Warm Monomictic Lake Research Articles

Diversity and structure of prokaryotic assemblages in a thermally stratified tropical lake

The structure and diversity of prokaryotic assemblages in inland waters are strongly related to their habitat. Yet, there is a lack of integrative studies surveying them in different compartments in tropical lakes. This study estimated the diversity and composition of prokaryotic assemblages in sediments, microbialites, and water column in the tropical warm monomictic Lake Alchichica and identified the most representative phylotypes while thermally stratified. The 16S rRNA gene survey showed a distinctive prokaryotic assemblage for each compartment, with 60–78 % of the phylotypes being unique to each one. Sediments accounted for the major prokaryotic diversity in the lake, with chemoorganotrophic bacteria and sulphate reducers as major biogeochemical components. Microbialites showed a high abundance of oxygenic and anoxygenic phototrophs, as well as organotrophs. The prokaryoplancton in the water column was mainly composed of picocyanobacteria and Flavobacteria in the epilimnion, shifting to a high diversity of anaerobic microorganisms in the hypolimnion. These water column assemblages were significantly correlated to pH, ORP, DO, and nutrient concentration. This study shows an integral view of the prokaryotic assemblages in a deep tropical lake related to physicochemical characteristics, from their diversity to their biogeochemical implications, suggesting the role of lake sediments as a possible microbial seed bank.

Concentration of total microcystins associates with nitrate and nitrite, and may disrupt the nitrogen cycle, in warm-monomictic lakes of the southcentral United States

Cyanobacterial blooms and the toxins they produce pose a growing threat worldwide. Mitigation of such events has primarily focused on phosphorus management and has largely neglected the role of nitrogen. Previous bloom research and proposed management strategies have primarily focused on temperate, dimictic lakes, and less on warm-monomictic systems like those at subtropical latitudes. The in-lake conditions, concentration of total microcystins, and microbial functioning of twenty warm-monomictic lakes in the southcentral United States were explored in the spring and summer of 2021. Our data revealed widespread microcystins in lakes across this region, some of which exceeded regulatory limits. Microcystins were higher in the spring compared to the summer, indicating that warm-monomictic lakes, even across a large range of precipitation, do not follow the trends of temperate dimictic lakes. Microcystins were found in surface waters and bottom waters well below the photic zone, reflecting the persistence of these toxins in the environment. Principal components analyses showed a strong association between microcystins, nitrate + nitrite, and Planktothrix relative abundance and transcriptional activity. Many systems exhibited stronger denitrification in the spring, perhaps contributing to the decreased toxin concentrations in the summer. Counter to most sampled lakes, one lake with the highest concentration of total microcystins indicated nitrogen cycle disruption, including inhibited denitrification. These findings are relevant to mitigating cyanobacterial blooms and toxin production in warm-monomictic systems, and suggests a need to consider nitrogen, and not solely phosphorus, in nutrient management discussions.

Climate Change Impact on Inflow and Nutrient Loads to a Warm Monomictic Lake

This study analyses the impact of climate change on the inflows, sediment loads, and nutrient inputs to the Sabalan dam reservoir, a warm monomictic lake located northwest of Iran. For this purpose, the Soil and Water Assessment Tool (SWAT) was calibrated (2005–2018) and validated (2001–2004). Future climate-based data under the AR5 emission scenarios were obtained from the HadGEM2–ES general circulation model and then downscaled using the LARSWG 6.0. The tuned SWAT model was used to investigate the climate change impact on the hydrological processes and pollution loads to the Sabalan dam reservoir. Our findings based on the Nash–Sutcliffe efficiency coefficient and the coefficient of determination indicated an acceptable performance of the SWAT model in the simulation of inflows, sediment loads, and nutrient inputs to the reservoir. Inflow and sediment load to the reservoir will increase during the period of 2030–2070 compared to the base period (1998–2018). The annual total nitrogen (phosphorus) load to the reservoir will increase by 8.5% (9.4%), 7.3% (8.2%), and 5% (3.4%) under the emission scenarios of RCP2.6, RCP4.5, and RCP8.5, respectively. An increase in sediment loads and nutrient inputs to the Sabalan dam reservoir will significantly exacerbate the reservoir eutrophic condition, leading to water quality deterioration with acute consequences for the positive functions of the dam.

