Oxic lake surface waters are frequently oversaturated with methane (CH4). The contribution to the global CH4 cycle is significant, thus leading to an increasing number of studies and stimulating debates. Here we show, using a mass balance, on a temperate, mesotrophic lake, that ~90% of CH4 emissions to the atmosphere is due to CH4 produced within the oxic surface mixed layer (SML) during the stratified period, while the often observed CH4 maximum at the thermocline represents only a physically driven accumulation. Negligible surface CH4 oxidation suggests that the produced 110 ± 60 nmol CH4 L−1 d−1 efficiently escapes to the atmosphere. Stable carbon isotope ratios indicate that CH4 in the SML is distinct from sedimentary CH4 production, suggesting alternative pathways and precursors. Our approach reveals CH4 production in the epilimnion that is currently overlooked, and that research on possible mechanisms behind the methane paradox should additionally focus on the lake surface layer.

[1] A steady state bubble-plume model is evaluated using full-scale temperature, salinity, and dissolved oxygen data collected in a Swiss lake. The data revealed a plume-generated near-field environment that differed significantly from the ambient far-field water column properties. A near-field torus of reduced stratification developed around the plume, the extent of which is on the same lateral scale as the horizontal dislocations generated by persistent first-mode seiching. The plume fallback water was found to penetrate much deeper than expected, thereby maintaining reduced vertical gradients in the near-field torus. The plume entrains a portion of the fallback water leading to short-circuiting, which generates a complex plume-lake interaction and reduces far-field downwelling relative to the upward plume flow. As the integral plume model incorporates the entrainment hypothesis, it is highly sensitive to the near-field environmental conditions. After identifying appropriate near-field boundary conditions the plume model predictions agree well with the field observations.

[1] A steady state bubble-plume model is evaluated using full-scale temperature, salinity, and dissolved oxygen data collected in a Swiss lake. The data revealed a plume-generated near-field environment that differed significantly from the ambient far-field water column properties. A near-field torus of reduced stratification developed around the plume, the extent of which is on the same lateral scale as the horizontal dislocations generated by persistent first-mode seiching. The plume fallback water was found to penetrate much deeper than expected, thereby maintaining reduced vertical gradients in the near-field torus. The plume entrains a portion of the fallback water leading to short-circuiting, which generates a complex plume-lake interaction and reduces far-field downwelling relative to the upward plume flow. As the integral plume model incorporates the entrainment hypothesis, it is highly sensitive to the near-field environmental conditions. After identifying appropriate near-field boundary conditions the plume model predictions agree well with the field observations.

Concentration of dissolved reactive phosphorus ([DRP]) in rivers changes periodically (daily, weekly, seasonally) and is dependent on the weather and discharge Q. Accordingly, accurate estimation of the annual DRP load requires intensive sampling if not even continuous monitoring, which is laborious and expensive. We present a new, elaborated low cost technique based on passive samplers (P-traps), describing their design and chemical analysis. P-traps use iron(oxy)hydroxide as a sorbent, are inexpensive, easy to handle, and can be exposed for several months. We compare average DRP concentrations obtained from spot samples and P-traps and discuss the applicability and accuracy of the suggested method to measure annual P loads of rivers characterized by highly variable DRP concentrations.

[1] Eutrophic Lake Hallwil (Switzerland) is equipped with an artificial aeration system to prevent anoxic conditions developing in the deep water during stratification in summer. The aeration system consists of diffusers releasing oxygen-rich gas containing noble gases into the deep water at the bottom of the lake. The deep water is strongly supersaturated with He, Ne, and Ar, while Kr and Xe are present at concentrations corresponding to their respective atmospheric equilibria. The observed noble gas excesses are related to the operation of the aeration system and to the composition of the injected aeration gas. We show how noble gas data were used successfully to estimate the fraction of the injected aeration gas that effectively remains dissolved in the water body. In particular, as the physical properties of Ar (e.g., atomic mass, solubility, diffusion coefficient) are similar to those of oxygen, the measured noble gas concentrations allow the efficiency of the aeration system to be quantified.Citation: Holzner, C. P., Y. Tomonaga, A. Stöckli, N. Denecke, and R. Kipfer (2012), Using noble gases to analyze the efficiency of artificial aeration in Lake Hallwil, Switzerland, Water Resour. Res., 48, W09531,

Decline in total phosphorus (TP) during lake reoligotrophication does not apparently immediately influence carbon assimilation or deep-water oxygen levels. Traditional monitoring and interpretation do not typically consider the amount of organic carbon exported from the productive zone into the hypolimnion as a measure of net ecosystem production. This research investigated the carbon-to-phosphorus ratios of suspended particles in the epilimnion, (C:P) epi , as indicators of changing productivity. We report sestonic C:P ratios, phytoplankton biomass, and hypolimnetic oxygen depletion rates in Lake Hallwil, a lake whose recovery from eutrophic conditions has been documented in 35 years of historic water-monitoring data. This study also interpreted long-term (C:P) epi ratios from reoligotrophication occurring in four other lakes. Lake Hallwil exhibited three distinct phases. (i) The (C:P) epi ratio remained low when TP concentrations did not limit production. (ii) (C:P) epi increased steadily when phytoplankton began optimizing the declining P and biomass remained stable. (iii) Below a critical TP threshold of ∼15 to ∼20 mg P m −3 , (C:P) epi remained high and the biomass eventually declined. This analysis showed that the (C:P) epi ratio indicates the reduction of productivity prior to classic indicators such as deep-water oxygen depletion.

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