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Dissolved organic matter (DOM) composition in streams reflects the dynamic interplay between DOM sources, mobilization mechanisms, and biogeochemical transformations within soils and receiving water bodies. The information regarding DOM sources being mobilized during baseflow can improve our ability to predict hydrological and biogeochemical responses to environmental changes, with implications for catchment management strategies. The objective of this study was to characterize the spatial changes in DOM composition along a headwater stream in a small mountainous catchment during baseflow and to link the findings in‐stream with possible DOM sources in the catchment. DOM was monitored over 1.5 years at three sites in the Große Ohe catchment (Bavarian Forest National Park, Southeast Germany) which strongly varied in slope between upper and lower part of the catchment, using UV‐Vis absorption indicators of aromaticity (SUVA) and molecular weight (E2:E3) from high‐frequency probe measurements. Additionally, discrete samples were collected and analyzed by ultrahigh‐resolution Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). At baseflow conditions, dissolved organic carbon (DOC) concentrations, a proxy of DOM amount, ranged from 1.5 to 4.7 mg L−1, but were similar along the stream. However, DOM quality exhibited clear spatial patterns, with overall high aromatic and low molecular weight DOM in the lower part of the catchment, which has a higher proportion of hydromorphic soils. Moreover, molecular data revealed that oxygen‐rich, aromatic compounds increased in their abundance at high DOC concentrations in both the steep upper and the flatter lower part of the catchment, with also additional input of oxygen‐depleted aromatic compounds identified in the lower part. In contrast, nitrogen‐rich aliphatic compounds were negatively correlated with DOC concentration, indicating a higher contribution of deep groundwater at low DOC concentrations. Plain Language Summary: Dissolved organic matter (DOM) is a complex and heterogeneous mixture of substances present in water. Due to its complexity, the concentration of DOM is estimated based on the concentration of dissolved organic carbon. Monitoring DOM composition and concentration is important because of its impact on the aquatic ecosystem and in the treatment of drinking water from surface water reservoirs. In this study, we examined spatial changes in DOM chemical composition and dissolved organic carbon concentration along a headwater stream in the Bavarian Forest National Park (Southeast Germany), focusing on baseflow conditions. For this purpose, in‐stream sensors were used to record changes in dissolved organic carbon concentration and discrete water samples were collected along the stream for molecular analysis of DOM. The analyses revealed that at comparatively high dissolved organic carbon concentrations, the composition of DOM in‐stream reflects the composition of DOM stored in the superficial soil layers. Whereas at low concentrations, the DOM composition points to greater contribution from deep groundwater flows. Key Points: High‐frequency monitoring of UV absorbance and high‐resolution chemical analysis are highly complementary in the study of dissolved organic matter (DOM) dynamics in headwater streamsUltrahigh‐resolution mass spectrometry revealed a linkage between DOM molecular composition in‐stream and in soilAt comparatively higher dissolved organic carbon (DOC) concentration at baseflow, DOM composition reflects the contribution of more superficial layers of the soil [ABSTRACT FROM AUTHOR]

