Stratification, mixing and transport processes in Lake Kivu
01 Jan 2012-pp 13-29
TL;DR: In this paper, the authors summarized the knowledge on mixing and transport processes in Lake Kivu, and showed that the lake is permanently stratified, with density increasing stepwise from ∼998 kg m−3 at the surface to ∼1,002 kg m − 3 at the maximum depth of 485 m.
Abstract: This chapter summarizes the knowledge on mixing and transport processes in Lake Kivu. Seasonal mixing, which varies in intensity from year to year, influences the top ∼65 m. Below, the lake is permanently stratified, with density increasing stepwise from ∼998 kg m−3 at the surface to ∼1,002 kg m−3 at the maximum depth of 485 m. The permanently stratified deep water is divided into two distinctly different zones by a main gradient layer. This gradient is maintained by a strong inflow of relatively fresh and cool water entering at ∼250 m depth which is the most important of several subaquatic springs affecting the density stratification. The springs drive a slow upwelling of the whole water column with a depth-dependent rate of 0.15–0.9 m year−1. This upwelling is the main driver of internal nutrient recycling and upward transport of dissolved gases. Diffusive transport in the deep water is dominated by double-diffusive convection, which manifests in a spectacular staircase of more than 300 steps and mixed layers. Double diffusion allows heat to be removed from the deep zone faster than dissolved substances, supporting the stable stratification and the accumulation of nutrients and gases over hundreds of years. The stratification in the lake seems to be near steady-state conditions, except for a warming trend of ∼0.01°C year−1.
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TL;DR: In this paper, the authors compare calculated (pH and TA) and measured (equilibra- tor and headspace) water pCO2 in a large array of temperate and tropical freshwaters.
Abstract: Inland waters have been recognized as a signifi- cant source of carbon dioxide (CO2) to the atmosphere at the global scale. Fluxes of CO2 between aquatic systems and the atmosphere are calculated from the gas transfer velocity and the water-air gradient of the partial pressure of CO2 (pCO2). Currently, direct measurements of waterpCO2 remain scarce in freshwaters, and most published pCO2 data are calculated from temperature, pH and total alkalinity (TA). Here, we compare calculated (pH and TA) and measured (equilibra- tor and headspace) water pCO2 in a large array of temperate and tropical freshwaters. The 761 data points cover a wide range of values for TA (0 to 14 200 µmol L 1 ), pH (3.94 to 9.17), measured pCO2 (36 to 23 000 ppmv), and dissolved organic carbon (DOC) (29 to 3970 µmol L 1 ). Calculated pCO2 were > 10 % higher than measured pCO2 in 60 % of the samples (with a median overestimation of calculated pCO2 compared to measured pCO2 of 2560 ppmv) and were > 100 % higher in the 25 % most organic-rich and acidic samples (with a median overestimation of 9080 ppmv). We suggest these large overestimations of calculated pCO2 with respect to measured pCO2 are due to the combination of two cumulative effects: (1) a more significant contribution of or- ganic acids anions to TA in waters with low carbonate alka- linity and high DOC concentrations; (2) a lower buffering ca- pacity of the carbonate system at low pH, which increases the sensitivity of calculated pCO2 to TA in acidic and organic- rich waters. No empirical relationship could be derived from our data set in order to correct calculated pCO2 for this bias. Owing to the widespread distribution of acidic, organic-rich freshwaters, we conclude that regional and global estimates of CO2 outgassing from freshwaters based on pH and TA data only are most likely overestimated, although the magni- tude of the overestimation needs further quantitative analysis. Direct measurements of pCO2 are recommended in inland waters in general, and in particular in acidic, poorly buffered freshwaters.
255 citations
Cites background from "Stratification, mixing and transpor..."
...While the risk of a limnic eruption is minimal [25], large scale industrial extraction of CH4 from the deep layers of Lake Kivu is planned [26], [27] which could affect the ecology and biogeochemical cycling of C of the lake and change for instance the emission of greenhouse gases such as CH4 and CO2....
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06 Nov 2013
TL;DR: In this paper, the Consortium for Small-Scale Modelling model in climate mode coupled with the Freshwater Lake model (FLake) and Community Land Model (CLM) is used to dynamically downscale a simulation from the African Coordinated Regional Downscaling Experiment (CORDEX-Africa) to 7km grid spacing for the period of 1999-2008.
