shaping the evolution of a prey species.Peer Reviewe

Phylogenetic and Functional Diversity of Saprolegniales and Fungi Isolated from Temperate Lakes in Northeast Germany

Funding: This research was funded by the Iranian Ministry of Science, Research, and Technology and DAAD (German Academic Exchange Service). The APC was funded by the Leibniz Association. Acknowledgments: The publication of this article was funded by the Open Access Fund of the Leibniz Association.Peer reviewedPublisher PD

Experimental investigation on the controls of clumped isotopologue and hydrogen isotope ratios in microbial methane

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 237 (2018): 339-356, doi:10.1016/j.gca.2018.06.029.The abundance of methane isotopologues with two rare isotopes (e.g., 13CH3D) has been proposed as a tool to estimate the temperature at which methane is formed or thermally equilibrated. It has been shown, however, that microbial methane from surface environments and from laboratory cultures is characterized by low 13CH3D abundance, corresponding to anomalously high apparent 13CH3D equilibrium temperatures. We carried out a series of batch culture experiments to investigate the origin of the non-equilibrium signals in microbial methane by exploring a range of metabolic pathways, growth temperatures, and hydrogen isotope compositions of the media. We found that thermophilic methanogens (Methanocaldococcus jannaschii, Methanothermococcus thermolithotrophicus, and Methanocaldococcus bathoardescens) grown on H2+CO2 at temperatures between 60 and 80°C produced methane with Δ13CH3D values (defined as the deviation from stochastic abundance) of 0.5 to 2.5‰, corresponding to apparent 13CH3D equilibrium temperatures of 200 to 600°C. Mesophilic methanogens (Methanosarcina barkeri and Methanosarcina mazei) grown on H2+CO2, acetate, or methanol produced methane with consistently low Δ13CH3D values, down to -5.2‰. Closed system effects can explain part of the non-equilibrium signals for methane from thermophilic methanogens. Experiments with M. barkeri using D-spiked water or D-labeled acetate (CD3COO-) indicate that 1.6 to 1.9 out of four H atoms in methane originate from water, but Δ13CH3D values of product methane only weakly correlate with the D/H ratio of medium water. Our experimental results demonstrate that low Δ13CH3D values are not specific to the metabolic pathways of methanogenesis, suggesting that they could be produced during enzymatic reactions common in the three methanogenic pathways, such as the reduction of methyl-coenzyme M. Nonetheless C-H bonds inherited from precursor methyl groups may also carry part of non-equilibrium signals.Grants from the National Science Foundation (EAR-1250394 to S.O.), N. Braunsdorf and D. Smit of Shell PTI/EG (to S.O.), the Deep Carbon Observatory (to S.O., M.K., K.-U.H., D.S.G.), the Gottfried Wilhelm Leibniz Program of the Deutsche Forschungsgemeinschaft (HI 616-14-1 to K.- U.H.), and the Heisenberg Program (KO3651-3-1 to M.K.) of the Deutsche Forschungsgemeinschaft supported this study. D.S.G. was also supported by a National Science Foundation Graduate Research Fellowship, the Neil and Anna Rasmussen Foundation Fund, the Grayce B. Kerr Fellowship, and a Shell-MIT Energy Initiative Graduate Fellowship. D.T.W. was supported by a National Defense Science and Engineering Graduate Fellowship. L.C.S. was supported by a NASA Earth and Space Science Fellowship (grant NNX11AP78H)

Near-surface Heating of Young Rift Sediment Causes Mass Production and Discharge of Reactive Dissolved Organic Matter

Ocean margin sediments have been considered as important sources of dissolved organic carbon (DOC) to the deep ocean, yet the contribution from advective settings has just started to be acknowledged. Here we present evidence showing that near-surface heating of sediment in the Guaymas Basin, a young extensional depression, causes mass production and discharge of reactive dissolved organic matter (DOM). In the sediment heated up to ~100 °C, we found unexpectedly low DOC concentrations in the pore waters, reflecting the combined effect of thermal desorption and advective fluid flow. Heating experiments suggested DOC production to be a rapid, abiotic process with the DOC concentration increasing exponentially with temperature. The high proportions of total hydrolyzable amino acids and presence of chemical species affiliated with activated hydrocarbons, carbohydrates and peptides indicate high reactivity of the DOM. Model simulation suggests that at the local scale, near-surface heating of sediment creates short and massive DOC discharge events that elevate the bottom-water DOC concentration. Because of the heterogeneous distribution of high heat flow areas, the expulsion of reactive DOM is spotty at any given time. We conclude that hydrothermal heating of young rift sediments alter deep-ocean budgets of bioavailable DOM, creating organic-rich habitats for benthic life

Multiple document comprehension of university students and its training

Akademisches Lesen erfordert von Studierenden häufig, mehrere Dokumente zu einemThema zu verstehen und miteinander in Beziehung zu setzen. Viele Studierende beherrschen ein solches Lesen jedoch zumindest zu Beginn ihres Studiums nicht in ausreichendem Maße. Klassische Lesekompetenz kann sie bei der Bewältigung anstehender Anforderungen unterstützen, reicht aber oft allein nicht aus. Im vorliegenden Beitrag werden die Fähigkeit des Verstehens multipler Dokumente sowie Befunde zum Zusammenhang mit anderen Variablen dargestellt und Fördermöglichkeiten skizziert. Dabei wird insbesondere auf einen Ansatz eingegangen, in dem Studierenden eine Möglichkeit zur eigenständigen Leistungsüberprüfung (Self-Assessment) mit anschließendem Feedback und darauf abgestimmtem Fördermaterial angeboten wird.Academic reading often requires students to understand and relate multiple documents on a topic. However, many students have not mastered such academic reading to a sufficient degree, at least at the beginning of their studies. Classical reading skills can support them in coping with the demands at hand but are often not sufficient on their own. In this article, the ability to comprehend multiple documents as well as findings on its relationship to other variables are presented. Support options are outlined. In particular, an approach is discussed in which students are offered a self-assessment program with subsequent feedback and support material tailored to their needs