Autumn Overturn Research Articles

Limnological dynamics of methane (CH4) and carbon dioxide (CO2) emissions from a tropical hypertrophic reservoir lake

ABSTRACT Methane (CH4) and carbon dioxide (CO2) emissions from tropical freshwater ecosystems have been understudied, particularly in terms of their interaction with limnological dynamics, their cycling, and the emission mechanisms of CH4. To help reduce that knowledge gap, this study addressed these processes in Valle de Bravo (VB), a tropical (19° 11. 65′ N) reservoir lake, that provides water supply to Mexico City metropolitan area. CH4 and CO2 concentrations and emissions from VB were measured during four field campaigns distributed along the annual limnological cycle of the reservoir. Dissolved CH4 concentration varied over four orders of magnitude (0.015–176.808 μmol L−1), and dissolved CO2 varied from below atmospheric saturation (15.062 μmol L−1) to 10 times that concentration (219.505 μmol L−1). CH4 fluxes ranged from 23.25 to 1220.80 μmol m−2 day−1, while CO2 fluxes ranged from −60.11 to 254.99 mmol m−2 day−1. Seasonal monitoring also allowed the assessment of the annual emissions as well as the greenhouse gas (GHG) storage during thermal stratification, which accounted for >58% of the total GHG annual emissions from VB. Overall, VB is a source of GHG, and its major contribution is the CH4 released during the autumn overturn.

Mechanistic Modeling of the Variability of Methane Emissions from an Artificial Reservoir

The mechanistic model LAKE2.3 was tested for its capability to predict of methane (CH4) emissions from reservoirs. Estimates of CH4 emissions from the Mozhaysk reservoir (Moscow region) provided by the model showed good agreement with instrumental in situ observations for several parameters of the water ecosystem. The average CH4 flux calculated by the model is 37.7 mgC-CH4 m−2 day−1, while according to observations, it is 34.4 mgC-CH4 m−2 day−1. Ebullition makes the largest contribution to the emissions from reservoirs (up to 95%) due to low methane solubility in water and the high oxidation rate of diffusive methane flux. During the heating period, an increase in methane emission is observed both in the model and empirical data, with a maximum before the onset of the autumn overturn. An effective parameter for calibrating the diffusive methane flux in the model is the potential rate of methane oxidation. For ebullition flux, it is the parameter q10 (an empirical parameter determining the relationship between methane generation and temperature) because methane production in bottom sediments is the most important. The results of this research can be used to develop mechanistic models and provide a necessary step toward regional and global simulations of lacustrine methane emission using LAKE2.3.

Seasonal Evolution of Stable Thermal Stratification in Central Area of Lake Ladoga

The complete climatic courses of the parameters of stable thermal stratification for the central part of Lake Ladoga, the largest European lake, are presented on the basis of empirical relationships, taking into account the physical processes governing water temperature variations. For the first time, the seasonal cycle of the surface water temperature, the temperature and the depth of the thermocline, and the hypolimnion temperature are calculated using the vertical profiles of the temperature obtained from the central area of Lake Ladoga. Temperature data are used for the period of in situ observations from 1897 to the present. The proposed functional forms of the temporal temperature cycle and the course of thermocline’s boundaries deepening are useful for examination and simulation of the heat vertical transport from air to water. Approximation curves for the parameters of heating and cooling periods were developed with high significant determination coefficients. Time dependencies of the climatic rates of change in water temperature and the depth of the thermocline boundaries were determined from the onset of stable stratification to its dissipation. The highest rate of water temperature change in the heating stage takes place in late June–early July, which at the water surface, is 0.32 °C/day, while in the thermocline layer, it is 0.18 °C/day. The peak velocity during the cooling stage at the surface occurs in late August–early September and is 0.14 °C/day, whereas in the thermocline, it is 0.08 °C/day and takes place between September and early October. During the period of heating, the deepening parameters of the thermocline layer do not fluctuate very much, only within the range of 0.1–0.3 m/day. During the cooling period, under the influence of free convection, rates increase drastically. The maximum rates of deepening during the period of full autumn mixing reach 1.8 m/day. When the autumn overturn occurs, the epilimnion thickness equals the bottom depth, and the bottom temperature reaches its maximum during the annual cycle. Climatic norms of the stratification parameters against which it is necessary to assess climate change are calculated.

