Humic Lake Research Articles

Snow removal cools a small dystrophic lake

Snow removal cools a small dystrophic lake

Year-round sampling of the fish community in a boreal lake: differences between summer and winter influence estimates of species composition, catch, and fish size.

Boreal lakes experience pronounced seasonal variation in abiotic factors, especially light, temperature, and oxygen. A deep boreal humic lake was sampled year-round to test putative changes in total fish catch, species composition, catch-per-unit-effort (CPUE), habitat use, fish size, and condition. Monthly sampling was conducted in Lake Pääjärvi, southern Finland, during one full year in 2020-2021 as well as in March and August 2021 and 2022. The fish community was dominated by cyprinid species in all months, but the percentage of percid fish caught increased during the warm summer period. Most fish were caught in littoral habitats and the highest catches occurred in summer, but some species (e.g., ruffe, Gymnocephalus cernua, and pikeperch, Sander lucioperca) remained abundant in the winter catch. The body size of fish was larger in the winter catch, while condition factor was higher in summer for most species. Fish species proportions in total catch, CPUE, and average size of fish were closest to the annual mean values in September, which may be used as the optimal period to monitor fish communities of similar deep boreal lakes. Our findings highlight the need for year-round research to reveal the impacts of rising temperatures and diminishing ice-covered periods in fish communities and lake food webs.

Eggs on a plate: Population‐dependent effects of humic substance rich water on perch egg size and hatching success

Abstract The ability of fish to sustain environmental perturbations during fragile early‐life stages can be of vital importance for populations and species facing rapid global environmental change. One of the potential challenges for aquatic organisms, including teleost fish, is rising concentration of dissolved organic carbon (DOC) in freshwater environments, known as brownification, which causes a cascade of changes including a decreased visibility and pH. Conversely, humic substances are increasingly recognised for their potential antimicrobial and immunostimulant properties. The questions remain whether exposure to humic waters high in DOC during the development causes significant perturbation, and to what extent genetic and parental factors may maintain optimal development in such a possibly challenging environment. To address these questions, we generated pure and hybrid (inter‐population) crosses of Eurasian perch (Perca fluviatilis) originating from clear‐water and humic lakes. Fertilised eggs from each cross were exposed to natural humic and clear‐water lake water. Intra‐chorionic space diameter (embryo diameter) and hatching success were recorded. Perch embryos successfully developed even in a small volume of water (5 mL) and majority hatched (70%) by 13 day post‐fertilisation. Embryo diameter was influenced by the interaction between cross type and treatment where embryos with maternal clear‐water origin were smaller than embryos of humic water origin in both clear‐water and humic water treatments. The probability of hatching was not related to female size, but rather to embryo diameter at the eyed stage and the interaction between cross type and treatment, resulting in pure clear‐water crosses hatching earlier and at higher numbers. Our study shows that effects of humic water are not uniformally negative in perch and discloses population‐dependent responses to humic water exposure during development. Our experimental design also illustrates the utility of multi‐well plates for conducting complex high‐throughput experiments beyond traditional model species and opens new possibilities for testing perch genotype–environment associations in early‐life traits. Taken together, results of this study add to the growing evidence that humic substances may have complex effects on fish during early life, motivating further investigations on their role in shaping natural phenotypes and potential use in the applied sector.

Plastic and terrestrial organic matter degradation by the humic lake microbiome continues throughout the seasons.

Boreal freshwaters go through four seasons, however, studies about the decomposition of terrestrial and plastic compounds often focus only on summer. We compared microbial decomposition of 13C-polyethylene, 13C-polystyrene, and 13C-plant litter (Typha latifolia) by determining the biochemical fate of the substrate carbon and identified the microbial decomposer taxa in humic lake waters in four seasons. For the first time, the annual decomposition rate including separated seasonal variation was calculated for microplastics and plant litter in the freshwater system. Polyethylene decomposition was not detected, whereas polystyrene and plant litter were degraded in all seasons. In winter, decomposition rates of polystyrene and plant litter were fivefold and fourfold slower than in summer, respectively. Carbon from each substrate was mainly respired in all seasons. Plant litter was utilized efficiently by various microbial groups, whereas polystyrene decomposition was limited to Alpha- and Gammaproteobacteria. The decomposition was not restricted only to the growth season, highlighting that the decomposition of both labile organic matter and extremely recalcitrant microplastics continues throughout the seasons.

