Oligotrophic Lake Research Articles

The Ecosystem of Lake Tolpanjärvi, Western Karelia, Preserved in Natural State

The results of the study of Lake Tolpanjärvi’s ecosystem are reported. Its hydrological and geochemical indices, as well as the current condition of biotic communities (zooplankton, benthos and fish population), were studied and analyzed. Analysis of the chemical composition of water indicates the low organic matter content (total phosphorus 0.007 mg/l, nitrogen 0.26 mg/l) of the lake. The Tolpanjärvi is an oligotrophic lake, as indicated by the abundance of zooplankton (with a biomass of less than 1 g/m3) and zoobenthos (biomass 0.44 g/m2). The lake is of an oligosaprobic class (2nd quality class, clean natural water), as evidenced by its saprobity index estimated using Pantle–Bukk’s method. The quantitative indices of zoobenthos varied from 120 individuals/m2 and 0.08 g/m2 in the profundal zone to 1200 inds./м2 and 0.86 g/m2 in the quiet littoral zone. The lake is more similar in the chironomid index (К – 2.00) to clean natural water. It is inhabited by 8 fish species of 5 families. The fish population is dominated by the whitefish Coregonus lavaretus, the perch Perca fluviatilis and the roach Rutilus rutilus. Two ecological forms of whitefish, differing in both the number of gill rakers (sparsely-rakered 18–24 and medium-rakered 28–36) and biological indices (linear-weight growth рост, maturation, fertility and feeding), occur in the lake. Stable highly diverse ecosystems were shown to form the basis for the preservation of the gene pool and for assessment of the current condition of water bodies heavily affected by human activities.

Biofilm community composition is changing in remote mountain lakes with a relative increase in potentially toxigenic algae

Mountain lakes provide clear drinking water to humankind but are strongly impacted by global change. Benthic biofilms are crucial for maintaining water quality in these oligotrophic lakes, yet little is known about the effects of global change on mountain biofilm communities. By combining analyses of metabarcoding data on 16S and 18S rRNA genes with climatic and environmental data, we investigated global change effects on the composition of biofilm prokaryotic and micro-eukaryotic assemblages in a five-year monitoring program of 26 Pyrenean lakes (2016–2020). Using time-decay relationships and within-lake dissimilarity modelling, we show that the composition of both prokaryotic and micro-eukaryotic biofilm communities significantly shifted and their biodiversity declined from 2016 to 2020. In particular, analyses of temporal trends with linear mixed models indicated an increase in the richness and relative abundance of cyanobacteria, including potentially toxigenic cyanobacteria, and a concomitant decrease in diatom richness and relative abundance. While these compositional shifts may be due to several drivers of global change acting simultaneously on mountain lake biota, water pH and hardness were, from our data, the main environmental variables associated with changes for both prokaryotic and micro-eukaryotic assemblages. Water pH and hardness increased in our lakes over the study period, and are known to increase in Pyrenean lakes due to the intensification of rock weathering as a result of climate change. Given predicted climate trends and if water pH and hardness do cause some changes in benthic biofilms, those changes might be further exacerbated in the future. Such biofilm compositional shifts may induce cascading effects in mountain food webs, threatening the resilience of the entire lake ecosystem. The rise in potentially toxigenic cyanobacteria also increases intoxication risks for humans, pets, wild animals, and livestock that use mountain lakes. Therefore, our study has implications for water quality, ecosystem health, public health, as well as local economies (pastoralism, tourism), and highlights the possible impacts of global change on mountain lakes.

