Bertram Boehrer

Water column anammox and denitrification in a temperate permanently stratified lake (Lake Rassnitzer, Germany)

We studied microbial N(2) production via anammox and denitrification in the anoxic water column of a restored mining pit lake in Germany over an annual cycle. We obtained high-resolution hydrochemical profiles using a continuous pumping sampler. Lake Rassnitzer is permanently stratified at ca. 29m depth, entraining anoxic water below a saline density gradient. Mixed-layer nitrate concentrations averaged ca. 200 micromol L(-1), but decreased to zero in the anoxic bottom waters. In contrast, ammonium was <5 micromol L(-1) in the mixed layer but increased in the anoxic waters to ca. 600 micromol L(-1) near the sediments. In January and October, (15)N tracer measurements detected anammox activity (maximum 504 nmol N(2)L(-1)d(-1) in (15)NH(4)(+)-amended incubations), but no denitrification. In contrast, in May, N(2) production was dominated by denitrification (maximum 74 nmol N(2)L(-1)d(-1)). Anammox activity in May was significantly lower than in October, as characterized by anammox rates (maximum 6 vs. 16 nmol N(2)L(-1)d(-1) in incubations with (15)NO(3)(-)), as well as relative and absolute anammox bacterial cell abundances (0.56% vs. 0.98% of all bacteria, and 2.7x10(4) vs. 5.2x10(4)anammox cells mL(-1), respectively) (quantified by catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) with anammox bacteria-specific probes). Anammox bacterial diversity was investigated with anammox bacteria-specific 16S rRNA gene clone libraries. The majority of anammox bacterial sequences were related to the widespread Candidatus Scalindua sorokinii/brodae cluster. However, we also found sequences related to Candidatus S. wagneri and Candidatus Brocadia fulgida, which suggests a high anammox bacterial diversity in this lake comparable with estuarine sediments.

Physical characteristics of Acidic Mining Lake 111

Measurements of physical properties have been conducted in Mining Lake 111 (ML111), located in Lusatia, Germany over the time period 1996–2002. In the deepest area of the ML111, a monimolimnion was observed, that persisted for the years 1996–1999. It disappeared in 2000 and again formed in 2001. The definition of the main physical properties, such as the temperature compensation for electrical conductivity, in acidic mining lakes required a lake specific approach. The relation between conductivity, temperature and density was determined for the acidic ML111. The variation in dissolved substances affected these relationships such that conductivity varied with temperature even in different layers of the water column and the limitations for a lake wide correlation was evident. Variation in the conductivity of the epilimnion could be verified, and agreed with the estimates of evaporation from the lake surface during summer stratification. Calculations, following the gradient flux method, indicated vertical transport coefficients between 10–7 and 10–6m2/s throughout the hypolimnion. The heat budget indicated that heat was transferred into the lake bed or the ground during spring.

Similarities and Differences in the Annual Temperature Cycles of East German Mining Lakes

The annual temperature stratification of five mining lakes within the Eastern part of Germany was surveyed in the years 1997 and 1998. An intercomparison of the field data of daily average temperatures showed that surface temperatures of all five lakes ran in parallel throughout the year. Differences between lakes were smaller than the interannual variability of a single lake. This indicated that surface temperature could be considered a climatic magnitude only weakly depending on the particular mixing characteristics within the lakes, and thus would be suited for a boundary condition in numerical models. Under certain conditions, surface temperature data could be transferred from one lake to another, if the data set for a numerically modelled lake, e.g. for prognostication purposes, were incomplete or only very short.

ON THE RELEVANCE OF MEROMIXIS IN MINE PIT LAKES 1

Worldwide the number of mine pit lakes is growing. Due to their steep slopes, their relatively great depth and their exposure to highly mineralized inflows, a remarkable portion of these lakes tend to be meromictic. Meromixis indicates that the deepest part of the water body - the monimolimnion - is excluded from seasonal overturn and thus from contact to the atmosphere. Although this phenomenon is not common in natural lakes, it is well known. Meromixis is accompanied by some important consequences with respect to water quality: (1) strong anoxia in the monimolimnion, (2) enrichment of products of microbial decay in the monimolimnion and (3) occurrence of hydrogen sulfide and precipitation of metal sulfides in the monimolimnion. In some cases, advantage can be taken from the enrichment of substances in the monimolimnion due to the very low exchange with the rest of the lake. As a consequence, hazards may be avoided. On the contrary, if a sudden total overturn happens, e.g. induced by a heavy storm, fish kills and other catastrophic events may be the consequence.

Vertical structure of currents in western Lake Constance

Simultaneous measurements of current velocity and density profiles at the Sill of Mainau (western Lake Constance) enabled the observation of the internal response of the lake and the calculation of gradient Richardson number over the entire water column at the sampling site. Periodic changes of current direction corresponded with predicted periods of the two-layer oscillation and could be confirmed with thermocline oscillation data. In general, the current profiles confirmed a two-layer structure. However, an activating wind of speeds higher than 4 m/s produced three-layer profiles for a period of several hours at the sampling site, when the velocities due to the two-layer oscillation traversed zero. At times the implied current shear produced supercritical values of gradient Richardson number on a vertical scale of a few meters. This occurred during winds in the epilimnion and after strong (> 4 m/s) winds in the lower thermocline and the hypolimnion. In the 20 m above the lake bed, topographical conditions helped the gradient Richardson number to go supercritical sporadically. Across the thermocline, no supercritical gradient Richardson numbers could be confirmed at the measuring site. A lower boundary for vertical mixing in the measuring site due to current shear could be evaluated.

