Variable sediment oxygen uptake in response to dynamic forcing

Seiche-induced turbulence and the vertical distribution of dissolved oxygen above and within the sediment were analyzed to evaluate the sediment oxygen uptake rate (JO2), diffusive boundary layer thickness (δDBL), and sediment oxic zone depth (zmax) in situ. High temporal-resolution microprofiles across the sediment-water interface and current velocity data within the bottom boundary layer in a medium-sized mesotrophic lake were obtained during a 12-h field study. We resolved the dynamic forcing of a full 8-h seiche cycle and evaluated JO2 from both sides of the sediment-water interface. Turbulence (characterized by the energy dissipation rate, ε), the vertical distribution of dissolved oxygen across the sediment-water interface (characterized by δDBL and zmax), JO2, and the sediment oxygen consumption rate (RO2) are all strongly correlated in our freshwater system. Seiche-induced turbulence shifted from relatively active (ε = 1.2 × 10-8 W kg-1) to inactive (ε = 7.8 × 10-12 W kg-1). In response to this dynamic forcing, δDBL increased from 1.0 mm to the point of becoming undefined, zmax decreased from 2.2 to 0.3 mm as oxygen was depleted from the sediment, and JO2 decreased from 7.0 to 1.1 mmol m-2 d-1 over a time span of hours. JO2 and oxygen consumption were found to be almost equivalent (within ~ 5% and thus close to steady state), with RO2 adjusting rapidly to changes in JO2. Our results reveal the transient nature of sediment oxygen uptake and the importance of accurately characterizing turbulence when estimating JO2

Effects of upstream hydropower operation and oligotrophication on the light regime of a turbid peri-alpine lake

Abstract.: Anthropogenic activities in catchments can alter the light regimes in downstream natural waters, affecting light attenuation and the perceived optical properties of the waters. We analyzed the effects of upstream hydropower operation and oligotrophication on light attenuation and reflectance in Lake Brienz (Switzerland). For this purpose, we reconstructed its light regime for the pre-dam condition and for periods of 4-fold increased primary productivity, based on direct observations of light and beam attenuation as well as concentrations of optically active compounds, especially observed and simulated mineral particle concentrations. Based on our assessment, light attenuation before the construction of upstream dams was double the current value during summer and nearly half in winter. This result is consistent with pre-dam measurements of Secchi depths in the early 1920s. Using a simple optical model, a significant increase in reflectance since the 1970s was estimated, assuming a 4-fold decrease of optical active organic compounds within the lake. As reflectance is perceived by human eyes as turbidity, this may explain subjective reports by local residents of increasing turbidity in recent year

Mixing in Stratified Lakes and Reservoirs

Aquatic physics in inland water is a crucial subject for studying aquatic ecosystems. Transport and mixing are of tremendous importance for the pace at which chemical and biological processes develop. Recent observations allow to distinguish mixing and transport processes in stratified lakes and reservoirs. The surface and bottom boundary layer are turbulent while the lake interior remains comparatively quiescent

Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing

To prevent the recurrence of a disastrous eruption of carbon dioxide (CO2) from Lake Nyos, a degassing plan has been set up for the lake. Since there are concerns that the degassing of the lake may reduce the stability of the density stratification, there is an urgent need for a simulation tool to predict the evolution of the lake stratification in different scenarios. This paper describes the development of a numerical model to predict the CO2 and dissolved solids concentrations, and the temperature structure as well as the stability of the water column of Lake Nyos. The model is tested with profiles of CO2 concentrations and temperature taken in the years 1986 to 1996. It reproduces well the general mixing patterns observed in the lake. However, the intensity of the mixing tends to be overestimated in the epilimnion and underestimated in the monimolimnion. The overestimation of the mixing depth in the epilimnion is caused either by the parameterization of the k-epsilon model, or by the uncertainty in the calculation of the surface heat fluxes. The simulated mixing depth is highly sensitive to the surface heat fluxes, and errors in the mixing depth propagate from one year to the following. A precise simulation of the mixolimnion deepening therefore requires high accuracy in the meteorological forcing and the parameterization of the heat fluxes. Neither the meteorological data nor the formulae for the calculation of the heat fluxes are available with the necessary precision. Consequently, it will be indispensable to consider different forcing scenarios in the safety analysis in order to obtain robust boundary conditions for safe degassing. The input of temperature and CO2 to the lake bottom can be adequately simulated for the years 1986 to 1996 with a constant sublacustrine source of 18 l s−1 with a CO2 concentration of 0.395 mol l−1 and a temperature of 26 °C. The results of this study indicate that the model needs to be calibrated with more detailed field data before using it for its final purpose: the prediction of the stability and the safety of Lake Nyos during the degassing proces