From oligotrophy to dystrophy: the ontogeny of a humic lake in an extrazonal boreal taiga in Central Europe

ABSTRACTWhile humic lakes are common in northern Europe, in central Europe they are rare and our understanding of their ontogeny is insufficient. Here, we present the reconstruction of the development of a humic lake in the Tatra Mountains (Slovakia) over the last 8000 years using chironomids along with diatoms, pollen and non‐pollen palynomorphs. The compositional changes in aquatic proxies suggest three developmental stages. Since its formation the lake has been oligotrophic (phase 1), and its chemistry was controlled by subsurface waters rich in base cations buffering the effect of acid humic substances from forest and peat that were already present in the catchment. The beginning of the transition to phase 2 (~4400 cal a bp) is marked by simultaneous alterations in all proxies. Climate change combined with the local geochemistry was able to alter the limnological conditions supporting unique chironomid assemblages that have no analogues in recent Carpathian lakes. At ~3400 cal a bp the lake shifted to a phase when acidophilic/acidotolerant taxa indicate characteristic humic lake conditions (phase 3). Some chironomid taxa appearing ~2000 years ago for the first time indicate a dystrophic state in which the lake has remained until now. Human activities have been detected in the area since the Aeneolithic but had no direct influence on the lake.

Ciliate Grazing on the Bloom-Forming Microalga Gonyostomum semen

The freshwater raphidophyte Gonyostomum semen forms extensive summer blooms in northern European humic lakes. The development of these blooms might be facilitated by a lack of natural top-down control, as few zooplankton species are able to prey on these large algal cells (up to 100 μm) that expel trichocysts upon physical stress. In this study, we describe a small ciliate species (< 17 μm) that preys on G. semen by damaging the cell membrane until cytoplasm and organelles spill out. Sequencing of clonal cultures of the ciliate tentatively identified it as the prostomatid species Urotricha pseudofurcata. Grazing experiments illustrated that feeding by U. cf. pseudofurcata can significantly reduce cell concentrations of the microalga. However, differences in cell size and growth rate between two investigated ciliate strains resulted in noticeably different grazing pressure. Environmental sequencing data from five different lakes supported potential interactions between the two species. Urotricha cf. pseudofurcata might, thus, play an important role in aquatic ecosystems that are regularly dominated by G. semen, reducing the abundance of this bloom-forming microalga and enabling transfer of organic carbon to higher trophic levels.

Deep chlorophyll maximum in temperate lakes with different trophic conditions − a rare or common phenomenon?

Deep chlorophyll maxima (DCM) are a common phenomenon in low-trophic stratified lakes, and there is much less information about the occurrence of DCM in eutrophic and dystrophic lakes. Therefore, we performed in situ continuous measurements of chlorophyll concentration by a submersible spectrofluorometer in 23 temperate lakes with different trophic conditions (mesotrophic, eutrophic, and dystrophic). We perform chemical analyses of available nutrient forms to better understand the mechanism of DCM formation. We found the highest concentration of phytoplankton in the metalimnion or upper hypolimnion of most studied lakes, regardless of trophic conditions. Nevertheless, the differences in the number of phytoplankton between DCM and epilimnion were largest in low-trophic lakes, where the chl a concentration in DCM was even 15 times higher than that in the epilimnion. The mesotrophic lakes showed vertical niche segregation by phytoplankton, with a general pattern − green algae in the upper layer, below diatoms, then cryptophytes (and cyanobacteria). Our results indicated that the main factor for DCM in mesotrophic and eutrophic lakes was epilimnetic nutrient depletion, while in dystrophic lakes, it seems to be caused by other factors due to the depletion of dissolved nutrients in whole water profiles.