Spatial and Temporal Dynamics of Key Water Quality Parameters in a Thermal Stratified Lake Ecosystem: The Case Study of Lake Mead

Lake Mead located in the Arizona–Nevada region of the Mohave Dessert is a unique and complex water system whose flow follows that of a warm monomictic lake. Although monomictic lakes experience thermal stratification for almost the entire year with a period of complete mixing, the lake on occasion deviates from this phenomenon, undergoing incomplete turnovers categorized with light stratifications every other year. The prolonged drought and growing anthropogenic activities have the potential to considerably impact the quality of the lake. Lake Mead and by extension the Boulder Basin receive cooler flow from the Colorado River and flow with varying temperatures from the Las Vegas Wash (LVW), which impacts its stratification and complete turnovers. This study analyzes four key water quality parameters (WQPs), namely, total dissolved solids (TDS), total suspended solids (TSS), temperature, and dissolved oxygen (DO), using statistical and spatial analyses to understand their variations in light of the lake stratifications and turnovers to further maintain its overall quality and sustainability. The study also evaluates the impacts of hydrological variables including in and out flows, storage, evaporation, and water surface elevation on the WQPs. The results produced from the analysis show significant levels of TDS, TSS, and temperature from the LVW and Las Vegas Bay regions compared with the Boulder Basin. LVW is the main channel for conveying effluents from several wastewater treatment facilities into the lake. We observed an increase in the levels of TDS, TSS, and temperature water quality in the epilimnion compared with the other layers of the lake. The metalimnion and the hypolimnion layer, however, showed reduced DO due to depletion by algal blooms. We observed statistically significant differences in the WQPs throughout various months, but not in the case for season and year, an indication of relatively consistent variability throughout each season and year. We also observed a no clear trend of influence of outflows and inflows on TDS, temperature, and DO. TSS concentrations in the lake, however, remained constant, irrespective of the inflows and outflows, possibly due to the settling of the sediments and the reservoir capacity.

Potential risk of eutrophication in the deepest lake of Southwest China: Insights from phosphorus enrichment in bottom water

Large deep lakes in plateau regions provide crucial ecosystem services but are susceptible to eutrophication due to their long water residence time. To date, the water quality of deep lakes has not received as much attention as that of shallow lakes owing to logistical challenges. This study investigated the seasonal variation and vertical distribution of phosphorus and related environmental variables in a large deep lake in the Yunnan Plateau, China (Fuxian Lake). Generally, the concentrations of total phosphorus (TP, R2 = 0.862), total dissolved phosphorus (TDP, R2 = 0.922), and dissolved inorganic phosphorus (DIP, R2 = 0.889) exhibited a linear increase with the greater water depth, whereas the pH and dissolved oxygen (DO) showed decreasing trends. The TP, TDP, and DIP values were 0.012, 0.006, and 0.004 mg/L, respectively, in surface waters (0.5 m depth), and increased to 0.074, 0.065, and 0.062 mg/L, respectively, at 140.0 m depth. The averaged over ordering method demonstrated that DO and air temperature accounted for a higher proportion of the explained variance of TP, TDP, and DIP in the shallow water layer (0.5–20.0 m). In contrast, DO and pH accounted for a higher proportion of the explained variance of TP, TDP, and DIP in deeper water layers (40.0–150.0 m). As a warm monomictic lake, the higher observed phosphorus concentrations in deeper water and sediment potentially pose a risk of future eutrophication in the Fuxian Lake. Our findings demonstrate that more efficient technical and management measures should be taken to reduce the external phosphorus load to Fuxian Lake, so that the load to and from the sediment will decrease eventually.