Interactions between light and wave exposure differentially affect epilithic algal biomass and productivity in two large lakes of different trophy

Light and nutrient availability determine the depth distribution of littoral attached algae, but erosional forces, such as wind waves, structure shallow, nearshore habitats. We measured the biomass and productivity of algae on cobbles (epilithon) in the wave-influenced erosional zones of two large lakes with contrasting trophic status, eutrophic Lake Erie and oligotrophic Lake Huron. We used fetch and wind speed to create an index of wave exposure for 10 sample sites and used regression models to determine the variation in algal biomass and productivity explained by physicochemical parameters. Lake Erie epilithon had greater biomass (average = 59–249 mg chlorophyll a m−2) and higher productivity (76–92 mg C m−2 hr−1) than Lake Huron epilithon (1–147 mg m−2 and 14–57 mg C m−2 hr−1, respectively). In Lake Erie, light and wave exposure had significant and opposite effects on algal biomass and productivity. Maximum algal biomass occurred between 1.5 and 3 m in Lake Erie, where the effects of waves begin to diminish and light was still relatively abundant. Conversely, Lake Huron biomass was uniformly low along the depth gradient. Although depth or wave exposure did not explain variation in biomass, Lake Huron epilithic productivity was best explained by the interaction between depth and wave exposure. Rocky nearshore areas in large lakes are dynamic habitats for attached algae, leading to high spatial variation in biomass and productivity. Understanding how physicochemical dynamics influence benthic algal biomass and productivity are necessary to predict their distributions in a changing world.

Contrasting dissolved organic matter biodegradation and bacterial cytometric features in oligotrophic and ultraoligotrophic Patagonian lakes

Abstract Biodegradation process of dissolved organic matter (DOM) could be affected by its concentration and quality. Two biodegradation experiments were conducted in two contrasting seasons in two oligotrophic and one ultraoligotrophic temperate lakes, which comprise a natural gradient of dissolved organic carbon (DOC) concentration and quality (including lakes of < 83.33 μM). We examined associations between biodegradation and bacterial cytometric descriptors (i.e. abundance, DNA relative content, etc.) and with different DOM characteristics. The biolabile DOC (BDOC) was higher in shallow oligotrophic lakes, which displayed higher DOC concentration than the deep ultraoligotrophic one. The BDOC was mainly relatable not only to DOC concentration but also with DOM quality [fluorescent terrestrial-humic component normalized by DOC (C2:DOC), lignin content (a*350) and biological index]. We found a positive association between biodegradation process and high nucleic acid–content bacterial group. The bacterioplankton cytometric features were shaped by DOM concentration and quality along the biodegradation process. We concluded that in oligotrophic lakes, DOC concentration was the main driver of BDOC, whereas in ultraoligotrophic lakes, DOM quality played a more central role. This study contributes to better understand the biodegradation process in pristine low-DOC lakes with natural variation in allochthonous DOM inputs and with implications in the biogeochemical cycles.

Innovative receiving phase for Chemcatcher® passive sampler for phosphorus in the water environment: Calibration of sampling rate by water temperature and pH

Passive sampling is a technique for monitoring orthophosphate (PO4-P) in the water environment. Compared with traditional grab sampling followed by PO4-P quantification, kinetic-type passive samplers such as Chemcatcher® express representative concentrations of PO4-P as time-weighted average concentrations (CTWA). They can also potentially evaluate much lower PO4-P concentrations, but the available receiving phases of Chemcatcher® used for PO4-P were extremely limited. We developed a new receiving phase, the PSfZS sheet, comprising a zirconium sulfate–surfactant micelle mesostructure and polysulfone matrix. We examined its performance in terms of PO4-P sorption characteristics, PO4-P selectivity, and PO4-P sampling rate (Rs). Its capacity was adequate (12.0 μg-P/cm2) and selectivity for PO4-P uptake was good. The Rs for PO4-P increased with increasing water temperature (8.1–29.1 °C) and decreasing pH (4.1–9.7) in a laboratory calibration, and ranged from 5.27 × 10−2 L/d to 1.66 × 10−1 L/d. We placed the samplers in a municipal wastewater treatment plant, a shallow eutrophic lake, and an oligotrophic caldera lake. The Rs in the deployment sites was calibrated by monitored water temperature and pH. The estimated CTWA of PO4-P in the municipal wastewater treatment plant was similar to the averaged concentration of soluble reactive phosphorus determined by multiple grab samplings. In the lake deployments, we found that the new sampler can quantify CTWA values of PO4-P below 10 μg/L, and thus it provides more technical monitoring options and contributes to the conservation and management of the water environment.