THE PHYTOCOENOTIC STUDY REGARDING THE WET HABITATS FOUND ON THE COPĂNIȚA ISLAND, DOLJ COUNTY, ROMANIA

The territory under study is an island located very close to the flow of the Jiu into the Danube, in the lower course of the Danube, and represents a protected area for important species and habitats of community and priority interest, as well as an important migration area for an impressive number of protected birds.This territory is part of the ROSCI0045 Coridorul Jiului Protected Area. Considering the extremely varied and rich biodiversity of this area, we started in 2022 the realization of complex studies regarding the flora, vegetation as well as the habitats of community and priority interest found on Copanița Island. In the present work, a study is presented regarding the phytocenology of wet habitats of conservation interest as well as the anthropogenic impact on them. Thus, the following wetland habitats were identified: 3160 - Natural dystrophic lakes and ponds (CLAS. PAL.: 22.14), 3150 - Natural eutrophic lakes with Magnopotamion or Hydrocharition-type vegetation (CLAS. PAL.: 22.13 x (22.41 sau 22.421), 3130 - Oligotrophic to mesotrophic standing waters with vegetation of the Littorelletea uniflorae and/or Isoëto-Nanojuncetea (CLAS. PAL.: 22.12 x (22.31 şi 22.32) and 3270 - Rivers with muddy banks with Chenopodion rubri p.p. and Bidention p.p. vegetation (CLAS. PAL.: 24.52). From the phytocoenotic point of view, the following aquatic and palustre plant communities were identified: Myriophyllo verticillati-Nupharetum luteae Koch 1926, Trapo-Nymphoidetum Oberd. 1957, Lemno-Spirodeletum Koch 1954, Spirodelo-Salvinietum natantis Slavnič 1965, Lemno-Salvinietum natantis Miyawaki et Tüxen 1960, Cyperetum flavescentis Koch ex Aichinger 1933 and Bidenti-Polygonetum hydropiperis Lohm. in Tüxen 1950, Polygono lapathifolio – Bidentetum Klika 1935; Echinochloo – Polygonetum lapathifolii Soo et Csurrs 1974. The climate changes of recent years had a particularly negative effect on these habitats with a particular impact on the surfaces of the habitats but also on the structure and physiognomy of the edifying plant communities.

Physico-chemical characterisation of protected lake habitats: A matter of dystrophy

Oligotrophic waters containing very few minerals of sandy plains (code 3110) and Natural dystrophic lakes and ponds (code 3160) are widespread freshwater lake habitats within Atlantic blanket bog habitats. Despite being protected under Annex I of the European Union (EU) Habitats Directive, the conservation conditions of these lake habitats is considered bad and inadequate respectively. A lack of field data (including physico-chemical, algal, macrophytes and invertebrates) is preventing appropriate conservation condition assessments, and recommendations to maintain / restore favourable conservation condition. This research aims to characterise these lake habitats with water chemistry data from favourable conservation condition sites and determine their natural state, and to consider seasonal and regional variability to evaluate whether two lake habitats exist. Several water chemistry parameters were monitored in 24 waterbodies monthly for 12 months. Lake habitat 3110 were characterised by higher levels of pH, alkalinity, silica and dissolved oxygen, whereas, contrary to previous reports, levels of true colour and dissolved organic matter (DOM) were lower in lake habitat 3160. However, all 24 sites were acidic with low alkalinity and slightly coloured, hence exhibiting dystrophic conditions. Principal Component Analysis highlighted a strong regional influence separating larger, coloured sites from smaller acidic and altitudinal sites. Differences in true colour were due to strong positive relationships between DOM and catchment area confirming their influence on a regional scale. This research highlights an urgent need to gather physico-chemical field data from a wider range of protected lake habitats 3110 and 3160, and from across the EU to discern whether a gradient of dystrophy exists along which both habitats occur. Outcomes of such research would have implications on the reporting and conservation assessment of these lake habitats by Member States and help harmonise monitoring methods and provide more accurate assessment tools for their reporting under the EU Habitats Directive.