Effects of Seasonal Variation on Water Quality Parameters and Eutrophication in Lake Yangzong

Understanding the seasonal variation characteristics and trends in water quality is one of the most important aspects for protecting and conserving lakes. Lake Yangzong water quality parameters and nutrients, including water temperature, dissolved oxygen (DO), pH, conductivity, Chlorophyll-a, phycocyanin, total nitrogen (TN) and total phosphorus (TP), were monitored in different seasons from 2015 to 2021. Based on the monitoring data, the temporal and spatial variations of various parameters were analyzed. The results showed that Lake Yangzong is a warm monomictic lake. The Pearson correlation coefficient and correlation analysis showed water quality parameters were significantly correlated and probably affected by temperature. Cyanobacteria were at risk of blooming in spring and autumn. The contents of TN and TP in winter were significantly higher than in summer, especially TN, with both reaching a peak at the epilimnion and hypolimnion in December 2020 (TN = 1.3 mg/L, TP = 0.06 mg/L). We also observed a dual risk of endogenous release and exogenous input. Therefore, strengthening the supervision for controlling eutrophication caused by human activities and endogenous release is urgently needed.

Light as a controlling factor of winter phytoplankton in a monomictic reservoir

Factors affecting the seasonal succession of plankton communities in freshwater temperate lakes have been thoroughly studied for decades. However, there are still relatively few data describing the winter season patterns in detail, as the focus has been mostly on spring to autumn conditions. Ice cover is often the crucial factor limiting light availability for winter phytoplankton, but in warm monomictic lakes is usually lacking and the gradually increasing solar radiation should, theoretically, drive phytoplankton growth. In this study conducted in 2002–2010, we documented regular sharp increases in phytoplankton chlorophyll a, starting just after the winter solstice and lasting throughout the total circulation and/or unstable inverse stratification period in the monomictic Slapy reservoir (Czechia). Chlorophyll a concentrations analysed in one-week intervals reached their yearly minimum of 0.2–0.8 µg L−1 in the solstice period, and the spring peak occurred before the onset of stable thermal stratification. The regular pattern was slightly disrupted in some years, associated with short periods of ice cover. Winter phytoplankton were species poor and dominated by diatoms, cryptophytes, green algae, and cyanobacteria. Using semiparametric regression approach, we aimed to test if selected environmental parameters had a significant effect on the observed winter trend. The resulting model revealed that solar radiation and water temperature positively influenced log chlorophyll a concentrations, whereas water age had a significant negative effect. On the other hand, zooplankton density and ice cover effects were not significant. The shapes of the marginal effects of water temperature and solar radiation were nonlinear, and the interaction of these two major factors was significant. The model-based estimated chlorophyll a concentrations showed a shift from radiation dominance to temperature-positive effects along the temperature gradient. This might represent as yet neglected pattern of phytoplankton seasonal development in warm monomictic lakes worldwide.

Study of vertical distribution dynamics of zooplankton in a thermally stratified warm monomictic lake of Kashmir Himalaya

AbstractIn the present study, we investigated how stratification patterns and the related physico‐chemical characteristics impact the vertical distribution of zooplankton community during the two consecutive stratifications in a warm monomictic lake (Manasbal Lake) in the Ganderbal area of Central Kashmir. Two consecutive events of stratification and deepening of thermocline recorded during the sampling period led to significant changes in the vertical distribution of zooplankton community. Physico‐chemical characteristics displayed significant changes with the stratification pattern. Temperature of the surface water (0 m) increased gradually from 13°C to 29°C, resulting in a higher thermal gradient. pH and dissolved oxygen (O2) displayed clinograde curves with vertical depth. Keratella cochlearis and Polyarthra vulgaris (Rotifers) were most dominant in terms of density and abundance at the thermocline and significantly decreased at the hypolimnion. Cladocera with significant difference along the vertical column showed an epilimnetic distribution with Ceriodaphnia reticulata and Ceriodaphnia quadriangularis being the major contributors. Among copepods, Cyclops viridis was recorded at all depths in greater dominance. Acanthodiaptomus denticornis (Diaptomidae) showed higher abundance from epilimnion to thermocline but decreased at hypolimnion. The results showed that the strong vertical gradient of environmental factors occurs in the Manasbal lake. Among them, dissolved oxygen, water temperature, and nutrients controlled the zooplankton community with higher aggregation above the thermocline. Therefore, the present investigation regarding the dynamic patterns of zooplankton community along the vertical gradient in a stratified lake could be important in monitoring habitats and to understand the ecology of warm monomictic lakes.