Reconstructing the Paleoenvironmental Evolution of Lake Kolon (Hungary) through Integrated Geochemical and Sedimentological Analyses of Quaternary Sediments

Lake Kolon (Hungary), situated in the middle of the Turjánvidék area between the saline lakes of the Danube valley and the Homokhátság, is one of the most significant natural aquatic habitats in the Danube–Tisza Interfluve region. The central question of this study is how the lake changed, and how environmental factors and human activities have influenced these paleoenvironmental changes in Lake Kolon. A multiproxy analysis of a core sequence (loss on ignition, grain size, magnetic susceptibility, and geochemistry) provided crucial insights. Notably, correlations are observed in the following relationships: (1) clay, organic matter, and elements derived from organic sources, such as Na, K, and Zn; (2) MS, sand, inorganic matter, and elements originating from inorganic sources, such as Fe, Al, Ti, Na, K, and P; and (3) carbonate content and elements originating from carbonate sources, such as Ca and Mg. The lake’s paleoenvironment underwent significant changes in the past 27,000 years. Late-Pleistocene wind-blown sand provided the bottom for an oligotrophic lake (17,700 BP), followed by a calcareous mesotrophic Chara-lake phase (13,800 BP). Peat accumulation, along with the eutrophic lake phase, began at the Pleistocene–Holocene boundary around 11,700 BP. From 10,300 BP, with the emergence of an extended peatland phase, the percentage of organic matter (peat) increased significantly. Anthropogenic changes occurred from around 9000–8000 BP due to the different emerging cultures in the Carpathian basin, and from 942–579 BP due to the Hungarian settlements and activity nearby, respectively.

Lake surface water temperature and oxygen saturation resistance and resilience following extreme storms: chlorophyll a shapes resistance to storms

ABSTRACT Extreme storms are becoming more frequent and intense with climate change. Assessing lake ecosystem responses to extreme storms (resistance) and their capacity to recover (resilience) is critical for predicting the future of lake ecosystems in a stormier world. Here we provide a systematic, standardized, and quantitative approach for identifying critical processes shaping lake ecosystem resistance following extreme storms. We identified 576 extreme wind storms for 8 lakes in Europe and North America. We calculated the resistance and resilience of each lake’s surface water temperature and oxygen saturation following each storm. Sharp decreases and increases in epilimnetic temperature and oxygen saturation caused by extreme storms resulted in unpredictable changes in lake resilience values across lakes, with a tendency not to return to pre-storm conditions. Resistance was primarily shaped by mean annual chlorophyll a concentration and its overall relationship with other physiochemical lake and storm characteristics. We modeled variation in resistance as a function of both lake and storm conditions, and the results suggested that eutrophic lakes were consistently less resistant to extreme storms compared to oligotrophic lakes. The lakes tended to be most resistant to extreme storms when antecedent surface waters were warm and oxygen saturated, but overall resistance was highest in lakes with low mean annual concentrations of chlorophyll a and total phosphorus. Our findings suggest physiochemical responses of lakes to meteorological forcing are shaped by ecological and/or physical feedback and processes that determine trophic state, such as the influence of differences in nutrient availability and algal growth.

Interactions between temperature and predation impact insect emergence in alpine lakes

AbstractWhile insect populations are simultaneously threatened by many local‐ and global‐scale stressors, the interacting effects of these factors remain poorly understood. These interactions between stressors, whether additive or antagonistic, may have profound effects on our assumptions about and predictions for any given system. Here we address this gap by exploring the interactive effects of introduced predators and elevated temperatures on emerging aquatic insects across alpine lakes. Using a crossed factorial field survey, we examine whether warmer temperatures either directly affect the magnitude or diversity of insect emergence, or mediate the impacts of predation via interactions with predator presence. Based on data from more than 11,000 insects collected across a two‐year period, we find a 71% reduction in insect emergence associated with an additional 3.6°C of water temperature in oligotrophic alpine lakes. While our work confirms prior findings that predator presence drives strong reductions in insect emergence, we find that the effects of predation are significantly weaker in warmer lakes (2% reduction in warmest lakes studied vs. 75% reduction in coldest). Combining our results with regional studies of predator communities to identify mechanisms suggests that the interaction effects between temperature and predator presence are likely due to changes in predator behavior across temperatures. Critically, to fully understand the multiple stressors that impact insect populations and predict their future effects, we must consider the interactions between stressors across spatial scales.