Convection in Lakes

Lakes and other confined water bodies are not exposed to tides, and their wind forcing is usually much weaker compared to ocean basins and estuaries. Hence, convective processes are often the dominant drivers for shaping mixing and stratification structures in inland waters. Due to the diverse environments of lakes—defined by local morphological, geochemical, and meteorological conditions, among others—a fascinating variety of convective processes can develop with remarkably unique signatures. Whereas the classical cooling-induced and shear-induced convections are well-known phenomena due to their dominant roles in ocean basins, other convective processes are specific to lakes and often overlooked, for example, sidearm, under-ice, and double-diffusive convection or thermobaric instability and bioconvection. Additionally, the peculiar properties of the density function at low salinities/temperatures leave distinctive traces. In this review, we present these various processes and connect observations with theories and model results

Effects of Lake–Reservoir Pumped-Storage Operations on Temperature and Water Quality

Pumped-storage (PS) hydropower plants are expected to make an important contribution to energy storage in the next decades with growing market shares of new renewable electricity. PS operations affect the water quality of the connected water bodies by exchanging water between them but also by deep water withdrawal from the upper water body. Here, we assess the importance of these two processes in the context of recommissioning a PS hydropower plant by simulating different scenarios with the numerical hydrodynamic and water quality model CE-QUAL-W2. For extended PS operations, the results show significant impacts of the water exchange between the two water bodies on the seasonal dynamics of temperatures, stratification, nutrients, and ice cover, especially in the smaller upper reservoir. Deep water withdrawal was shown to strongly decrease the strength of summer stratification in the upper reservoir, shortening its duration by ~1.5 months, consequently improving oxygen availability, and reducing the accumulation of nutrients in the hypolimnion. These findings highlight the importance of assessing the effects of different options for water withdrawal depths in the design of PS hydropower plants, as well as the relevance of defining a reference state when a PS facility is to be recommissioned

Utilisation thermique des eaux superficielles

Les eaux superficielles suisses renferment d’immenses réserves d’énergie thermique renouvelable, dont une fraction pourrait servir à chauffer et refroidir les infrastructures proches. Une telle utilisation pourrait avoir des impacts, notamment via les rejets d’eau réchauffée ou refroidie. En se basant sur de nombreuses études, cet article détaille ces impacts et propose des recommandations concrètes visant à les minimiser et à garantir une exploitation durable

The Fate of Trace Pollutants in Natural Waters – Lakes as 'Real-World Test Tubes'

Lakes play an important role as ecosystems and drinking-water supplies, but they are also ideal 'real-world test tubes' for studying the fate and behavior of trace pollutants in natural waters. The trace metals Cu, Zn, and Cd and the organic herbicide atrazine are used to illustrate the combined approach of field measurements and mathematical modeling to assess the behavior of pollutants in natural waters. In contrast to fast flowing waters (i.e., rivers), lakes act as integrators of pollutant inputs from surface waters of the respective catchment area, thus being regional indicators of human activities

THERMISCHE NUTZUNG VON OBERFLÄCHENGEWÄSSERN

Les eaux superficielles suisses renferment d’immenses réserves d’énergie thermique renouvelable, dont une fraction pourrait servir à chauffer et refroidir les infrastructures proches, remplaçant combustibles et électricité. Une telle utilisation repose sur des techniques éprouvées et de nombreux systèmes sont en fonctionnement ou planifiés en Suisse et à travers le monde. Le principe consiste à utiliser l’eau d’un lac, d’une rivière ou d’une nappe phréatique pour en extraire ou y rejeter de la chaleur, selon les besoins. Les lacs profonds et les grandes rivières sont particulièrement adaptés à cet objectif. La technique implique une modification de la température de l’eau utilisée: généralement un refroidissement en hiver et un réchauffement en été. Les rejets de l'eau utilisée peuvent potentiellement avoir des conséquences physicochimiques, mais aussi écologiques pour les organismes et écosystèmes aquatiques. Différentes études indiquent que le risque principal se situe dans la sensibilité de plusieurs espèces aux températures trop chaudes – typiquement au-dessus de 25 °C. C’est cependant essentiellement le changement climatique qui met les écosystèmes sous cette pression. Un réchauffement additionnel pourrait ainsi péjorer la situation des espèces vulnérables (p. ex. les truites) au profit d’autres (p. ex. les carpes). Par ailleurs, une altération de la température peut perturber le développement, le comportement et la reproduction des organismes, avec au final d’éventuels effets sur la composition et le fonctionnement de l’écosystème. Les refroidissements modérés générés lors de l’utilisation pour le chauffage sont souvent peu critiques. Une conception réfléchie des systèmes d’extraction de chaleur et de froid permettrait de minimiser les impacts possibles et d’exploiter ces ressources thermiques de manière durable