Thermal mixing of Lake Erhai (Southwest China) induced by bottom heat transfer: Evidence based on observations and CE-QUAL-W2 model simulations

The thermal structure of a lake is strongly associated its hydrodynamics and climatic conditions, and is an important basis to understand the physical, chemical, and ecological processes of the lake. Lake Erhai (100°05′–100°17′N and 25°36′–25°58′E) is a large and deep lake in Yunnan Province, located on the Yunnan–Guizhou Plateau of Southwest China. Some deep lakes located in areas adjacent to Lake Erhai with similar climatic conditions and water depths are warm monomictic lakes. However, the historical data of the surface water temperature and bottom water temperature for the past 15 years (2004–2018) showed small differences (<1 °C). However, ours and some other studies on the vertical water temperature profile showed mixing of the entire water column in different seasons. We have applied a well-established hydrodynamics model (CE-QUAL-W2) to explore the main process that induces the particular thermal structure of Lake Erhai. Model calibration was conducted twice, once with the extinction coefficient (Kd) and fraction of solar radiation absorbed in the surface layer (BETA) as the main calibrated parameters, and the other considering the process of bottom-water heat exchange, represented by the sediment temperature (TSED) parameter in the CE-QUAL-W2 model. The simulation results showed that the model can reproduce water level and surface water temperature accurately with the first calibration (TSED is 14.8 °C), however, only when the TSED is greater than 24 °C, the RMSE can be less than 2 °C. The model reproduces the bottom water temperature well when the TSED value was 29.6 ± 0.37 °C. Moreover, a series of scenarios were conducted to quantify the effect of wind speed and/or tributary inflows temperature on the thermal structure of Lake Erhai, and two stability indices (Schmidt stability, Birgean work) were calculated to quantify the energy required for stratification and mixing. These results demonstrated that the most likely process to establish the holomixis of Lake Erhai is geothermal heat transfer from sediment to water column, and an additional 12% heat flux from sediment plus the actual external energy supplies from sediment are required to mix the entire water column of Lake Erhai in summer.

Benthic macroinvertebrate communities of three tropical, warm monomictic Mexican lakes

Most studies of benthic macroinvertebrate communities are from shallow lakes or restricted to the littoral zone of deep, temperate lakes, with just a few dealing with the deep benthos. Furthermore, the deep benthic macroinvertebrate communities of tropical lakes are almost unknown. The present work describes the benthic macroinvertebrate communities of three tropical, warm monomictic lakes in “Lagunas de Montebello” National Park, Mexico, by describing the differences along the bathymetric profile, from the littoral down to the profound benthos. We studied the benthic macroinvertebrate communities in the two contrasting hydrodynamic periods of the warm monomictic lakes: a) stratification, when the hypolimnion becomes anoxic, and b) mixing, when the water column becomes oxygenated. We expected: 1) a reduction in the benthic macroinvertebrate taxonomic richness, density, and biomass from the littoral to the deep zone, 2) an impoverished benthic macroinvertebrate community while stratified (anoxia) compared to mixing (oxygenated), and 3) depletion in the taxonomic richness, density, and biomass of the profundal benthic macroinvertebrates in the tropical compared to temperate lakes. We found: 1) a decreasing trend in taxonomic richness (6 ± 2–3 ± 1 taxa), density (1868.7 ± 1069.7–349.1 ± 601.8 in. m−2) and biomass (277.8 ± 188.9–85.1 ± 95.6 mg C m−2) from the littoral to the deep zone; chironomids dominated the littoral zone, while oligochaetes dominated the deep zone. 2) Lower density and biomass but not taxonomic richness while stratified (4 ± 3 taxa; 586.2 ± 527.6 in. m−2; 81.6 ± 164.3 mg C m−2) compared to mixing (4 ± 3 taxa; 877.5 ± 1051.4 in. m−2; 190.1 ± 131.1 mg C m−2). 3) lower taxonomic richness and density but not biomass in tropical Montebello oligotrophic lakes (3 ± 3 taxa; 349.1 ± 601.8 in. m−2; 85.1 ± 195.6 mg C m−2) compared to temperate analogous (2–48 taxa; 492−83,189 8 in. m−2; 0.13−201.5 mg m−2). We conclude the early onset and long-lasting hypolimnetic anoxia restrict the benthic macroinvertebrate community radiation and diversification in tropical, oligotrophic, warm monomictic lakes.