Diel variations in dissolved oxygen concentration and algal growth in the Laver pit lake, northern Sweden

The diel oxygen technique relying on automated in-situ measurements of dissolved oxygen was used as an indicator of seasonal and diurnal variations of photosynthesis in the circumneutral Laver pit lake in northern Sweden. From July to September 2017, surface water temperature, electrical conductivity, pH, and dissolved oxygen were continuously measured at a time resolution of 30 min using sensors mounted on a floating buoy. The data could be monitored in real-time using a browser-based software and permitted calculation of gross primary production (GPP), net ecosystem production (NEP), and respiration (R) in the lake. The dissolved O2 concentration showed a consistent pattern of diel variations that reached up to 0.5 mg L−1 during the warmer summer period (July–August). Towards the end of August these variations decreased in magnitude and remained at ∼0.1 mg L−1 throughout September. pH showed diel variations that mimicked those of dissolved O2, with maximum daily variations of 0.4–0.5 pH units during July and August. A seven-day moving average of GPP showed a peak during July to mid-August, and the maximum GPP value of 0.55 mg O2 L−1 day−1 is similar to those found in natural oligotrophic lakes. A phytoplankton sample showed a total biomass concentration of 24 μg L−1, with the species Chrysophyceae, Chlorophyta, and Bacillariophyta occurring in the water. Diel oxygen data indicated that respiration by autotrophs and respiration of autochthonous labile organic matter by heterotrophs dominated in the lake, as is often the case in lakes where planktonic primary production is the main supplier of labile organic carbon. A close coupling between R and GPP suggests that nearly all GPP was respired in the epilimnion. The study shows that the diel oxygen technique can be used for real-time monitoring of seasonal and diurnal variations of dissolved oxygen and pH in pit lakes. This would be a useful technique in pit lake remediation projects where fertilization is used to stimulate algal growth and metal sequestering by algae.

Excessive use of chemical fertilizers in catchment areas raises the seasonal pH in natural freshwater lakes of the subtropical monsoon climate region

In recent years, the pH of some oligotrophic natural freshwater lakes in the subtropical monsoon climate region has generally reached an extremely high level of approximately 9.0 during the summer and autumn. It has not been proven that the explanation for why carbon assimilation in productive lakes exceeds carbon supply and causes a rise in pH also applies to oligotrophic freshwater lakes. Regardless, this may have been caused by changes in the solute composition of inflowing rivers as a result of changes in land use in catchment areas. Beimiao Reservoir in Baoshan City, Yunnan Province, where pH has increased significantly in summer and fall over the past few years, was chosen for this research in order to verify this hypothesis. This paper evaluates the status quo of water quality and nutrition in this reservoir. In addition, this paper analyzes the effects of water temperature, the metabolism of aquatic organisms, and river input on the horizontal and vertical spatial differences, diurnal variations, seasonal changes, and annual changes in the pH of the reservoir. It was discovered that one of the primary causes of the pH increase in natural freshwater lakes in the subtropical monsoon climate region during summer and autumn can be summarized as follows: A large amount of nitric acid and sulfuric acid are produced in the soil when chemical fertilizers are overused in runoff areas, particularly during intensive horticultural crop cultivation. Therefore, the subtropical monsoon climate region's soil loses calcium and magnesium ions, whose combined equivalent concentrations significantly exceed those of bicarbonate ions. The combined equivalent concentrations of calcium ions and magnesium ions in the inflowing rivers in summer and autumn are also significantly higher than those in the lake, resulting in the depletion of free carbon dioxide caused by calcium carbonate precipitation and an increase in pH. The conclusion of this research casts new light on the preservation and treatment of the ecological environment of lake water with similar issues.

