Search

The Qinghai‐Tibet Plateau possesses the largest alpine lake system, which plays a crucial role in the land‐atmosphere interaction. We report first observations on the thermal and radiation regime under ice of the largest freshwater lake of the Plateau. The results reveal that freshwater lakes on the Tibetan Plateau fully mix under ice. Due to strong solar heating, water temperatures increase above the maximum density value 1–2 months before the ice break, forming stable thermal stratification with subsurface temperatures >6°C. The resulting heat flow from water to ice makes a crucial contribution to ice cover melt. After the ice breakup, the accumulated heat is released into the atmosphere during 1–2 days, increasing lake‐atmosphere heat fluxes up to 500 W m−2. The direct biogeochemical consequences of the deep convective mixing are aeration of the deep lake waters and upward supply of nutrients to the upper photic layer. Plain Language Summary: The Qinghai‐Tibet Plateau possesses the largest alpine lake system, which plays a crucial role in the land‐atmosphere interaction. Data on thermal properties of Tibetan lakes during the ice‐covered season are extremely scarce. The first observations on the thermal and radiation regime under ice of the largest freshwater lake of the Plateau reveal that freshwater (and apparently the majority of brackish) lakes gain an extremely large amount of solar radiation penetrating the highly transparent ice cover. As a result, lakes fully mix under ice and get heated up to >6°C. The accumulated heat makes a crucial contribution to ice cover melt. The quick release of the heat to the atmosphere during 1–2 days after the ice breakup strongly increases lake‐atmosphere heat fluxes, potentially affecting regional weather conditions. Warm and well‐mixed conditions are favorable for aquatic life in the extreme environment of Tibet. Key Points: An abnormal thermal regime under the ice cover of Tibetan lakes is revealedThe lakes get heated above their maximum density temperature by extremely high level of solar radiation penetrating the ice coverThe stored heat shortens the ice‐covered period and is quickly released into the atmosphere after ice‐off, affecting local climate [ABSTRACT FROM AUTHOR]

Summary: We explored the diversity and community composition of bacteria along a vertical gradient in Lake Issyk Kul, Kyrgyzstan, one of the world's largest and deepest brackish lakes. We identified 4904 bacterial amplicon sequence variants based on the 16S rRNA gene analysis and determined significant changes in the composition, responding mainly to depth and salinity. A higher abundance of Proteobacteria and Bacteroidetes was observed in the surface waters and lake tributaries. Cyanobacteria were more abundant in the deep chlorophyll maximum from 28.5 to 128 m, while Planctomycetes and Chloroflexi were dominant in the deepest layers, from 128 to 600 m. According to our machine learning analyses, depth and temperature were the most critical environmental factors, with strong effects on Proteobacteria, Planctomycetes and Chloroflexi, while oxygen was associated with the variations in Cyanobacteria. We also observed that with increasing depth, the alpha diversity values increased. The dominance of Planctomycetes and Chloroflexi in the deepest layers can only be seen in a few lakes of the world. However, the lake is facing increasing anthropogenic and climatic pressure. There is an urgent need to understand better the ecological role and function of these unique deep‐water microbial communities. [ABSTRACT FROM AUTHOR]

Qinghai Lake is the largest lake in China. However, its influence on the local climate remains poorly understood. By using an atmosphere‐lake coupled model, we investigated the impact of the lake on the local climate. After the adjustment of four key parameters, the model reasonably reproduced the lake‐air interaction. Superimposed by the orographic effects on lake‐land breeze circulation, the presence of the lake enhanced precipitation over the southern part of the lake and its adjacent land, while slightly reduced precipitation along the northern shore of the lake. The lake effect on local precipitation revealed a distinct seasonal and diurnal variability, reducing precipitation in May (−6.6%) and June (−4.5%) and increasing it from July (5.7%) to November (125.6%). During the open water season, the lake's daytime cooling effect weakened and the nighttime warming effect strengthened, affecting spatial distribution and intensity of lake‐induced precipitation. In early summer, precipitation slightly decreased over the north part of the lake due to the lake's daytime cooling. In turn, lake‐induced nighttime warming increased precipitation over the southern section of the lake and its adjacent land. With the start of the autumn cooling in September, heat and moisture fluxes from the lake resulted in precipitation increase in both daytime and nighttime over the entire lake. In October, the background atmospheric circulation coupled with the strong lake effects lead to a small amount but high proportion of lake‐induced precipitation spreading evenly over the lake. Key Points: The coupled lake‐atmosphere model after adjusting four key parameters performs well in quantifying the lake's regional climate effectsQinghai Lake decreased the local precipitation during April–June while increased that during July–NovemberThroughout the simulation period, precipitation declined slightly in the north and rose greatly in the south due to the lake's presence [ABSTRACT FROM AUTHOR]