Seasonal variation of &lt;em&gt;Microcystis aeruginosa&lt;/em&gt; and factors related to blooms in a deep warm monomictic lake in Mexico

The occurrence of cyanobacterial blooms has increased globally over the last decades, with the combined effect of climate change and eutrophication as its main drivers. The seasonal dynamic of cyanobacterial blooms is a well-known phenomenon in lakes and reservoirs in temperate zones. Nevertheless, in the tropics, most studies have been performed in shallow and artificial lakes; therefore, the seasonal dynamic of cyanobacterial blooms in deep and eutrophic tropical lakes is still under research. We studied the seasonal variation of the phytoplankton community and the factors associated with Microcystis aeruginosa blooms along the water column of Lake Alberca de Tacámbaro, a warm monomictic crater lake located in Mexico, during 2018 and 2019. According to previous studies performed in 2006 and 2010, this lake was mesotrophic-eutrophic, with Chlorophyta and Bacillariophyta as the dominant groups of the phytoplankton community. During 2018 and 2019, the lake was eutrophic and occasionally, hypertrophic, a phenomenon likely associated with the increase of farmland area around the lake. The dominant species was M. aeruginosa, forming blooms from the surface to 10 m depth in winter, in the hypolimnion in spring and summer, and along the full water column in autumn. These findings suggest that M. aeruginosa in Lake Alberca de Tacámbaro displays seasonal and spatial population dynamics. Total phosphorus, dissolved inorganic nitrogen, water temperature and photosynthetically active radiation were the environmental factors related to M. aeruginosa blooms. Our results suggest that the changes in the structure of the phytoplankton community through time, and M. aeruginosa blooms in Lake Alberca de Tacámbaro, are mainly related to changes in land use from forest to farmland in areas adjacent to the lake, which promoted its eutrophication in the last years through runoffs. Comparative studies with other deep and eutrophic lakes will allow us to gain a deeper understanding of the dynamic of cyanobacterial blooms in natural and artificial water reservoirs strongly stressed by human activities.

Variations in dissolved O2 in a Chinese lake drive changes in microbial communities and impact sedimentary GDGT distributions

Glycerol dialkyl glycerol tetraethers (GDGTs) are widespread but unique membrane-spanning lipids of many Archaea and some Bacteria. However, their specific biological sources and the associated environmental controls on their distribution remain unclear, especially in lacustrine settings, hindering our understanding of these compounds and their application as environmental proxies. Here we investigated the GDGT distributions across a large bottom water dissolved oxygen (DO) gradient (0.10–7.20 mg/L) using surface sediments of Lake Yangzonghai, a warm monomictic lake in southwestern China. We show that the distributions of both branched (br) and isoprenoidal (iso) GDGTs co-vary significantly with bottom water DO concentration. The relative abundances of tetramethylated brGDGTs, 5-methyl penta- and hexamethylated brGDGTs and isoGDGT-0 show a stepwise increase as bottom water DO concentration decreases. On the other hand, the relative abundances of 6-methyl penta- and hexamethylated brGDGTs and crenarchaeol exhibit a stepwise decline with a decrease in bottom water DO concentration. Genetic data indicate these DO-induced changes in GDGT parameters are related to changes in the bacterial and archaeal communities across the oxycline in the lake. For example, the high abundance of isoGDGT-0 in low DO samples coincides with a high abundance of methanogenic archaea and Bathyarchaeota. We propose that brGDGT-producing bacteria might include a diversity of other phyla in addition to the proposed source organism acidobacteria; different groups of anaerobic and aerobic bacteria are likely to contribute to the increased abundance of 5-methyl brGDGTs and 6-methyl brGDGTs in low and high DO zones, respectively. Consequently, the MBT′ and MBT′6ME brGDGT indices display strong correlations with DO concentration. Importantly, the MBT′5ME index is not significantly influenced by changes in DO concentration, suggesting that this index might be more resilient to these impacts and more suitable to reconstruct temperature in lake systems.