Catchment and lake network modify export of anaerobic oxidation capacity in boreal freshwaters

Anaerobic terminal electron acceptors (aTEAs, i.e. NO3, Fe, SO4) enable anaerobic respiration, and each has a specific ability to oxidize reduced compounds. However, little is known about how seasonal and lake-specific aTEA fluxes form anaerobic oxidation capacity (AOC) to oxidize organic carbon in boreal systems. We compiled 26 years of data from two interconnected semi-pristine boreal lakes and defined mean daily imports, pools, and exports of aTEAs. In both lakes, the export of NO3 formed 2 %–3 % of the total AOC in summer and autumn, and up to 11 % in winter and spring. In a predominantly monomictic humic lake surrounded by peatlands, Fe was responsible for 15 %–31 % of the seasonal export of AOC, with a large proportion of Fe originating from the lake bottom. A dimictic clear-water lake downstream retained Fe and exported 87 %–95 % of AOC as SO4. In the humic lake, the annual SO4:Fe:NO3 export ratio for AOC was 10:3:1 and in the clear-water lake 15:0.4:1. In the monomictic lake, exports were specifically regulated by stratification; in the dimictic lake, exports were more regulated by spring flooding and the ascending and descending side of the peak flood. These events modified lake dynamics and caused lake-specific NO3, Fe, and SO4 exports which continued for months. We conclude that a catchment and lake network can cause spatial and temporal variation in exports of NO3, Fe, and SO4 affecting AOC export. Such natural variations in exports have significant potential to modify the system's capacity to oxidize C and resist changes in oxidation-reduction reactions coupled to nutrient cycling and the formation of greenhouse gases in downstream water bodies.

Regional differences in water beetle communities networks settling in dystrophic lakes in northern Poland

The relationships between the species that form the networks in small dystrophic lakes remain poorly recognised. To investigate and better understand the functioning of beetle communities in different ecosystems, we created three network models that we subjected to graph network analysis. This approach displays correlation–based networks of connections (edges) between objects (nodes) by evaluating the features of the whole network and the attributes of nodes and edges in the context of their roles, expressed by centrality metrics. We used this method to determine the importance of specific species in the networks and the interspecific relationships. Our analyses are based on faunal material collected from 25 dystrophic lakes in three regions of northern Poland. We found a total of 104 species representing different ecological elements and functional trophic groups. We have shown that the network of relationships between the biomass of species differs considerably in the three study regions. The Kashubian Lakeland had the highest cohesion and density, while the network in the Suwalki Lakeland was the thinnest and most heterogeneous, which might be related to the fractal structure and the degree of development of the studied lakes. Small–bodied predators that congregated in different clusters with species with similar ecological preferences dominated all networks. We found the highest correlations in the Masurian Lakeland, where we obtained the highest centralisation of the network. Small tyrphophiles typically occupied the central places in the network, while the periphery of the network consisted of clusters with different habitat preferences, including large predators. The species that were most important for network cohesion and density were mainly tyrphophilous species, such as Anacaena lutescens, Hygrotus decoratus, Enochrus melanocephalus and Hydroporus neglectus. The values of attributes determining the role of species in community networks were influenced by both biotic and environmental factors.

Fish muscle mercury concentration and bioaccumulation fluctuate year-round - Insights from cyprinid and percid fishes in a humic boreal lake