Spatial-temporal variability of methane fluxes in lakes varying in latitude, area, and depth

Many previous studies have found spatial and seasonal variabilities in CH4 fluxes, which could significantly affect lake-wide CH4 budgets. However, the ways in which the spatial and seasonal patterns of CH4 fluxes vary among lakes on a global scale is largely unknown. We compiled literature on CH4 flux data from global lakes and analyzed the spatial and seasonal variabilities for lakes varying in latitude, maximum depth, and area. Spatially, we found a significant linear relationship between the ratio of littoral to profundal fluxes and lake morphology (more related to area than depth), while globally, half of the lakes would have within 5% error of CH4 emission estimation under single-zone sampling. Seasonally, mid-latitude lakes showed higher CH4 fluxes in the summer and autumn, indicating the influence of temperature and autumn overturn, and the latter being largely related to maximum depth. Globally, due to abundant shallow lakes in the mid-latitude zone, approximately 99% of lakes had higher fluxes in the summer, while 75% of lakes showed errors in CH4 emission estimation within 20% when only the summer flux was investigated. In the high-latitude lakes, CH4 evasion during the spring ice-off period was significantly correlated with lake maximum depth, while lake area was also important when analyzing the CH4 diffusive flux. Our study yields preliminary conclusions about spatial and seasonal patterns of CH4 flux in different lake types, which are fundamental to building an effective sampling strategy and to determining an accurate CH4 budget from global lakes.

Interannual Variability of Methane Storage and Emission During Autumn Overturn in a Small Lake

AbstractMethane emissions from freshwater systems, and especially from small lakes, source a significant proportion of naturally produced atmospheric methane. In small temperate lakes, storage flux, that is, the diffusive emission of methane that was stored in anoxic waters during the seasonal overturn of the water column, can contribute a large fraction of annual methane emissions. Here, we use an extensive field data set to quantify methane storage in anoxic deep water and identify as well as quantify the sources and losses of this methane. The comparison of measurements from 4 years in a small temperate lake (Illmensee) shows that methane storage can differ substantially between years. In 2012 and 2018, the annual maximum of the methane stored in the entire water column was around twice as high (5,350 and 5,822 kg) as in 2013 and 2017 (2,722 and 2,295 kg). A methane mass balance approach suggests that the variability of methane storage in the anoxic water between the years was not caused by the methane flux at the anoxic–oxic water interface, but rather was related to changes in the methane source from the anoxic sediments. The interannual differences in sediment flux could not be explained by sediment temperature, but rather by the differing supply of organic matter. Our findings suggest that phytoplankton blooms promote methane storage within the same year and thus cause interannual variability in emissions during autumn overturn.

Interannual variability of methane storage and emission during autumn overturn in a small lake

Interannual variability of methane storage and emission during autumn overturn in a small lake