First observation of incomplete vertical circulation in Lake Biwa

Vertical circulation reaches the bottom of the deepest (> 90 m) part of the northern basin of Lake Biwa, a warm monomictic lake, during winter (January to March) every year. In winter 2019, we found non-circulated water masses at the bottom of the lake where the basin is 90 m deep. This is the first observation of an incomplete vertical circulation event since we started monitoring Lake Biwa in 1979. Analysis of water quality monitoring and meteorological data shows that weaker cooling and weaker winds during winter 2019 affected vertical circulation. Dissolved oxygen concentrations in the bottom layer recovered even after the appearance of a seasonal thermocline. However, the concentrations remained below that in the surface layer.

Assessing the Nutrient Dynamics in a Himalayan Warm Monomictic Lake

Thermal stratification is considered to be the most important limnological feature of deep lake ecosystems affecting a water column’s chemical characteristics thereby directly connected with lake management. The lake is also attracting the attention of management authorities, being a growing tourist destination with sustainable environmental management plan. Lake Manasbal is only true warm monomictic urban lake in Kashmir and remains thermally stratified for 8–9 months, with separate layers of epilimnion, metalimnion and hypolimnion.We noticed that in the lower part of the hypolimnion, the process of stratification is characterized by an oxygen deficit (O2 < 1 mg/L). The rate of oxygen depletion reported in the lake’s bottom layers is indicative of the water body’s more eutrophic nature. A clinograde type of pH curve with a wide range of fluctuation from top to bottom layers of water was observed. Thermal stratification of the lake allows ammonium and phosphates to pool in the hypolimnion for much of the stratification period, resulting in concentrations higher than in the epilimnion. Consequently, the lake was characterized by a nutrient-enriched hypolimnion and an epilimnion deprived of nutrients. With the advent of the lake stratification, the thermocline was formed on 15th March between 2 and 3 m and on 7th April between 4 and 6 m, which gradually moved down to 10 m on 4th July and remained at that depth until circulation. The concentration of phosphorus and nitrogen in Lake Manasbal showed an increase towards the bottom, thus revealing an inverse clinograde depth distribution. Due to stratification phenomena in this lake, the internal release and subsequent distribution of nutrients throughout water column especially phosphorus from sediment water interface after mixing is a cause of concern for management authorities as it has the potential to diminish the ecological and aesthetics of the lake.Therefore, we argue that this study will provide some valuable insights on how stratification is affecting the nutrient dynamics and will offer inputs for better management accordingly both from ecological and economic perspectives.

Mixing patterns and deep chlorophyll a maxima in an eutrophic tropical lake in western Mexico

In many temperate oligotrophic lakes, algal accumulations can form below the mixing zone. However, Deep Chlorophyll Maxima (DCM) have also been found in some eutrophic, tropical lakes and in this paper we aim to identify if they are recurrent features in these kinds of lakes and to recognize the factors that favor their formation. We analyzed 5 years of thermal stratification, water quality, and chlorophyll a concentrations in a tropical eutrophic lake in Central Mexico. Thermal stratification patterns were characteristic of warm monomictic lakes. Full water column deoxygenation during winter mixing was recorded in 3 of the analyzed years, and an increase of ~ 1 °C in the hypolimnion was detected between 2011 and 2015. DCM were detected in 4 out of the 5 studied years, at the top of the hypolimnion when the water column was stratified (spring–summer). This study is the first report of recurrent DCM formation in the northern limit of the Neotropics. It confirms that high light penetration is a necessary condition for DCM. Stratified nutrients with epilimnetic P depletion are also factors favoring DCM formation.

Seasonal vertical stratification and the potential impacts of its fading on algal blooms in winter: Cases of Lake Chenghai and Lake Yangzonghai, Yunnan Plateau