Extreme carbon dioxide concentrations in acidic pit lakes provoked by water/rock interaction.

We quantify the gas pressure and concentration of a gas-charged acidic pit lake in SW Spain. We measured total dissolved gas pressure, carbon dioxide (CO2) concentration, major ion concentration, isotopic composition of dissolved inorganic carbon (δ(13)C(DIC)), and other physicochemical parameters. CO2 is the dominant dissolved gas in this lake and results mainly from carbonate dissolution during the interaction of acidic water with wall rocks, followed by diffusive and advective transport through the water column. The δ(13)C(DIC) values suggest that the biological contribution is comparatively small. Maximum CO2 concentrations higher than 0.1 M (∼5000 mg/L) have been measured, which are only comparable to those found in volcanic crater lakes. The corresponding gas pressures of CO2 alone (pCO2 ∼3.6 bar) imply 60% saturation relative to local pressure at 50 m depth. High CO2 concentrations have been observed in other pit lakes of the region. We recommend gas-specific monitoring in acidic pit lakes and, if necessary, the design of feasible degassing strategies.

Modal response of a deep stratified lake: western Lake Constance

The orthogonality relation of the internal wave equation has been derived and has been included in normalizing the particular modes. Under the assumption that observed currents in western Lake Constance all result from basin-scale internal waves, they can be decomposed into contributions of the particular modes. In all cases of a response to a wind stress, at first the velocity profiles indicate a quite pure first-mode wave. After strong winds, a second-mode wave contributes remarkably and consistently to the velocity profile later during the response. During the zero transition of the first mode, the observed velocity profile can be attributed mainly to a second-mode wave.

Complementary ecophysiological strategies combine to facilitate survival in the hostile conditions of a deep chlorophyll maximum

In the deep, cooler layers of clear, nutrient-poor, stratified water bodies, phytoplankton often accumulate to form a thin band or “deep chlorophyll maximum” (DCM) of ecological importance. Under such conditions, these photosynthetic microorganisms may be close to their physiological compensation points and to the boundaries of their ecological tolerance. To grow and survive any resulting energy limitation, DCM species are thought to exhibit highly specialised or flexible acclimation strategies. In this study, we investigated several of the adaptable ecophysiological strategies potentially employed by one such species, Chlamydomonas acidophila: a motile, unicellular, phytoplanktonic flagellate that often dominates the DCM in stratified, acidic lakes. Physiological and behavioural responses were measured in laboratory experiments and were subsequently related to field observations. Results showed moderate light compensation points for photosynthesis and growth at 22°C, relatively low maintenance costs, a behavioural preference for low to moderate light, and a decreased compensation point for photosynthesis at 8°C. Even though this flagellated alga exhibited a physiologically mediated diel vertical migration in the field, migrating upwards slightly during the day, the ambient light reaching the DCM was below compensation points, and so calculations of daily net photosynthetic gain showed that survival by purely autotrophic means was not possible. Results suggested that strategies such as low-light acclimation, small-scale directed movements towards light, a capacity for mixotrophic growth, acclimation to low temperature, in situ exposure to low O2, high CO2 and high P concentrations, and an avoidance of predation, could combine to help overcome this energetic dilemma and explain the occurrence of the DCM. Therefore, corroborating the deceptive ecophysiological complexity of this and similar organisms, only a suite of complementary strategies can facilitate the survival of C. acidophila in this DCM.

Density Stratification and Stability

Most lakes show periods of density stratification. The stratification controls currents and vertical circulation, and hence is crucial for understanding and predicting the evolution of water quality in lakes. The availability of nutrients and oxygen, for example, impacts on organisms that can live in the lake. Water properties above the lake bed control recycling of settling material and recycling by bioturbation from buried material in the sediment. Exposure of the lake surface to seasonal changes of weather conditions, inflows and outflows as well as internal chemical transformations create gradients in the distribution of heat and dissolved substances. Both factors contribute to density. Diffusive processes, wind impact on the lake surface, as well as surface cooling and geothermal heating, in general, erode this stratification. In most lakes, mixing overcomes the stratifying processes at times and lakes show circulation periods alternating with stratification periods during an annual cycle. Some lakes, however, can stay permanently stratified. To understand the density stratification, various approaches have been developed to evaluate density at required accuracy. Most usually, temperature and conductivity measurements are used to calculate (potential) density by applying empirical regression curves. In many cases, lake-specific coefficients need to be evaluated. The resulting profiles of stability quantities can be used to estimate vertical transport in a stratified lake. Bulk quantities for the stability indicate the strength of wind forcing that is required for intense upwelling.

Convection in a long cavity with differentially heated end walls

The fluid-filled long cavity with differentially heated end walls is a common and simple model for flows both in the environment and in technical applications. A delineation of the flow regime showing confined flowing layers from those showing cavity-filling counter currents is provided. Theoretical considerations yield a description of the flowing layer thickness and an approximate value for the longitudinal temperature gradient in the core of the tank. This way, a good description of the major features in the long cavity is achieved, which is confirmed by experimental data.