Seasonal thermal regime and climatic trends in lakes of the Tibetan highlands

Abstract. The hydrology of the lake-rich Tibetan Plateau is important for the global climate, yet little is known about the thermal regime of Tibetan lakes due to scant data. We (i) investigated the characteristic seasonal temperature patterns and recent trends in the thermal and stratification regimes of lakes on the Tibetan Plateau and (ii) tested the performance of the one-dimensional lake parameterization scheme FLake for the Tibetan lake system. For this purpose, we combined 3 years of in situ lake temperature measurements, several decades of satellite observations, and the global reanalysis data. We chose the two largest freshwater Tibetan lakes, Ngoring and Gyaring, as study sites. The lake model FLake faithfully reproduced the specific features of the high-altitude lakes and was subsequently applied to reconstruct the vertically resolved heat transport in both lakes during the last 4 decades. The model suggested that Ngoring and Gyaring were ice-covered for about 6 months and stratified in summer for about 4 months per year with a short spring overturn and a longer autumn overturn. In summer the surface mixed boundary layer extended to 6–8 m of depth and was about 20 % shallower in the more turbid Gyaring. The thermal regime of the transparent Ngoring responded more strongly to atmospheric forcing than Gyaring, where the higher turbidity damped the response. According to the reanalysis data, air temperatures and humidity have increased, whereas solar radiation has decreased, since the 1970s. Surprisingly, the modeled mean lake temperatures did not change, nor did the phenology of the ice cover or stratification. Lake surface temperatures in summer increased only marginally. The reason is that the increase in air temperature was offset by the decrease in radiation, probably due to increasing humidity. This study demonstrates that air temperature trends are not directly coupled to lake temperatures and underscores the importance of shortwave radiation for the thermal regime of high-altitude lakes.

Effect of a moderate-size reservoir on transport of trace elements in a watershed

ABSTRACTWildman RA, Forde NA. 2016. Effect of a moderate-size reservoir on transport of trace elements in a watershed. Lake Reserve Manage. 32:353–365.We assessed the extent to which Grand Lake, Oklahoma (>30 m deep, >80 km long), retains Fe, Mn, P, As, Zn, Pb, and Cd. Filtered water samples and suspended sediment samples were collected upstream of, within, and downstream of the reservoir. We then estimated instantaneous, seasonal, elemental fluxes. In winter and spring, when storms brought high flows to the reservoir, Grand Lake modified flood water minimally. During these seasons, trace element distributions were determined by the passage of storm inflows through the reservoir. In summer, Fe, Mn, P, and As were enriched in anoxic bottom water and exported through the dam, which draws water from below the surface mixed layer. Concentrations of aqueous elements in the water column were lower following autumn overturn, perhaps due to precipitation of metal oxides and settling. Unlike Fe, Zn was retained in Grand Lake during all seasons. Concentrations of Cd and Pb in filtered samples were often below our detection limit. Logistic regression indicated that Zn predicted detectable Cd, and so Grand Lake probably sequesters Cd. Sequestration of Pb was unclear because detectable Pb was predicted by both Zn and Fe. This study shows that watershed hydrology determines the transport of trace elements through a reservoir during times of high flow but that vertical circulation and biogeochemistry dominate during summertime and autumn low flows. Understanding these mechanisms can aid reservoir managers who seek to reduce downstream loads of trace elements.

Abundance and size change of Hannaea baicalensis in Lake Baikal

Hannaea baicalensis is a benthic pennate diatom that predominantly grows at depths of less than a metre attached to rocks and filamentous algae in Lake Baikal, Russia. This narrow zone at the edge of the lake is subject to frequent wave action and lake level fluctuations, which combine with other factors to affect seasonal abundance. During ice cover from January to May in 2008, when lake levels decreased from 42 to 14 cm above datum, H. baicalensis cell abundance remained low (0.39 × 106 cells cm–2). The main period of net cell increase occurred in autumn, when there was a period of stable lake level (±10 cm changes in water depth) that coincided with the return of nutrients during autumn overturn. Cell abundance reached 1.52 × 106 cells cm–2 on 31 October. Alongside the changes in abundance, cyclic size changes in cell apical lengths were found (40 to 144 µm), which were associated with timing of the length of the life cycle. Size decline occurred in both spring and autumn, with an average decrease in apical length of 36 µm per year. It took two years for the mean apical length of a single cohort to decrease from 128 µm to 56 µm, which was then below the threshold (< 65 µm) for initiation of size regeneration.