基于2014年10月-2016年7月在云南程海和阳宗海开展了4个季度（秋季，10月；冬季，1月；春季，4月；夏季，7月）的调查，研究了两个湖泊的水体分层特征，探讨了热力分层及其变化与其他环境因子对浮游植物生物量的潜在影响，结果显示：程海和阳宗海水体分层的特征均为冬季混合、春季形成分层、夏秋季分层稳定，两湖均属暖单次混合型湖泊；程海分层期温跃层的平均深度（顶界）、厚度和强度分别为17.70±3.89 m、5.54±4.44 m和0.67±0.43℃/m，阳宗海的分别为12.53±3.35 m、8.25±4.85 m和0.53±0.43℃/m.在热力分层稳定期，两湖底层达到缺氧甚至厌氧状态，底层的电导率总体较表层高.调查期间，两个湖泊水柱表层浮游植物生物量（以叶绿素a浓度表征）均在冬季出现峰值，程海和阳宗海的分别为19.22±11.08和45.82±9.41 μg/L；进一步分析发现，热力分层的消退可能是导致水体表层无机营养盐升高（底层供给）的重要原因，加之适宜的光热条件可诱导两湖冬季水华的发生；在其他季节转化期间，浮游植物生物量变化的主要影响因子亦具有一定的共性及湖泊与季节异质性.;The thermal stratification and its potential impacts on the phytoplankton biomass were studied through the four seasonal field investigations (October 2014-July 2016) in Lake Chenghai and Lake Yangzonghai, two mesotrophic deep lakes on the Yunnan Plateau, southwest China. The results revealed that these two lakes are both warm monomictic lakes, where the vertical stratification was occurred in spring, stabilized in summer and autumn, and disappeared in winter. The mean values of thermocline depth (the upper boundary), thickness and strength during the stratification periods (spring, summer and autumn) were 17.70±3.89 m, 5.54±4.44 m and 0.67±0.43℃/m in Lake Chenghai and were 12.53±3.35 m, 8.25±4.85 m and 0.53±0.43℃/m in Lake Yangzonghai, respectively. During the stabilized thermal stratification periods, the dissolved oxygen in the bottom of the water column reached less than 3 mg/L even 1 mg/L, and the specific conductance in the bottom was generally larger than that in the surface water column. In these two lakes, the maximum values of the seasonal-average algal biomass (represented by chlorophyll-a concentration) were both in winter, which was 19.22±11.08 μg/L in Lake Chenghai and 45.82±9.41 μg/L in Lake Yangzonghai. Moreover, the fading of thermal stratification may be an important factor that increases the inorganic nutrients in the surface of water column (from the bottom of water column). Thereafter, higher nutrients combined with suitable light conditions and water temperature could promote the occurrence of algal blooms in winter. Besides a certain similarity, the major influencing factors of seasonal phytoplankton biomass also had inter-lake and seasonal heterogeneity during the other periods of season changes.

Stratification of microbiomes during the holomictic period of Lake Fuxian, an alpine monomictic lake

AbstractIn warm monomictic lakes, the hypolimnion is important for accumulating and decomposing organic matter derived from surface production, and the regenerated nutrients will be supplied to the epilimnion through winter vertical mixing. So far, we know little about microbial community composition and function in the hypolimnion when the significant thermal stratification disappears. In this study, we investigated microbial community compositions and functional gene contents by means of metagenomics along a depth profile in the warm monomictic alpine Lake Fuxian during holomictic period. Overall, bacteria were the dominant microbial group at different water depths, while phages had their high relative abundance in the epilimnion. We observed slight thermal but strong chemical stratification even during this typical winter overturn. The anaerobic respiration with nitrate and sulfate as the terminal electron acceptors was accumulated at bottom of hypolimnionin as indicated through metabolic pathway reconstruction. We were able to get 440 metagenome‐assembled genomes (MAGs) and unraveled a high genomic diversity of freshwater pelagic microbiomes along this depth profile. We furthermore defined a new class of “Plancto_FXH1” of Planctomycetes from these MAGs, of which a distinct nitrate reduction operon was identified. Representatives of this phylum mainly thrive in the hypolimnion as previously suspected, but few lineages were detected in the epilimnion. In summary, metagenomics enabled us to find a new group of Planctomycetes, probably involved in denitrification in the hypolimnion in Lake Fuxian, which expand our knowledge on denitrifying bacterial diversity and their denitrification potential in deep freshwater lakes.

Comparative study of biochemical, histological and molecular biomarkers of heavy metal contamination in Cyprinus carpio collected from warm-monomictic lake and government culture pond.