Late Pleistocene to Holocene Palaeohydrological History of the Thermal-Spring-Fed Lake Pețea (NW Romania) Revealed by Radiocarbon Dating and Complex Sedimentological Investigations

Understanding sedimentation processes in response to past hydrogeological and climatic changes and capturing millennial-scale variations is a key focus of lacustrine paleoenvironmental research. This study presents the first high-resolution chronology and sedimentary data for the small thermal-spring-fed Lake Pețea, NW Romania, and unravels the evolutionary history of the lake harboring a unique endemic fauna. Its small size and single source of water make it particularly sensitive to hydrological changes. In the recent past, over-exploitation of the thermal water has led to the complete drying up of the lake and the extinction of its fauna. Nevertheless, past spatio-temporal variation of environmental factors, in particular the fluctuation of lake levels and water temperature, must have had a significant impact on the survival and evolution of the endemic mollusk fauna. This fact makes this study particularly important. Based on our results, a three-stage sedimentary evolution occurred, mainly controlled by major climate-driven hydrological changes also seen in regional records, i.e., 17.5–14.5 ka shallow eutrophic lake, 14.5–5.5 ka oligotrophic carbonate-rich lake, and 5.5–0.5 ka shallow eutrophic lake. A major lowstand at 11.7–10.2 ka due to drier climate was followed by progressively rising water levels up to 5 ka followed by a drop. The main control on lake level fluctuations and sedimentary phases was the varying input of thermal water due to recurring increased/decreased recharge of the underground shallow karst water system. The driving factor of thermal water discharge was different during the Late Glacial than the Holocene. It was the warming of the climate at 14.5 ka cal BP and melting of regional ice sheets in addition to increased precipitation that created an oligotrophic lake by recharging the underground thermal water system. Conversely, during the Holocene, increasing/decreasing moisture availability driven by major climate forcings was in control of thermal water recharge, erosion, and fluctuating lake levels.

MACROPHYTES OF THE LITTORAL ZONE IN LAKES OF SPECIALLY PROTECTED NATURAL AREAS OF THE ARKHANGELSK REGION

The article analyzes distribution of 94 species of aquatic plants in 18 lakes, different in size, origin, trophic status, hydrological and hydrochemical conditions. Macrophytes and their communities were studied in 11 large and small lakes in the Kenozersky National Park, in 2 large lakes in the Onezhskoye Pomorye National Park, and in 5 small lakes in the Pinezhsky State Nature Reserve. The eutrophic lake Porzhenskoye in the Kenozersky National Park stands out with the greatest species number, 52 species. Fewer species of macrophytes were identified in oligotrophic lakes. It is shown that the majority of aquatic plants of the Arkhangelsk Region inhabit narrow range from low to medium total dissolved solids – 54 species. Eurybiont species, occurring in a large range of total dissolved solids from 1 to 1020 mg/l, show a great species diversity – 24 species. Only 11 species tend to grow in karst lakes with elevated level of total dissolved solids from 500 to 1020 mg/l – vascular plants: Ranunculus kauffmannii, Eleocharis acicularis, E. quinqueflora, Myriophyllum verticillatum, Potamogeton obtusifolius, Stuckenia pectinata, Sparganium hyperboreum; mosses: Calliergon cordifolium, Scorpidium scorpioides; algae: Chara aspera, C. subspinosa. The majority of species in the lakes of specially protected natural areas of the Arkhangelsk Region, growing in a narrow water pH range, are neutrophytes – 31 and neutro-alkalophytes – 27. Thirteen protected species of macrophytes were discovered in the flora of lakes in the Arkhangelsk Region – Chara aspera, C. strigosa, C. subspinosa, C. virgata, Fissidens fontanus, Fontinalis dalecarlica, F. hypnoides, Isoëtes echinospora, I. lacustris, Lobelia dortmanna, Nymphaea candida, N. tetragona, Sparganium gramineum.