Abstract: AbstractAlthough the African Great Lakes are important regulators for the East African climate, their influence on atmospheric dynamics and the regional hydrological cycle remains poorly understood. This study aims to assess this impact by comparing a regional climate model simulation that resolves individual lakes and explicitly computes lake temperatures to a simulation without lakes. The Consortium for Small-Scale Modelling model in climate mode (COSMO-CLM) coupled to the Freshwater Lake model (FLake) and Community Land Model (CLM) is used to dynamically downscale a simulation from the African Coordinated Regional Downscaling Experiment (CORDEX-Africa) to 7-km grid spacing for the period of 1999–2008. Evaluation of the model reveals good performance compared to both in situ and satellite observations, especially for spatiotemporal variability of lake surface temperatures (0.68-K bias), and precipitation (−116 mm yr−1 or 8% bias). Model integrations indicate that the four major African Great Lakes almos...
115 citations
TL;DR: In this paper, for the first time, a set of one-dimensional lake models are evaluated for Lake Kivu (2.28°S; 28.98°E), East Africa.
Abstract: The African great lakes are of utmost importance for the local economy (fishing), as well as being essential to the survival of the local people. During the past decades, these lakes experienced fast changes in ecosystem structure and functioning, and their future evolution is a major concern. In this study, for the first time a set of one-dimensional lake models are evaluated for Lake Kivu (2.28°S; 28.98°E), East Africa. The unique limnology of this meromictic lake, with the importance of salinity and subsurface springs in a tropical high-altitude climate, presents a worthy challenge to the seven models involved in the Lake Model Intercomparison Project (LakeMIP). Meteorological observations from two automatic weather stations are used to drive the models, whereas a unique dataset, containing over 150 temperature profiles recorded since 2002, is used to assess the model’s performance. Simulations are performed over the freshwater layer only (60 m) and over the average lake depth (240 m), since salinity increases with depth below 60 m in Lake Kivu and some lake models do not account for the influence of salinity upon lake stratification. All models are able to reproduce the mixing seasonality in Lake Kivu, as well as the magnitude and seasonal cycle of the lake enthalpy change. Differences between the models can be ascribed to variations in the treatment of the radiative forcing and the computation of the turbulent heat fluxes. Fluctuations in wind velocity and solar radiation explain inter-annual variability of observed water column temperatures. The good agreement between the deep simulations and the observed meromictic stratification also shows that a subset of models is able to account for the salinity- and geothermal-induced effects upon deep-water stratification. Finally, based on the strengths and weaknesses discerned in this study, an informed choice of a one-dimensional lake model for a given research purpose becomes possible. Keywords: lake modelling, model intercomparison, surface–atmosphere interactions, tropical lakes, Lake Kivu (Published: 5 February 2014) Citation: Tellus A 2014, 66 , 21390, http://dx.doi.org/10.3402/tellusa.v66.21390 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here
98 citations
Cites background or result from "Stratification, mixing and transpor..."
...2), whereas below 65 m, the stabilizing salinity gradient causes a permanent stratification and therewith inhibits seasonal temperature variability (Schmid and Wüest, 2012)....
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...The excess heat is removed both by the upward motion and enhanced diffusion through the doublediffusive staircases (Schmid and Wüest, 2012)....
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...…thermal, chemical and biological lake characteristics across all subbasins except Bukavu and Kabuno bay, and attribute horizontal homogeneity in the monimolimnion to long residence times (Sarmento et al., 2006; Pasche et al., 2009; Tassi et al., 2009; Pasche et al., 2011; Schmid and Wüest, 2012)....
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...The excess heat is removed both by the upward motion and enhanced diffusion through the doublediffusive staircases (Schmid and Wüest, 2012)....
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...…homogeneous salt content in this layer, water temperatures vary according to meteorol ogical variability (Section 3.2), whereas below 65 m, the stabilizing salinity gradient causes a permanent stratification and therewith inhibits seasonal temperature variability (Schmid and Wüest, 2012)....
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TL;DR: In this paper, the ability of the one-dimensional lake model FLake to represent the mixolimnion temperatures for tropical conditions was tested for three locations in East Africa: Lake Kivu and Lake Tanganyika's northern and southern basins.