The study investigated the metallothionein (MT) and glutathione peroxidase (GPX) genes expression in freshwater fish Cyprinus carpio dwelling in Warm-monomictic Lake (Dal) and Government culture pond. Oxidative stress induced by heavy metals in the fish body manipulates stress genes expression resulting in the production of scavenger protein (for free metal ions) metallothionein. It interacts with Cu, Cr, Ni and Cd via metal-thiolate bond relieving the metal load from fish body. Maximum fold change was observed in liver, muscle and midgut tissue. Similar rise seen in GPX indicates defence against lipid peroxidation. MT and GPX genes data were compared with beta-actin gene used as an internal control. Limnological studies of both the sites (temperature, dissolved oxygen, pH, total dissolved solids, conductivity), were essential to ensure the quality of water in which the dominant species (C. carpio) was thriving, as these fishes are transported for human consumption. Heavy metal concentration (water, tissues, gut content), bioindices, biochemical parameters and histological alterations were studied to observe the impact of elements selected.

Factors explaining the yearly changes in minimum bottom dissolved oxygen concentrations in Lake Biwa, a warm monomictic lake

Vertical profiles of dissolved oxygen (DO) and water temperature (WT) measured bi-monthly for 36 years (1980–2015) near the deepest part of a warm monomictic lake were analyzed with special reference to yearly minimum DO at bottom (DOmin). DOmin changed yearly (3.0 ± 1.2 mg l−1) and significant differences in DOmin were not observed between Period I (1980–1993; cooler and worse in water quality) and Period II (1994–2015; warmer and better in water quality). This unclear trend in DOmin was probably due to the offsetting influences between warming induced by global warming and oligotrophication attempted by local governments etc. for the study period. DOmin was positively correlated with disturbance time (timing of last cold water intrusion observed from Mar to Aug), which could be related to the start of DO depletion at bottom. Thus, the linear model using this parameter could predict yearly DOmin fairly well for the entire study period (r2 = 0.60). In addition, DOmin and time of disturbance were correlated negatively with water density at bottom in Jan and positively with water density equilibrated to air temperature (AT) in Mar. Higher lake water density after full depth mixing advances the disturbance time. In contrast, lower AT in Mar and/or higher density of influent water after Mar delays the time likely due to the larger amount of snowfall in the watershed. Further, DOmin was positively correlated with maximum wind velocity in Sep which probably induced the recovery of DO. Multiple-regression models to predict DOmin using these meteorological and water quality parameters were developed (r2 ≥ 0.38, worse performances than the model using disturbance time) to forecast future trends of DOmin through global warming and/or climate change. Significant influences of water or sediment oxygen demands on DOmin were not detected. We also discuss the applicability of the proposed models.

Quantifying the role of organic phosphorus mineralisation on phytoplankton communities in a warm-monomictic lake

ABSTRACTPhosphorus (P) is a key driver of phytoplankton growth in lentic systems. During summer-stratified periods when dissolved inorganic P (DIP) is low, the phytoplankton assemblage often has elevated P uptake rates. Under these conditions, dissolved organic P (DOP) can become an important source of P for phytoplankton and a driver of phytoplankton succession. We examined how DOP mineralisation by alkaline phosphatase (AP) activity (APA), relative to phytoplankton P release (i.e., from exudation and lysis), meets phytoplankton P uptake and how it is associated with phytoplankton community composition changes under thermally stratified DIP-depauperate conditions in a large warm-monomictic lake. We found that when phytoplankton internal P stores were low (<0.65 µg P µg−1 Chl-a), APA increased, resulting in potential DOP mineralisation rates of 88 µg L−1 h−1 P, which is at the upper end of rates in the literature. APA represented up to 89% of the total phytoplankton P uptake and was much higher than the rate of phytoplankton P release. We found that elevated APA coincided with peak phytoplankton biovolume and dominance of the phytoplankton assemblage by cyanobacteria. Further, APA was strongly positively associated with the majority of the 10 most dominant cyanobacteria, including the key toxin-producer Raphidiopsis raciborskii (previously Cylindrospermopsis raciborskii). Our results indicate that under DIP-depauperate conditions, DOP mineralised by APA may be a major source of P for phytoplankton, and the ability to produce AP may be a key factor in the occurrence of bloom-forming cyanobacteria.