Water temperature and characteristics of thermal stratification in Nam Co, Tibet

水温变化是湖泊的重要物理特性,对湖泊的水质特征、湖水能量循环、水生生态系统研究具有重要意义.基于不同季节的实地观测资料,分析青藏高原高海拔、深水大湖纳木错的水温变化特征及季节差异,并着重分析湖水热力学分层的季节变化.结果显示纳木错中部、东部两个湖盆冬季封冻,夏季存在稳定分层,春、秋季混合,是一个典型双季对流、完全混合型湖泊.但两个湖盆水温变化与热力学分层又有各自的特征,东部浅湖盆湖水在春季升温快,夏季分层与秋季翻转均比中部湖盆早,且秋季翻转时水温也比中部湖盆高.初步分析认为两个湖盆不同的湖盆形状及水深分布可能是造成其热力学特征差异的主要原因.;Variation of water temperature is a critical physical property of a lake, which has great significance for other research, such as lake water quality, energy cycle, aquatic ecosystem. Illustrated by Nam Co, characteristics and seasonal differences of lake water temperature distribution were studied in a high altitude deep lake on the Tibetan Plateau, with in-situ monitoring data from different seasons. Then, seasonal variability of thermal stratification was emphatically analyzed. Results imply that in Nam Co, both the middle and eastern parts are frozen up in winter, completely mixed in spring and autumn, and steadily stratified in summer. Thus, Nam Co is a typical dimictic lake, and a holomictic lake at the same time. However, the two parts of Nam Co have unique characteristics in season variability of water temperature and thermal stratification respectively. In spring, temperature rises faster in the relatively shallow basin than that in the middle lake. Meanwhile, the date of summer stratification and autumn overturn is earlier in the shallow basin, in which water temperature is higher when circulated in autumn as well. By preliminary analysis, different topography under water and bathymetric distribution of the two basins probably contribute the most to the difference in the thermodynamic characteristic.

Importance of the autumn overturn and anoxic conditions in the hypolimnion for the annual methane emissions from a temperate lake.

Changes in the budget of dissolved methane measured in a small temperate lake over 1 year indicate that anoxic conditions in the hypolimnion and the autumn overturn period represent key factors for the overall annual methane emissions from lakes. During periods of stable stratification, large amounts of methane accumulate in anoxic deep waters. Approximately 46% of the stored methane was emitted during the autumn overturn, contributing ∼80% of the annual diffusive methane emissions to the atmosphere. After the overturn period, the entire water column was oxic, and only 1% of the original quantity of methane remained in the water column. Current estimates of global methane emissions assume that all of the stored methane is released, whereas several studies of individual lakes have suggested that a major fraction of the stored methane is oxidized during overturns. Our results provide evidence that not all of the stored methane is released to the atmosphere during the overturn period. However, the fraction of stored methane emitted to the atmosphere during overturn may be substantially larger and the fraction of stored methane oxidized may be smaller than in the previous studies suggesting high oxidation losses of methane. The development or change in the vertical extent and duration of the anoxic hypolimnion, which can represent the main source of annual methane emissions from small lakes, may be an important aspect to consider for impact assessments of climate warming on the methane emissions from lakes.

Seasonal formation of clastic-biogenic varves: the potential for palaeoenvironmental interpretations

Clastic-biogenic varved lake sediments from Nautajärvi, Finland, covering the last ca. 10 000 years have previously been investigated with respect to varve chronology, laminae composition and thickness variations, palaeomagnetic secular variation, the pollen record and the diatom composition of selected sections. In this study, we provide detailed information on the seasonal sedimentation of different varve components and palaeolimnological indicators (diatoms, chrysophycaean cysts) using near-bottom sediment-trap monitoring. The monitoring results confirm previous interpretations of the formation of clastic-biogenic varves in Fennoscandian lakes. The results also indicate that seasonal sediment fluxes correspond with environmental changes. Higher snow storage in winter and the discharge intensity during the following spring snowmelt clearly increase the suspended sediment load, transportation and net accumulation of detrital mineral matter during spring and early summer. A prolonged autumn and subsequent freezing and thawing cycles in winter, conversely, result in an incremental but slow accumulation of assorted mineral matter and organic particles during winter, whereas the subsequent spring flooding and detrital sediment yield are diminished and thinner clastic laminae are formed. The accumulation rates of diatoms and chrysophyte cysts suggest that these algal groups are more dependent on seasonal processes (e.g. spring and autumnal overturn) than on rapid, short-lived environmental episodes such as the spring discharge peak.