Boreal lakes demonstrate pronounced seasonality, where the warm open-water season and subsequent cold and ice-covered season dominate natural cycles. While fish muscle total mercury concentration (mg/kg) [THg] is well documented in open-water summer months, there is limited knowledge on the ice-covered winter and spring mercury dynamics in fish from various foraging and thermal guilds. This year-round study tested how seasonality influences [THg] and its bioaccumulation in three percids, perch (Perca fluviatilis), pikeperch (Sander lucioperca), ruffe (Gymnocephalus cernua), and three cyprinids, roach (Rutilus rutilus), bleak (Alburnus alburnus), and bream (Abramis brama) in deep boreal mesotrophic Lake Pääjärvi, southern Finland. Fish were sampled and [THg] was quantified in the dorsal muscle during four seasons in this humic lake. Bioaccumulation regression slopes (mean ± STD, 0.039 ± 0.030, range 0.013–0.114) between [THg] and fish length were steepest during and after spawning and shallowest during autumn and winter for all species. Fish [THg] was significantly higher in the winter-spring than summer-autumn in all percids, however, not in cyprinids. The lowest [THg] was observed in summer and autumn, likely due to recovery from spring spawning, somatic growth and lipid accumulation. Fish [THg] was best described by multiple regression models (R2adj: 52–76%) which included total length and varying combinations of seasonally changing environmental (water temperature, total carbon, total nitrogen, and oxygen saturation) and biotic factors (gonadosomatic index, and sex) in all species. The seasonal variation in [THg] and bioaccumulation slopes across multiple species suggests a need for standardized sampling seasons in long-term monitoring to avoid any seasonality bias. From the fisheries and fish consumption perspective in seasonally ice-covered lakes, monitoring of both winter-spring and summer-autumn would improve knowledge of [THg] variation in fish muscle.

Humic lakes with inefficient and efficient transfer of matter in planktonic food webs

Humic compounds and related factors are the main constraints for the development of zooplankton in humic lakes, leading to low transfer efficiency in food webs. The results of this study indicated that some zooplankton species could have an advantage under these conditions. We found that the mass development of omnivorous Asplanchna priodonta in temperate humic lakes could be caused by the domination of high nutritional algae such as Gonyostomum semen and Botryococcus braunii. These algae are too large for most zooplankton to ingest, but A. priodonta can feed on a wide range of particles and benefit from this high-nutritional food. Small cladocerans (Ceriodaphnia, Bosmina) might be favored when picoplankton and small algae-dominate humic lakes. Therefore, some zooplankton species could have an advantage and control the development of phytoplankton, leading to the effective transfer of matter and energy in the planktonic food web in humic lakes.

Caracterización limnológica, estructura del fitoplancton y batimetría de Laguna Negra, un lago coloreado del Parque Nacional Tierra del Fuego (Argentina)

The 4% of the Argentinian sector from Tierra del Fuego is covered with water. Colored lakes are among the most conspicuous water bodies in the Andean area, and Laguna Negra, in the National Park Tierra del Fuego, is one of them. The main physical and chemical variables were monitored and phytoplankton samples were obtained with a monthly frequency during the ice-free period, between September 2016 and March 2018 (n=15). Laguna Negra was deeper than expected, with a maximum depth of 15.5 m. In addition, this lake has high pH values (7.5±0.55), which were quite stable during the study (variation coefficient, VC=8.3%). These characteristics preclude us from considering this lake as a shallow peatland lake. Dissolved and total nutrients were low and did not show a temporal pattern of fluctuation during the study period. The same trend of no clear fluctuation was also shared by dissolved organic carbon concentration (DOC=7.7±0.78 mg/L). Taxa richness ranged between 6 and 13 throughout the study. The biovolume was inversely correlated with the vertical attenuation coefficient of light (Kd). Phytoplankton biomass (chlorophyll-a concentration, abundance, and biovolume) was generally low, with Dinobryon divergens, Mallomonas sp. and Parvodinium cf. inconspicuum being the dominant taxa. Depending on the classification used, Laguna Negra might be regarded as a humic or dystrophic lake because its color is at the edge of values used in the current literature to consider a lake as humic.

The role of diatoms algae in phytoplankton of dystrophic lakes

The paper presents seasonal indicators of the abundance and biomass of the dominant groups of algae in the phytoplankton of lakes Kurganistoye and Yazhginya. In both lakes the largest values of summer abundance and biomass are formed by cryptophyte algae. A group of diatoms formed the peaks of only the May values of the abundance and biomass of phytoplankton in Lake Yazhginya.

Decomposition rate and biochemical fate of carbon from natural polymers and microplastics in boreal lakes.