Abstract: . The ability of the one-dimensional lake model FLake to represent the mixolimnion temperatures for tropical conditions was tested for three locations in East Africa: Lake Kivu and Lake Tanganyika's northern and southern basins. Meteorological observations from surrounding automatic weather stations were corrected and used to drive FLake, whereas a comprehensive set of water temperature profiles served to evaluate the model at each site. Careful forcing data correction and model configuration made it possible to reproduce the observed mixed layer seasonality at Lake Kivu and Lake Tanganyika (northern and southern basins), with correct representation of both the mixed layer depth and water temperatures. At Lake Kivu, mixolimnion temperatures predicted by FLake were found to be sensitive both to minimal variations in the external parameters and to small changes in the meteorological driving data, in particular wind velocity. In each case, small modifications may lead to a regime switch, from the correctly represented seasonal mixed layer deepening to either completely mixed or permanently stratified conditions from ~ 10 m downwards. In contrast, model temperatures were found to be robust close to the surface, with acceptable predictions of near-surface water temperatures even when the seasonal mixing regime is not reproduced. FLake can thus be a suitable tool to parameterise tropical lake water surface temperatures within atmospheric prediction models. Finally, FLake was used to attribute the seasonal mixing cycle at Lake Kivu to variations in the near-surface meteorological conditions. It was found that the annual mixing down to 60 m during the main dry season is primarily due to enhanced lake evaporation and secondarily to the decreased incoming long wave radiation, both causing a significant heat loss from the lake surface and associated mixolimnion cooling.
90 citations
Cites background or methods from "Stratification, mixing and transpor..."
...At this depth, the upwelling of deep, saline waters (0.5 m yr −1; Schmid and Wüest, 2012) equilibrates with mixing forces....
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...At this depth, the upwelling of deep, saline waters (0.5 m yr−1; Schmid and Wüest, 2012) equilibrates with mixing forces....
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...Lake Kivu and Lake Tanganyika are both characterised by long lake water retention times (∼ 100 yr and∼ 800 yr, respectively; Schmid and Wüest, 2012; Coulter, 1991), hence the impact of riverine in- and outflow is of little importance to the circulation within these lakes....
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...Extrapolating the average LHF at Ishungu to the entire Lake Kivu surface (estimated by the control simulation and using a surface area of 2370 km 2; Schmid and Wüest, 2012) leads to a preliminary estimate of the total annual evaporative flux of 2....
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...Note that, based on calculations from Bultot (1971), Schmid and Wüest (2012) estimate a total annual evaporation of 3.0–4.0 km3 yr−1 for Lake Kivu....
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TL;DR: Modern environments resembling the redox conditions of these ancient oceans can offer insights into the past significance of photoferrotrophy and help to explain how this metabolism operated as an important source of organic carbon for the early biosphere.
Abstract: Photoferrotrophy, the process by which inorganic carbon is fixed into organic matter using light as an energy source and reduced iron [Fe(II)] as an electron donor, has been proposed as one of the oldest photoautotrophic metabolisms on Earth. Under the iron-rich (ferruginous) but sulfide poor conditions dominating the Archean ocean, this type of metabolism could have accounted for most of the primary production in the photic zone. Here we review the current knowledge of biogeochemical, microbial and phylogenetic aspects of photoferrotrophy, and evaluate the ecological significance of this process in ancient and modern environments. From the ferruginous conditions that prevailed during most of the Archean, the ancient ocean evolved toward euxinic (anoxic and sulfide rich) conditions and, finally, much after the advent of oxygenic photosynthesis, to a predominantly oxic environment. Under these new conditions photoferrotrophs lost importance as primary producers, and now photoferrotrophy remains as a vestige of a formerly relevant photosynthetic process. Apart from the geological record and other biogeochemical markers, modern environments resembling the redox conditions of these ancient oceans can offer insights into the past significance of photoferrotrophy and help to explain how this metabolism operated as an important source of organic carbon for the early biosphere. Iron-rich meromictic (permanently stratified) lakes can be considered as modern analogs of the ancient Archean ocean, as they present anoxic ferruginous water columns where light can still be available at the chemocline, thus offering suitable niches for photoferrotrophs. A few bacterial strains of purple bacteria as well as of green sulfur bacteria have been shown to possess photoferrotrophic capacities, and hence, could thrive in these modern Archean ocean analogs. Studies addressing the occurrence and the biogeochemical significance of photoferrotrophy in ferruginous environments have been conducted so far in lakes Matano, Pavin, La Cruz, and the Kabuno Bay of Lake Kivu. To date, only in the latter two lakes a biogeochemical role of photoferrotrophs has been confirmed. In this review we critically summarize the current knowledge on iron-driven photosynthesis, as a remains of ancient Earth biogeochemistry.
56 citations
Cites background from "Stratification, mixing and transpor..."