Rainstorm events trigger algal blooms in a large oligotrophic reservoir

Algal blooms occasionally occur in oligotrophic lakes or reservoirs; however, the mechanisms and conditions that trigger bloom formation remain unclear. Therefore, high-frequency (every 3 days) monitoring and high-resolution (100 sites) surveying of the algal biomass were performed during the summer of 2018–2020 to explore rainfall effects on algal growth and its associated mechanism in Xin'anjiang Reservoir, a large, deep, and oligotrophic reservoir in the subtropical region of China. The results showed that, in the short-term, rainfall inhibited algal growth in the river zone, while promoted algal growth in the lake zone. Path analysis (PLS-PM) and correlation analysis revealed that rainfall inhibited algal growth in the river zone by increasing discharge and decreasing light availability (turbidity) and water temperature, while promoted algal growth in the lake zone by increasing the nutrient concentrations, especially for phosphorus. In addition, we found that the stronger the rainfall is, the wider regions of the reservoir it impacted. Monthly observations at 100 sites showed substantial increases in both algal biomass (mean Chla concentration doubled) and nutrients concentrations in almost the whole reservoir after rainstorm events in Xin'anjiang Reservoir, indicating that rainstorm-induced nutrient pulsing loading could trigger algal blooms in oligotrophic reservoirs in the long-term. Extreme weather events are predicted to occur more frequently as the global climate changes in the future, and oligotrophic reservoirs are facing a greater risk of algal blooms.

Sediment oxygen consumption in Antarctic subglacial environments

AbstractOxygen consumption in aquatic sediments is an indicator of overall biological activity of the ecosystem. As such, rates of sedimentary oxygen utilization are well documented for much of the open oceans and freshwater lakes. However, there are few direct measurements of sedimentary oxygen consumption from Antarctic subglacial aquatic sediments. We report the first microsensor oxygen profiles and derived sedimentary oxygen consumption rates from beneath the Ross Ice Shelf and a subglacial lake beneath the West Antarctic Ice Sheet. Rates of oxygen consumption in these two environments are relatively low, but comparable to those reported from ice‐free polar oceans and oligotrophic Arctic lakes. Our study demonstrates the presence of oxygen within Antarctic subglacial aquatic sediments and its importance for oxygen‐consuming microorganisms living in these ecosystems.

Species‐specific functional traits rather than phylogenetic relatedness better predict future range‐shift responses of odonates

Abstract Climate change has the potential to modify habitat characteristics and, consequently, induce species responses to ongoing environmental changes. Functional traits determine both if a species can persist and maintain stable population sizes in particular ecological conditions, and its capacity to disperse to more favourable habitats. Given that functional traits evolve over time, one could expect closely related species to show similar responses to climate change, which should identify vulnerable lineages. Alternatively, species‐specific functional traits may anticipate species responses to climate change, and therefore, trait composition should be a strong predictor. We compiled a comprehensive dataset of functional traits of 84 Iberian and Moroccan odonates species and built a phylogenetic tree to determine if dissimilarity of traits and phylogenetic relatedness are relevant to better discriminate species range‐shift responses to climate change. Modelling results for 66 species showed clear impacts of the increase in temperature and drought events on their potential distribution. The traits that best‐discriminated species that expanded their ranges were multivoltinism, short‐life cycles and preference for temporary habitats, whereas species with a reduced and displaced potential distribution were mostly semivoltine, with a short flying season, oviposition on gravel and restricted to permanent streams, small rivers or oligotrophic lakes. Trait conservatism was rejected and phylogenetic relatedness was a poor predictor of range shifts. Considering odonates as model organisms, traits such as voltinism, beginning and prolongation of flight period and preference of temporal habitats should be examined to anticipate range‐shift responses of freshwater insects to climate change.