Ice cover plays a critical role in physical, biogeochemical, and ecological processes in lakes. Despite its importance, winter limnology remains relatively understudied. Here, we provide a primer on the predominant drivers of freshwater lake ice cover and the current methodologies used to study lake ice, including in situ and remote sensing observations, physical based models, and experiments. We highlight opportunities for future research by integrating these four disciplines to address key knowledge gaps in our understanding of lake ice dynamics in changing winters. Advances in technology, data integration, and interdisciplinary collaboration will allow the field to move toward developing global forecasts of lake ice cover for small to large lakes across broad spatial and temporal scales, quantifying ice quality and ice thickness, moving from binary to continuous ice records, and determining how winter ice conditions and quality impact ecosystem processes in lakes over winter. Ultimately, integrating disciplines will improve our ability to understand the impacts of changing winters on lake ice. Plain Language Summary: Lakes are experiencing accelerated rates of warming, including shorter seasonal duration of ice cover, later ice‐on, earlier ice‐off, and in some years no ice cover at all. Lake ice has been historically studied independently by four subdisciplines: observations by in situ and remote sensing scientists, controlled mesocosm experiments by limnologists, and process‐based models by physical modelers. Here, we highlight opportunities for collaboration between disciplines and provide guidelines to successfully integrate the disciplines to tackle the most urgent questions surrounding lake ice loss in warming climates. Key Points: Ripe opportunity to merge disciplines and harmonize data streams to further understand lake ice dynamics in changing wintersDevelop integrated plans for future data collection by moving from ice cover and phenology to multifaceted descriptions of ice conditionsOperationalize development of skill sets to work in multidisciplinary teams by constructing a workforce and developing a common language [ABSTRACT FROM AUTHOR]

We describe a low‐cost three‐dimensional underwater particle tracking velocimetry system to directly measure particle settling rate and flux in low‐turbulence aquatic environments. The system consists of two waterproof cameras that acquire stereoscopic videos of sinking particles at 48 frames s−1 over a tunable sampling volume of about 45 × 25 × 24 cm. A dedicated software package has been developed to allow evaluation of particle velocities, concentration and flux, but also of morphometric parameters such as particle area, sinking angle, shape irregularity, and density. Our method offers several advantages over traditional approaches, like sediment trap or expensive in situ camera systems: (1) it does not require beforehand particle collection and handling; (2) it is not subjected to sediment trap biases from turbulence, horizontal advection, or presence of swimmers, that may alter particulate load and flux; (3) the camera system enables faster data processing and flux computation at higher spatial resolution; (4) apart from the particle settling rates, the particle size distribution, and morphology is determined. We tested the camera system in Lake Stechlin (Germany) in low turbulence and mean flow, and analyzed the morphological properties and settling rates of particles to determine their sinking behavior. The particle flux assessed from conventional sediment trap measurements agreed well with that determined by our system. By this, the low‐cost approach demonstrated its reliability in low turbulence environments and a strong potential to provide new insights into particulate carbon transport in aquatic systems. Extension of the method to more turbulent and advective conditions is also discussed. [ABSTRACT FROM AUTHOR]

Extreme weather events can pervasively influence ecosystems. Observations in lakes indicate that severe storms in particular can have pronounced ecosystem-scale consequences, but the underlying mechanisms have not been rigorously assessed in experiments. One major effect of storms on lakes is the redistribution of mineral resources and plankton communities as a result of abrupt thermocline deepening. We aimed at elucidating the importance of this effect by mimicking in replicated large enclosures (each 9 m in diameter, ca. 20 m deep, ca. 1300 m3 in volume) a mixing event caused by a severe natural storm that was previously observed in a deep clear-water lake. Metabolic rates were derived from diel changes in vertical profiles of dissolved oxygen concentrations using a Bayesian modelling approach, based on high-frequency measurements. Experimental thermocline deepening stimulated daily gross primary production ( GPP) in surface waters by an average of 63% for >4 weeks even though thermal stratification re-established within 5 days. Ecosystem respiration ( ER) was tightly coupled to GPP, exceeding that in control enclosures by 53% over the same period. As GPP responded more strongly than ER, net ecosystem productivity ( NEP) of the entire water column was also increased. These protracted increases in ecosystem metabolism and autotrophy were driven by a proliferation of inedible filamentous cyanobacteria released from light and nutrient limitation after they were entrained from below the thermocline into the surface water. Thus, thermocline deepening by a single severe storm can induce prolonged responses of lake ecosystem metabolism independent of other storm-induced effects, such as inputs of terrestrial materials by increased catchment run-off. This highlights that future shifts in frequency, severity or timing of storms are an important component of climate change, whose impacts on lake thermal structure will superimpose upon climate trends to influence algal dynamics and organic matter cycling in clear-water lakes. [ABSTRACT FROM AUTHOR]

Using airborne thermal infrared imaging and horizontally resolved in situ temperature monitoring at the lake surface, we estimated strength and duration of regular wind-driven upwelling of dense deep water to the lake surface in two small (in terms of Rossby radius) temperate lakes during the initial phase of summer thermal stratification. The onset and duration of the upwelling events correlated well with the balance between stratification (in terms of Schmidt stability) and wind forcing, as expressed by Lake and Wedderburn numbers. The period of regular upwelling appearances lasted 7-15 days, identified by Schmidt stabilities around 30 J m−2 and Lake numbers between 0 and 1, and resulted in persistent temperature gradients of up to 2°C across the lake surface. Our results suggest that spring upwelling should inevitably take place in all freshwater temperate lakes with mean temperatures crossing the maximum density value of freshwater on annual cycle, whereas duration and intensity of the upwelling would vary depending on lake morphometry and weather conditions. Our results suggest major contribution of upwelling in nutrient supply to the upper waters, oxygenation of the deep water column, and air-lake gas exchange, in particular, the release of the sediment-produced methane into the atmosphere. [ABSTRACT FROM AUTHOR]