...…volcanic area with hydrothermal influences on lake deep water layers, which slightly disturb the meromictic conditions (Katsev et al., 2017) created by the vertical salinity gradient originated by salt inputs coming from subaquatic groundwater discharges (Schmid and Wüest, 2012; Ross et al., 2015)....
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References
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Book•
23 Feb 1973
TL;DR: In this article, the authors introduce linear internal waves and herar flows in a stratified fluid and double-diffusive convection in stably stratified fluids, and show that the shear flows can produce turbulence.
Abstract: Preface 1. Introduction and preliminaries 2. Linear internal waves 3. Finite amplitude motions in stably stratified fluids 4. Instability and the production of turbulence 5. Turbulent shear flows in a stratified fluid 6. Buoyant convection from isolated sources 7. Convection from heated surfaces 8. Double-diffusive convection 9. Mixing across density interfaces 10. Internal mixing processes Bibliography and author index Recent publications Subject index.
2,722 citations
TL;DR: In this article, small-scale turbulence observations allow the mixing regimes in lakes, reservoirs, and other enclosed basins to be categorized into the turbulent surface and bottom boundary layers as well as the comparably quiet interior.
Abstract: Recent small-scale turbulence observations allow the mixing regimes in lakes, reservoirs, and other enclosed basins to be categorized into the turbulent surface and bottom boundary layers as well as the comparably quiet interior. The surface layer consists of an energetic wave-affected thin zone at the very top and a law-of-the-wall layer right below, where the classical logarithmic-layer characteristic applies on average. Short-term current and dissipation profiles, however, deviate strongly from any steady state. In contrast, the quasi-steady bottom boundary layer behaves almost perfectly as a logarithmic layer, although periodic seiching modifies the structure in the details. The interior stratified turbulence is extremely weak, even though much of the mechanical energy is contained in baroclinic basin-scale seiching and Kelvin waves or inertial currents (large lakes). The transformation of large-scale motions to turbulence occurs mainly in the bottom boundary and not in the interior, where the local shear remains weak and the Richardson numbers are generally large.
519 citations
13 Aug 2019
TL;DR: Tectonic setting of the East African lakes East African climate physical limnology aquatic chemistry food webs and fisheries sedimentary processes and desciphering the past in large lakes impact of man historical note.
Abstract: Tectonic setting of the East African lakes East African climate physical limnology aquatic chemistry food webs and fisheries sedimentary processes and desciphering the past in large lakes impact of man historical note.
304 citations
TL;DR: In this paper, the authors integrated geophysical, geochemical and biological data to unravel the origin and evolution of an unusual rift lake, Lake Kivu, and found that sharp boundaries in the vertical temperature and salinity structure of the water across the lake can best be explained as separate convecting layers.
Abstract: Geophysical, geochemical and biological data are integrated to unravel the origin and evolution of an unusual rift lake. The northern basin of Lake Kivu contains about 0.5 km of sediments which overlie a basement believed to be crystalline rocks of Precambrian age. Volcanic rocks at the northern end of the lake have created large magnetic anomalies of up to 300γ. Heat flow varies from 0.4 to 4 hfu. The extreme variability may be due in part to sedimentation or recent changes in the temperature of the bottom water. Sharp boundaries in the vertical temperature and salinity structure of the water across the lake can best be explained as separate convecting layers. Such convecting cells are the result of the increase in both temperature and salinity with depth. Concentrations of the major dissolved gases in the deep water, CO2 and CH4, approach saturation but do not exceed it at any depth. The salts are supplied mainly by hydrothermal discharges at the bottom of the lake which we calculate to have a salinity of 4‰ which is about 60% higher than the salinity of the bottom water. The annual discharge at the present time is about 0.5 km3. Zinc anomalies in the water are explained by the accumulation of sphalerite-containing globules at certain depths. Stratigraphic correlation of sediments is possible across the entire lake, based on physical, geochemical and paleontological criteria. Sedimentation rates are of the order of 30 cm/1000 years implying a Pliocene age for the deepest part of the lake. Periods of hydrothermal activities and heightened volcanism, as recorded in the sediments, appear to have coincided with pluvial times. Enrichment of the surface waters of Lake Kivu by nutrients has led to explosive speciation in the diatom genusNitzschia. Several new types of methane oxidizing and-producing bacteria were isolated. Bacterial degradation of recent plankton appears insufficient to explain the amount of methane in the lake, and some of it is derived diagenetically.
230 citations