Microbial mineralization of organic compounds is essential for carbon recycling in food webs. Microbes can decompose terrestrial recalcitrant and semi-recalcitrant polymers such as lignin and cellulose, which are precursors for humus formation. In addition to naturally occurring recalcitrant substrates, microplastics have been found in various aquatic environments. However, microbial utilization of lignin, hemicellulose, and microplastics as carbon sources in freshwaters and their biochemical fate and mineralization rate in freshwaters is poorly understood. To fill this knowledge gap, we investigated the biochemical fate and mineralization rates of several natural and synthetic polymer-derived carbon in clear and humic lake waters. We used stable isotope analysis to unravel the decomposition processes of different 13C-labeled substrates [polyethylene, polypropylene, polystyrene, lignin/hemicellulose, and leaves (Fagus sylvatica)]. We also used compound-specific isotope analysis and molecular biology to identify microbes associated with used substrates. Leaves and hemicellulose were rapidly decomposed compared to microplastics which were degraded slowly or below detection level. Furthermore, aromatic polystyrene was decomposed faster than aliphatic polyethylene and polypropylene. The major biochemical fate of decomposed substrate carbon was in microbial biomass. Bacteria were the main decomposers of all studied substrates, whereas fungal contribution was poor. Bacteria from the family Burkholderiaceae were identified as potential leaf and polystyrene decomposers, whereas polypropylene and polyethylene were not decomposed.

Humic Lake Exhibits Higher Microbial Functional Gene Diversity and Weaker Gene Interaction Efficiency than a Common Alkaline Lake.

Simple SummaryThe humic lake represents a special kind of aquatic ecosystem with high humic substances, low irradiance, and a high potential for greenhouse gas emissions. Despite the special environment and biogeochemical processes in humic lake water, knowledge about the underlying microbial-driven functions remains elusive. Here, we studied the compositions and functional gene structures of microbial communities in a humic lake (HL) and a reference weakly alkaline lake (RAL). We found that the high organic matter content in the HL supported higher gene diversity; and, specifically, the carbon and nitrogen fixations, the degradation of various types of carbon, methane oxidation and methanogenesis, ammonification, denitrification, and assimilatory N reduction might be enhanced more in the HL than in the RAL. By contrast, the humic fractions in the HL might reduce microbial metabolic potential for sulfur oxidation and phosphorus degradation. The potential interactions between different functional microorganisms might be down-regulated provided that there were more easily acquired nutrients in the HL. Overall, our results showed the functional gene “landscape” of microbial communities in the surface water of a humic lake, which helps understand the biogeochemical processes and the remediation of organic matter pollution in lacustrine ecosystems.Humic lakes (HLs) are special water bodies (high organic matter content, low pH, and low transparency) that are important sources of major greenhouse gases. The knowledge about microbial functional potentials and the interactions among different genes in HL water has been scarcely understood. In this study, we used 16S rRNA gene sequencing and the GeoChip 5.0 to investigate microbial community compositions and functional gene structures in an HL and a reference weakly alkaline lake (RAL). The HL microbial communities showed distinct compositions and functional gene structures than those in the RAL. The functional gene diversity was significantly higher in the HL than in the RAL. Specifically, higher gene relative intensities in carbon and nitrogen fixations, the degradation of various types of carbon, methane oxidation and methanogenesis, ammonification, denitrification, and assimilatory N reduction were observed in the HL samples. By contrast, the metabolic potentials of microorganisms involved in dissimilatory N reduction, phosphorus degradation, and sulfur oxidation were weaker in the HL than in the RAL. Despite higher functional gene diversity, the interaction efficiency among genes (reflected by network geodesic distance and clustering coefficient) might be reduced in the HL. Different functional microbes may develop less interdependent relationships in acquiring nutrients given the high resource availability in the HL. Overall, the enhanced microbial metabolic potentials and less efficient functional interactions might have great consequences on nutrient cycling and greenhouse gas emissions in the HL ecosystem.

Biodegradation of microplastic in freshwaters: A long-lasting process affected by the lake microbiome.