Information on the main drivers of subsurface flow generation on hillslopes of alpine headwater catchments is still missing. Therefore, the dominant factors controlling the water table response to precipitation at the hillslope scale in the alpine Bridge Creek Catchment, Northern Italy, were investigated. Two steep hillslopes of similar size, soil properties and vegetation cover but contrasting topography were instrumented with 24 piezometric wells. Sixty-three (63) rainfall-runoff events were selected over three years in the snow-free months to analyse the influence of rainfall depth, antecedent moisture conditions, hillslope topographic characteristics and soil depth on shallow water table dynamics. Piezometric response, expressed as percentage of well activation and water peak magnitude, was strongly correlated with soil moisture status, as described by an index combining antecedent soil moisture and rainfall depth. Hillslope topography was found to be a dominant control only for the convex-divergent hillslope and during wet conditions. Timing of water table response depended primarily on soil depth and topographic position, with piezometric peak response occurring later and showing a greater temporal variability at the hillslope bottom, characterized by thicker soil. The relationship between mean hillslope water table level and standard deviation for all wells reflected the timing of the water table response at the different locations along the hillslopes. The outcomes of this research contribute to a better understanding of the controls on piezometric response at the hillslope scale in steep terrain and its role on the hydrological functioning of the study catchment and of other sites with similar physiographic characteristics. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT The ecohydrological controls on soil erosion and landscape evolution are difficult to quantify and poorly understood. In many parts of the world, cyclone-induced tree throw is a major source of disturbance. Tree throw may increase sediment transport by exposing a mound of fresh soil as well as providing a pit which may act as a knickpoint triggering gully erosion. Alternatively, while tree throw provides characteristic pit-mound topography, the amount of soil disturbed or exposed in a mound is relatively small on the hillslope and catchment scale and the effects may be minimal. The April 2006 tropical cyclone Monica that impacted the coast of northern Australia with winds' speeds > 100 m s−1 uprooted approximately 50% of the trees in the study catchment. We use a landscape evolution model with repeated occurrence of the cyclone over a 1000-year simulated period to quantify the effect of pit-mound topography distributions on both sediment transport and landscape evolution by including the fallen trees into the digital elevation model both as a pit-mound and also as a pit-mound and tree trunk. The results show that the inclusion of pit-mound topography substantially reduced erosion for the first 10-15 years of its introduction and adding pit-mound-trunk topography reduced erosion rates even further. The pit-mound and pit-mound-trunk acted as sediment traps, capturing sediment from upslope and storing it in debris dams reducing hillslope connectivity. Model simulations predict average denudation rates for the catchment approximating field measured data. These findings suggest that any tree throw is unlikely to result in landscape instability. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

1. It has been proposed that phenolphthalein exerts its laxative effect via an intracellular cascade that begins with the activation of nitric oxide synthase (NOS) and ends with an inhibition of NaCl and water reabsorption from the colon. Phenolphthalein also promotes the release of potassium from cells, but it is not known how this is related to its effect on sodium and water uptake. 2. An established in vitro system was used to examine the role of nitric oxide (NO) in phenolphthalein-induced release of (86)Rb(+) from COS-7 cells. 3. Sodium nitroprusside, an NOS-independent NO source, was unable to mimic the effects of phenolphthalein and N(G)-nitro-L-arginine methyl ester, an NOS inhibitor, was unable to block the effect of phenolphthalein. 4. It is concluded that NO generation is not required for phenolphthalein-stimulated potassium release. It is proposed that the effect of phenolphthalein on cellular potassium release is mechanistically distinct from the effect on NaCl and water uptake by colonocytes.

1. Phenolphthalein, a well-known laxative, stimulates the secretion of Na+ and Cl- ions and accompanying water into the intestinal tract. Measurement of 86Rb+ efflux from several, but not all, cell types indicates that phenolphthalein also results in release of cellular K+ ions. 2. In the present study, the transport of 86Rb+ across human colonocyte cells (T84) cultured on trans-well inserts was examined. The T84 cells were cultured until they developed tight junctions and a high trans-epithelial resistance. 3. Results show that phenolphthalein applied to the apical, but not the basolateral, surface of cells causes the release of 86Rb+ from the apical surface. Basolateral treatment of cells with phenolphthalein had no effect on the release of 86Rb+. 4. Simultaneously with the increased 86Rb+ efflux, indirect evidence of enhanced Na+/K+-ATPase activity was also observed. 5. Although ouabain inhibited the increased Na+ pump activity, it did not affect apical 86Rb+ release. 6. As evidenced by near steady state 86Rb+ uptake data, the increased Na+/K+-ATPase activity was insufficient to restore intracellular concentrations of K+ in the presence of phenolphthalein. 7. 4,4(9-Fluorenylidene)diphenol, a homologue of phenolphthalein, had a similar effect on 86Rb+ transport by T84 cells. 8. These results indicate a primary stimulation of 86Rb+ efflux from the apical surface of polarized T84 cells by apically applied bis-phenolic compounds. 9. A secondary stimulation of the basolateral Na+/K+-ATPase is thought to result from intracellular Na+ increase, as documented in several other cell types exposed to bis-phenolic compounds, although not directly measured in these experiments. 10. The results also indicate that bis-phenolic compounds interact specifically with some apical but not basolateral membrane structures in regulating 86Rb+ efflux from polarized T84 cells.