Modeling 1993–2008 climatology of seasonal general circulation and thermal structure in the Great Lakes using FVCOM

An unstructured Finite Volume Coastal Ocean Model was applied to all five Great Lakes simultaneously to simulate circulation and thermal structure from 1993 to 2008. Model results are compared to available observations of currents and temperature and previous modeling work. Maps of climatological circulation for all five Great lakes are presented. Winter currents show a two-gyre type circulation in Lakes Ontario and Erie and one large-scale cyclonic circulation in Lakes Michigan, Huron, and Superior. During the summer, a cyclonic circulation remains in Lakes Superior; a primarily cyclonic circulation dominates upper and central Lake Huron; Lake Ontario has a single cyclonic circulation, while circulation in the central basin of Lake Erie remains two-gyre type; Lake Michigan has a cyclonic gyre in the north and an anti-cyclonic one in the south. The temperature profile during the summer is well simulated when a surface wind-wave mixing scheme is included in the model. Main features of the seasonal evolution of water temperature, such as inverse temperature stratification during the winter, the spring and autumn overturn, the thermal bar, and the stratification during summer are well reproduced. The lakes exhibit significant annual and interannual variations in current speed and temperature.

Seasonal dynamics of dissolved organic matter on a coastal salinity gradient in the northern Baltic Sea

During the phytoplankton growth season, the vertical distributions of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) were followed biweekly along with key physical, chemical and biological variables on a shore-to-open-sea salinity gradient in the Gulf of Finland. Moreover, bioavailable shares of DOC and DON were evaluated with natural bacterial surface and deepwater samples. Dissolved organic matter (DOM) accumulated in the surface layer throughout the productive season, and the accumulated DOM was N-rich (molar C:N ratio of 10) compared to the bulk DOM pool (C:N 20–29). Redundancy analysis showed a negative correlation between phytoplankton and DOM concentrations, suggesting that most DOM release occurred during declining phases of spring and late summer algal blooms. During the spring bloom, bioavailable shares of DOM were small, whereas during and after the late summer bloom of filamentous cyanobacteria, 3–9% and 10–20% of the respective total DOC and DON pools were degraded by bacteria within 2 weeks. Vertical mixing over the thermocline was estimated by constructing a steady state budget for water and salt mass flows between key GoF basin compartments (inflows equal outflows). The flow estimates suggested that the net changes in surface DOC and DON pools underestimated the respective total accumulations by ca 40% and 20%, respectively, over the 4-mo thermal stratification period. Thus, integral export of surface DOC and DON after autumn overturn amounted to ca 710mmolCm−2 and 40mmolNm−2, corresponding respectively to ca 12–25% and 11% of reported annual particulate organic carbon and nitrogen sedimentation in our study area.

Size Fractionation of Trace Elements in a Seasonally Stratified Boreal Lake: Control of Organic Matter and Iron Colloids