Compared to the well-studied open water of the "growing" season, under-ice conditions in lakes are characterized by low and rather constant temperature, slow water movements, limited light availability, and reduced exchange with the surrounding landscape. These conditions interact with ice-cover duration to shape microbial processes in temperate lakes and ultimately influence the phenology of community and ecosystem processes. We review the current knowledge on microorganisms in seasonally frozen lakes. Specifically, we highlight how under-ice conditions alter lake physics and the ways that this can affect the distribution and metabolism of auto- and heterotrophic microorganisms. We identify functional traits that we hypothesize are important for understanding under-ice dynamics and discuss how these traits influence species interactions. As ice coverage duration has already been seen to reduce as air temperatures have warmed, the dynamics of the under-ice microbiome are important for understanding and predicting the dynamics and functioning of seasonally frozen lakes in the near future. [ABSTRACT FROM AUTHOR]

The aim was to disclose the seasonality in the internal seiches during summer stratification in 2010 within the small, 3 km long, ellipse-shaped, temperate Lake Arendsee (Germany) with simple morphometry. We used observations of temperatures from two thermistor chains (09 April to 24 September), conductivity and temperature profiles, and wind speeds and directions (January to December). To analyze the seasonality in the periods, appearance and modal structure of the seiches, and the wind forcing, we applied spectral analysis on monthly time series of isotherms and wind speeds, and wavelet analysis on 6 month long time series of integrated potential energy and wind speeds. To determine the effect of earth's rotation on seiches, we applied two analytical models based on three-layer density stratification. Earth's rotation affected internal seiches significantly even in a small lake. That was shown by the models and the Burger number being < 1 during stratification. Vertical mode 1 Kelvin-type seiches dominated the internal seiche weather in Lake Arendsee, with their periods changing because of seasonal variations in stratification. Weak, free seiches of vertical mode 2 were amplified during three episodes: (1) in mid-summer, when their periods coincided with the inertial period at the lake's latitude, (2) in late spring, and (3) in autumn, both when their periods approached 24 h. The amplifications were caused by resonance between free seiches and wind-forced oscillations at diurnal and inertial periods. Such short-term resonance events, associated with enhanced mixing, are inherent for the seasonal stratification cycle in the most temperate lakes. [ABSTRACT FROM AUTHOR]

A field and theoretical study of the mean circulation patterns, gravity currents, and heat fluxes in a mid-latitude, ice-covered lake is presented. Thermistor chains captured the lake-wide pressure gradients and thermal regime, acoustic instruments measured bottom and interior velocities, and a sediment temperature probe measured sediment temperatures and heat fluxes. Typical Rossby numbers showed that the mean circulation was geostrophic, in the form of either an anti-cyclonic or a cyclonic basin-scale gyre. Each gyre reached a steady state after ~ 10-15 d, circulating at a maximum velocity of ~ 1 mm s-1. The gyres persisted throughout most of the winter and generally decayed with time. The cyclonic gyre was generated by the lateral temperature gradients created by sediment heat release. It was not possible to determine the driver of the anti-cyclonic gyre. Because most ice-covered lakes are controlled by the Earth's rotation, anti-cyclonic and cyclonic circulation is postulated as being the most common mean interior flow pattern. The horizontal heat fluxes were of the same order as the vertical heat fluxes, which is consistent with the findings of previous research. The sediment heat flux decayed with time, stabilizing after ~ 60 d. A scaling analysis of down-sloping advection with dynamic, seasonal-scale temperatures showed that a warmer lake and an increase in the length-to-depth geometric aspect ratio (both within a given lake) and an increase in lake size all increased advection time scales, and therefore circulation time scales, near the boundaries. [ABSTRACT FROM AUTHOR]

Using the carcass sinking rate and density determined in laboratory for several freshwater zooplankton species, we developed a model of zooplankton carcass sinking as affected by turbulence and stratification. The model was subsequently used to estimate the residence time of zooplankton carcasses in the water column of Lake Stechlin, a typical temperate dimictic lake in northeastern Germany. The residence time varied among the different species and was strongly affected by thermal stratification. At the peak of summer stratification, the carcasses stayed up to 5d in the 70m-deep water column before reaching the lake bottom. Residence time was long enough that zooplankton carcasses could serve as an important matter and energy source for bacteria in the lake's pelagic zone and hence have the potential to significantly affect aquatic carbon and nutrient cycling. The proposed model of sinking rates, based on physically sound relationships, can be easily applied to other passively sinking particles, and be integrated into large ecosystem models. [ABSTRACT FROM AUTHOR]