A Hybrid Chlorophyll a Estimation Method for Oligotrophic and Mesotrophic Reservoirs Based on Optical Water Classification

Low- and medium-resolution satellites have been a relatively mature platform for inland eutrophic water classification and chlorophyll a concentration (Chl-a) retrieval algorithms. However, for oligotrophic and mesotrophic waters in small- and medium-sized reservoirs, problems of low satellite resolution, insufficient water sampling, and higher uncertainty in retrieval accuracy exist. In this paper, a hybrid Chl-a estimation method based on spectral characteristics (i.e., remote sensing reflectance (Rrs)) classification was developed for oligotrophic and mesotrophic waters using high-resolution satellite Sentinel-2 (A and B) data. First, 99 samples and quasi-synchronous Sentinel-2 satellite data were collected from four small- and medium-sized reservoirs in central China, and the usability of the Sentinel-2 Rrs data in inland oligotrophic and mesotrophic waters was verified by accurate atmospheric correction. Second, a new optical classification method was constructed based on different water characteristics to classify waters into clear water, phytoplankton-dominated water, and water dominated by phytoplankton and suspended matter together using the thresholds of Rrs490/Rrs560 and Rrs665/Rrs560. The proposed method has a higher classification accuracy compared to other classification methods, and the band-ratio algorithm is simpler and more effective for satellite sensors without NIR bands. Third, given the sensitivity of the empirical method to water variability and the ease of development and implementation, a nonlinear least squares fitted one-dimensional nonlinear function was established based on the selection of the best-fitting spectral indices for different optical water types (OWTs) and compared with other Chl-a estimation algorithms. The validation results showed that the hybrid two-band method had the highest accuracy with squared correlation coefficient, root mean squared difference, mean absolute percentage error, and bias of 0.85, 2.93, 32.42%, and −0.75 mg/m3, respectively, and the results of the residual values further validated the applicability and reliability of the model. Finally, the performance of the classification and estimation algorithms on the four reservoirs was evaluated to obtain images mapping the Chl-a in the reservoirs. In conclusion, this study improves the accuracy of Chl-a estimation for oligotrophic and mesotrophic waters by combining a new classification algorithm with a two-band hybrid model, which is an important contribution to solving the problem of low resolution and high uncertainty in the retrieval of Chl-a in oligotrophic and mesotrophic waters in small- and medium-sized reservoirs and has the potential to be applied to other optically similar oligotrophic and mesotrophic lakes and reservoirs using similar spectrally satellite sensors.

Cyclical browning in a subtropical lake inferred from diatom records

Changing climate and land use activity are altering inputs of colored dissolved organic carbon (cDOC) into lakes. Increased cDOC reduces water transparency (browning) and changes lake physicochemistry, with biological consequences. Identifying the drivers and effects of changing cDOC inputs is critical for mitigating the consequences of climate change on lake ecosystems through adaptive watershed management. This study focused on determining the drivers of lake browning by evaluating shifts in diatom assemblages in subtropical, oligotrophic Lake Annie (FL, United States), which is known to experience climate-driven oscillations in transparency associated with watershed inputs of cDOC. We combined long-term limnological monitoring data and paleolimnological techniques to determine how diatoms respond to changes in cDOC and to infer past cDOC fluctuations relative to records of past climate and land use changes in the watershed. Diatom assemblage composition in a 14-year phytoplankton dataset was strongly correlated with cDOC-driven transparency fluctuations. Likewise, diatom assemblages in the upper 35 cm of the sediment core, which corresponded to a 35-year lake monitoring record, were also strongly related to past water transparency, yielding a strong transfer function (paleo model, R 2 = 0.72). When the model was applied to the diatom record from a 166-cm sediment core, diatom-inferred transparency and rates of nutrient input showed that localized ditching in the 1930s enhanced the effect of climate oscillations on water transparency, intensifying cyclical browning thereafter. Integration of long-term monitoring and paleoecological data provided valuable insights into the history of the aquatic ecosystem, enabling implementation of adaptive management strategies to contend with a changing climate.

