Elevated concentrations of dissolved organic matter (DOM) such as humic substances in raw water pose significant challenges during the processing of the commercial drinking water supplies. This is a relevant issue in Saxony, Central East Germany, and many other regions worldwide, where drinking water is produced from raw waters with noticeable presence of chromophoric DOM (CDOM), which is assumed to originate from forested watersheds in spring regions of the catchment area. For improved comprehension of DOM molecular composition, the seasonal and spatial variations of humic-like fluorescence and elemental formulas in the catchment area of the Muldenberg reservoir were recorded by excitation emission matrix fluorescence (EEMF) and ultrahigh-resolution mass spectrometry (FT-ICR-MS). The Spearman rank correlation was applied to link the EEMF intensities with exact molecular formulas and their corresponding relative mass peak abundances. Thereby, humic-like fluorescence could be allocated to the pool of oxygen-rich and relatively unsaturated components with stoichiometries similar to those of tannic acids, which are suspected to have a comparatively high disinfection byproduct formation potential associated with the chlorination of raw water. Analogous relationships were established for UV absorption at 254 nm (UV(254)) and dissolved organic carbon (DOC) and compared to the EEMF correlation.
Although the occurrence of metalimnetic oxygen minima (MOM) in lakes during summer stratification was described by early limnologists, not much is known about the processes leading to its formation. Generally, dissolved oxygen (DO) consumption and net transport contribute to the observed DO decrease, but the latter was rarely considered in former studies. To examine the importance of both processes to MOM development in Lake Arendsee, Germany, we measured DO concentration and temperature with high spatial and temporal resolution during a complete stratification period. Vertical turbulent diffusivities were estimated from temporal changes of the heat content of the lake. The MOM development was not caused by locally enhanced DO consumption rates, but rather by a combination of vertical gradients in DO concentration and consumption. A vertical DO mass balance revealed that net DO transport into the metalimnion was in the same order of magnitude as DO consumption in the MOM. We found that DO consumption was governed by microbial respiration and the vertical variations of DO depletion rates in the metalimnion could be explained by a minor contribution of temperature and a higher contribution of turbidity, implying that the downward flux of particulate organic carbon promoted the MOM development. The intensive metalimnetic respiration in lakes forming a MOM can be expected to accelerate nutrient cycling close to the photic zone and thus, may further stimulate primary production.
We compared oxygen fluxes measured simultaneously at the pelagic and benthic oxycline in a lake and analyze their relation to hydrodynamic forcing conditions. While the mean oxygen fluxes did not differ significantly among both sites, the fluxes were highly variable in time. Short energetic periods contributed disproportionately to the overall oxygen flux above both the benthic and pelagic oxycline. In the pelagic region, mean fluxes across the oxycline were limited by low diffusivities (7 3 10 28 m 2 s 21 ) and were one to two orders of magnitude smaller than fluxes above the oxycline (0.5 and 32 mmol m 22 d 21 , respectively). A one-dimensional transport model was used to estimate sources and sinks of oxygen potentially causing this imbalance. The model results indicate that 92% of dissolved oxygen transported into the oxycline is used by the respiration of organic material imported into the oxycline from the epilimnion; chemical oxygen consumption associated with the upward flux of reduced substances is negligible. Our findings indicate that under such conditions, dissolved oxygen consumption and therewith mineralization within the oxycline can be comparable with the corresponding rates occurring in the sediments of eutrophic lakes with an oxic hypolimnion.Anoxic conditions prevail in bottom waters of many stratified water bodies like lakes, reservoirs, and oceans. These conditions arise from an imbalance in the biogeochemical uptake and downward vertical transport of dissolved oxygen (DO), which can result from eutrophication (Wetzel 2001) and/or limited vertical mixing (Boehrer and Schultze 2008). While the former enhances the input of organic matter to the bottom waters and leads to an increased DO demand, the latter restricts downward fluxes and oxygen replenishment. Deep-water DO can be depleted either temporarily, during the seasonal cycle of density stratification (Turner and Erskine 2005), or permanently if meromixis inhibits seasonal mixing (Rodrigo et al. 2001). The so-called oxycline forms at the boundary between oxic surface waters and anoxic bottom waters. We define the oxycline as the layer ranging from the depth where the DO is 62.5 mmol L 21 to the depth where the DO becomes 0 mmol L 21 ; it is congruent with the hypoxic zone. Pelagic oxyclines exhibit strong vertical gradients not only in dissolved substances, but also in biodiversity and ecosystem functioning. The lack of DO results in drastic habitat alterations (Zhang et al. 2009) and affects chemical cycling (Davison 1981). Inland water bodies are likely to be more susceptible to anoxia more frequently in the future since climate warming leads to longer stratification periods and thereby promotes the additional formation and persistence of anoxic conditions (Adrian et al. 1995;Livingstone 2003;Foley et al. 2012). Therefore, a process-based understanding of vertical oxygen transport between the oxic surface water and anoxic bottom layers becomes increasingly important. From a physical point of view, the transport across the oxyclin...
Environmental context The cycling of iron plays an important role in pelagic boundary zones such as the oxic–anoxic interface where physical and chemical gradients occur. The turnover of iron in this zone depends on oxygen fluctuation and the duration of the fluctuation event. This study increases the understanding of biogeochemical iron transformation in such hotspots. Abstract In stratified iron-rich lakes, the interface between oxic and anoxic water bodies, the oxycline, is accompanied by a steep gradient of dissolved iron, the ferrocline. It is a hotspot of biogeochemical transformations, namely the cycling of iron (Fe). The rate of iron oxidation, both chemical and microbial, depends on pH, iron and oxygen concentration, and microbial activity. We investigated the ferrocline of the meromictic Lake Waldsee to find out how the ferrocline is influenced by fluctuating oxygen concentrations. We measured diurnal fluctuations of Fe2+, O2 and pH along vertical profiles during two campaigns in July and September 2011 as well as rates of iron oxidation in laboratory incubations. The oxygen content of the water column varied both between the campaigns and diurnally. We observed a diurnal intrusion of O2 into the ferrocline. The diurnal signal was visible in the iron profile in July but not in September. Iron oxidation rates determined in the laboratory demonstrate the importance of microbial iron reduction and the strong pH dependency. We related the reaction timescales for iron oxidation to the characteristic timescale of oxygen fluctuations by calculating non-dimensional numbers. This analysis showed that an oxygenation event had to last at least 10h in order to affect the depth and vertical extent of the ferrocline, which was the case in July but not in September. Our results show that the duration of events can be an important parameter regulating biogeochemical interactions in pelagic redoxclines.
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