The colloidal distribution and size fractionation of organic carbon and trace elements were studied in a seasonally stratified, organic-rich boreal lake, Lake Maselga, located in the European subarctic zone (NW Russia, Arkhangelsk region). This study took place over the course of 5 years in winter (glacial) and summer periods and during the spring and autumn overturn. A newly developed in situ dialysis technique (1, 10, and 50 kDa) and traditional frontal filtration and ultrafiltration (20, 10, 5, 0.22, and 0.025 μm) were used to assess element concentrations at different depths. No significant changes in element concentrations occurred during filtration through sub-colloidal pore-size membranes (20–0.22 μm), suggesting a negligible amount of particulate Fe, OC, and associated trace metals. Large colloids (0.025–0.22 μm) were found to be the main carriers of poorly soluble elements (Fe, Al, Ti, Zr, REEs, Th, and U) during the summer and winter stratification. There was also a clear change in the vertical pattern of the percentage of colloidal Al, Ti, V, Cr, Fe, and Ni during different seasons, and the greatest proportion of colloidal forms was observed during the spring and autumn overturn. This pattern is most likely linked to the dominance of soil (allochthonous) organic carbon, which complexes with trace metals during these periods. During the summer seasons, autochthonous production of small exometabolites or photodegradation increases the concentration of the low-molecular weight fractions (<1 kDa) that dominate the speciation of divalent heavy metals in surface horizons. The colloidal status of As (30–60%), which was documented in different seasons along the full depth of the water column, is most likely linked to the presence of organic complexes. The overall results of this study suggest that changes in the colloidal speciation of trace elements with depth in different seasons depend on changes in the redox conditions, the input of soil OM, the biodegradation of plankton biomass releasing dissolved organic matter in the bottom horizons, and in upward diffusion from the sediments.

Seasonal increase of methane in sediment decreases δ13C of larval chironomids in a eutrophic shallow lake

Recent studies have shown that larval chironomids assimilate 13C-depleted carbon derived from biogenic methane by feeding on methane-oxidizing bacteria (MOB). The dietary contribution of MOB is known to be maximized in the autumn overturn period or winter in eutrophic dimictic lakes due to the increase of MOB biomass following the supply of oxygenated water, but in polymictic lakes, such seasonal variability has not been revealed. We investigated the seasonal patterns of larval δ13C and methane concentrations in the sediment of a eutrophic polymictic lake, Izunuma, Japan. Larval δ13C decreased in late summer and autumn. Methane concentrations above a 6 cm depth peaked in late summer or autumn, while those in the 10–11- and 20–21-cm layers peaked in October. Negative correlations between methane concentrations in the 5–6/10–11-cm layers and larval δ13C were found. This suggests that an increase in the supply of methane stimulated the activity of MOB in a polymictic lake, where water above the lake bottom rarely became anoxic because of frequent overturn, thus increasing the dietary contribution of MOB to larval chironomids.

Differential Response of High-Elevation Planktonic Bacterial Community Structure and Metabolism to Experimental Nutrient Enrichment

Nutrient enrichment of high-elevation freshwater ecosystems by atmospheric deposition is increasing worldwide, and bacteria are a key conduit for the metabolism of organic matter in these oligotrophic environments. We conducted two distinct in situ microcosm experiments in a high-elevation lake (Emerald Lake, Sierra Nevada, California, USA) to evaluate responses in bacterioplankton growth, carbon utilization, and community structure to short-term enrichment by nitrate and phosphate. The first experiment, conducted just following ice-off, employed dark dilution culture to directly assess the impact of nutrients on bacterioplankton growth and consumption of terrigenous dissolved organic matter during snowmelt. The second experiment, conducted in transparent microcosms during autumn overturn, examined how bacterioplankton in unmanipulated microbial communities responded to nutrients concomitant with increasing phytoplankton-derived organic matter. In both experiments, phosphate enrichment (but not nitrate) caused significant increases in bacterioplankton growth, changed particulate organic stoichiometry, and induced shifts in bacterial community composition, including consistent declines in the relative abundance of Actinobacteria. The dark dilution culture showed a significant increase in dissolved organic carbon removal in response to phosphate enrichment. In transparent microcosms nutrient enrichment had no effect on concentrations of chlorophyll, carbon, or the fluorescence characteristics of dissolved organic matter, suggesting that bacterioplankton responses were independent of phytoplankton responses. These results demonstrate that bacterioplankton communities in unproductive high-elevation habitats can rapidly alter their taxonomic composition and metabolism in response to short-term phosphate enrichment. Our results reinforce the key role that phosphorus plays in oligotrophic lake ecosystems, clarify the nature of bacterioplankton nutrient limitation, and emphasize that evaluation of eutrophication in these habitats should incorporate heterotrophic microbial communities and processes.