Role of formation and decay of seston organic matter in the fate of methylmercury within the water column of a eutrophic lake

Abstract. Anoxic microniches in sinking particles in lakes have been identified as important water phase production zones of monomethylmercury (MeHg). However, the production and decay of MeHg during organic matter (OM) decomposition in the water column and its relation to the total Hg concentration in seston are poorly understood. We investigated total Hg and MeHg in relation to chemical changes in sinking seston and hydrochemical settings in a small and shallow (12 m deep) eutrophic lake during phytoplankton blooms from April to November 2019. The results show that MeHg proportions reach up to 22 % in seston in oxygen super saturation at the water surface and highest values (up to 26 %) at the oxic–suboxic redox boundary. MeHg concentrations were highest in May and November when algal biomass production was low and seston were dominated by zooplankton. Biodilution of MeHg concentrations could not be observed in the months of the highest algal biomass production; instead, MeHg and THg concentrations in seston were comparatively high. During suboxic OM decomposition and with decreasing redox potential (Mn and nitrate reduction), the concentration and proportion of MeHg in seston strongly decreased (<0.5 %), whereas total Hg concentrations show a 3.8- to 26-fold increase with water depth. Here, it remains unclear to which extent biodilution on the one hand and OM decomposition on the other alter the MeHg and THg concentration in seston. Changes in OM quality were most intense within or slightly below the redox transition zone (RTZ). The concentrations of MeHg and THg in seston from the RTZ were comparable to those found in the sediment trap material which integrated the changes in seston composition during the entire sampling period, suggesting that changes in the MeHg and THg content in the hypolimnion below the RTZ are comparatively small. Our study suggests that, in shallow eutrophic lakes, the water phase formation and decomposition of MeHg is intense and controlled by the decomposition of algal biomass and is, assumedly, largely disconnected from Hg methylation in sediments, similar to what has been observed in deep oligotrophic lakes.

Spatial and seasonal variation of arsenic speciation in Pantanal soda lakes

The occurrence of high arsenic concentrations (up to 3000 μg L−1) in water of soda lakes of the Pantanal wetland is a remarkable case of natural arsenic contamination in South America. However, little is known about arsenic speciation in this environment, particularly regarding speciation changes related to lake trophic status and seasonal variations. To fill this gap, arsenic speciation analysis was carried out in surface (SW) and subsurface (SSW) waters sampled in five soda lakes with different eutrophication status, in two dry and one wet season. As(V) was the dominant species in these waters, while As(III), DMA, MMA and likely complex organic species were present in lower amounts. The results allow to conclude that the arsenic speciation in SW and SSW varies seasonally according to the regional wet or dry periods and lake water levels. In eutrophic turbid and in oligotrophic vegetated soda lakes, arsenic speciation was also characterized by spatial differences between edge and center or between the SW and SSW. Cyanobacteria or macrophytes/algae are involved in arsenic biotransformation in soda lakes through its metabolic and detoxification processes. Significant variation in surface water arsenic speciation occurs as a result of seasonal primary production fluctuation or water arsenic concentration changes in the soda lakes, increasing organoarsenics in dry periods, whereas in flood periods, As(V) prevails. Spatial distribution of arsenic species is significantly impacted by biogeochemical conditions at the water/sediment interface in soda lakes.

Navicula sinicomeniscus sp. nov. (Bacillariophyta; Naviculaceae), a new species from southwestern China

Based on light and scanning electron microscopy observations, Navicula sinicomeniscus sp. nov. is described here as a new species from oligotrophic Lake Fuxian in Yunnan Province, China. Characteristics of this new species used to assign it to the genus Navicula include filiform, almost straight to slightly curved raphe structure, lineolate striae, and a row of visible, horizontal areolae positioned on the valve mantle at the apices. The key diagnostic features between the new species and other similar species include: valve shape and size, stria and areola densities, as well as the straightness or degree of bending of the raphe. Comparison with morphologically similar species is discussed in detail. These results can help us further understand the morphological and ecological information of the genus Navicula in Fuxian Lake.