Seasonal differences in bacterial community composition following nutrient additions in a eutrophic lake

We examined the effects of nutrient amendments on epilimnetic freshwater bacteria during three distinct periods in the eutrophic Lake Mendota's seasonal cycle (spring overturn, summer stratification and autumn overturn). Microcosm treatments enriched solely with phosphorus containing compounds did not result in a large bacterial community composition (BCC) change or community activity response (assessed via alkaline phosphatase activity, APA) relative to the controls during any season. Treatments enriched with carbon- and nitrogen-containing compounds resulted in a dramatic BCC change and a large APA increase in the autumn and spring seasons, but only treatments receiving carbon, nitrogen and phosphorus (CNP) exhibited similar responses in the summer season. Despite the fact that the amendments created similar CNP concentration conditions across seasons, the BCC following amendment greatly varied among seasons. 16S rRNA gene sequence analysis indicated that many common freshwater bacterial lineages from the Alpha- and Betaproteobacteria class and Bacteroidetes phylum were favoured following nutrient (CNP) addition, but individual taxa were generally not favoured across all seasons. Targeted quantitative PCR analysis revealed that the abundance of the Actinobacteria acIB1 cluster decreased in all microcosms during all three seasons, while the Flavobacterium aquatile (spring) and ME-B0 (summer) clusters of Bacteroidetes increased following CNP addition. These results suggest a particular bacterial group is not universally favoured by increased nutrient loads to a lake; therefore, efforts to predict which bacteria are involved in nutrient cycling during these periods must take into account the seasonality of freshwater bacterial communities.

Vertical mixing, size change and resting stage formation of the planktonic diatom Aulacoseira baicalensis

Aulacoseira baicalensis (K. Meyer) Simonsen is a freshwater planktonic diatom that undergoes large seasonal changes in cell morphology related to changes in vertical mixing. Short cells (10–20 µm) with thin walls were formed under the ice of Lake Baikal but cell lengths increased up to 150 µm by the time mixing depth reached over 100 m in June. These long cells became resting stages that were packed with reserve products and had siliceous walls up to 4 µm thick. Increase in mixing depth gave access to sufficient silica for completion of resting stages in most years but not in high biomass years, which has long-term implications for the population. Wall thickening reduced the risk of dissolution during dormancy but it also reduced cell volume. Therefore, by increasing length, cells maintained storage space for reserves. Seasonal changes in valve length showed that individual valves did not last more than 6 months, equivalent to 5 to 10 divisions. Separation valves were important in determining the number of cells per filament during spring growth but cell breakage became more important during summer dormancy. Resting stages survived in cool, intermediate depths (50–150 m) during summer stratification and were returned to the surface during autumn overturn.

Some generalizations based on stratification and vertical mixing in meromictic Lake Shira, Russia, in the period 2002–2009

In a brackish, temperate, 24-m-deep Lake Shira, the profiles of salinity, temperature, oxygen and sulfide concentrations were measured on a seasonal basis from 2002 to 2009. The lake was shown to be meromictic with autumnal overturn restricted to mixolimnion. The depth of mixolimnion and position of oxic–anoxic interface varied annually. The spring mixing processes contribute to the formation of mixolimnion in autumn. The exceptionally windy spring of 2007 caused the deepening of mixolimnion in the winter of 2008. The winter position of oxic–anoxic interface was affected by the position of lower boundary of mixolimnion in all winters. The salinity in the winter mixolimnion increased compared with the autumn because of freezing out of salts from the upper water layers meters during ice formation and their dissolution in water below. The profiles of salinity and temperature were simulated by the mathematical 1-D model of temperature and salinity conditions taking into account ice formation. The simulated profiles generally coincided with the measured ones. The coincidence implies that simplified one-dimensional model can be applied to roughly describe salinity and density profiles and mixing behavior of Lake Shira.