Plastics have been produced for over a century, but definitive evidence of complete plastic biodegradation in different habitats, particularly freshwater ecosystems, is still missing. Using 13 C-labelled polyethylene microplastics (PE-MP) and stable isotope analysis of produced gas and microbial membrane lipids, we determined the biodegradation rate and fate of carbon in PE-MP in different freshwater types. The biodegradation rate in the humic-lake waters was much higher (0.45% ± 0.21% per year) than in the clear-lake waters (0.07% ± 0.06% per year) or the artificial freshwater medium (0.02% ± 0.02% per year). Complete biodegradation of PE-MP was calculated to last 100-200 years in humic-lake waters, 300-4000 years in clear-lake waters, and 2000-20,000 years in the artificial freshwater medium. The concentration of 18:1ω7, characteristic phospholipid fatty acid in Alpha- and Gammaproteobacteria, was a predictor of faster biodegradation of PE. Uncultured Acetobacteraceae and Comamonadaceae among Alpha- and Gammaproteobacteria, respectively, were major bacteria related to the biodegradation of PE-MP. Overall, it appears that microorganisms in humic lakes with naturally occurring refractory polymers are more adept at decomposing PE than those in other waters.

Sedimentary conditions based on the vertical distribution of radionuclides in small dystrophic lakes: a case study of Toporowe Stawy Lakes (Tatra Mountains, Poland)

The aim of this work was to prove the use of radionuclides deposited in sediment core taken from an overgrowing dystrophic lakes surrounded by marsh-peat vegetation to estimate sedimentary conditions. Sediment core samples were taken from the Toporowe Stawy Lakes (Niżni (TSN) and Wyżni (TSW); Tatra Mountains). The sampling was done using a Limnos corer. After the physical sample preparations, gamma measurements were performed. Radiochemical analysis was applied with the aim of determining 210Pb radioactivity by means of 210Po. The mean values for TSN lake are as follows: 137Cs ~ 123 Bq∙kg−1, 40 K ~ 389 Bq∙kg−1, 228Th ~ 55 Bq∙kg−1, 226Ra ~ 86 Bq∙kg−1, 241Am ~ 5 Bq∙kg−1, and 210Pbuns ~ 180 Bq∙kg−1. For TSW lake, the radioactivity levels of 226Ra and 241Am are comparable to the TSN. The mean values of 137Cs, 40 K, and 228Th are almost twice as high as in TSN. The level of 210Pb in uppermost layer of TSN is higher than in TSW. Sediments were dated by use of 210Pb method, and the rate of sedimentation of each layer was also estimated. Basic chemometric tools were used to confirm the way of deposition of radionuclide, find the correlations between variables, and compare analyzed lakes. It was concluded that the presented type of lakes are a valuable source of information and the vertical distribution of radionuclide can be used to interpret the source of material supply and factors that influence the sedimentation process in recent 150–200 years.

Whole-genome sequencing illuminates multifaceted targets of selection to humic substances in Eurasian perch.

Extreme environments are inhospitable to the majority of species, but some organisms are able to survive in such hostile conditions due to evolutionary adaptations. For example, modern bony fishes have colonized various aquatic environments, including perpetually dark, hypoxic, hypersaline and toxic habitats. Eurasian perch (Perca fluviatilis) is among the few fish species of northern latitudes that is able to live in very acidic humic lakes. Such lakes represent almost “nocturnal” environments; they contain high levels of dissolved organic matter, which in addition to creating a challenging visual environment, also affects a large number of other habitat parameters and biotic interactions. To reveal the genomic targets of humic‐associated selection, we performed whole‐genome sequencing of perch originating from 16 humic and 16 clear‐water lakes in northern Europe. We identified over 800,000 single nucleotide polymorphisms, of which >10,000 were identified as potential candidates under selection (associated with >3000 genes) using multiple outlier approaches. Our findings suggest that adaptation to the humic environment may involve hundreds of regions scattered across the genome. Putative signals of adaptation were detected in genes and gene families with diverse functions, including organism development and ion transportation. The observed excess of variants under selection in regulatory regions highlights the importance of adaptive evolution via regulatory elements, rather than via protein sequence modification. Our study demonstrates the power of whole‐genome analysis to illuminate the multifaceted nature of humic adaptation and provides the foundation for further investigation of causal mutations underlying phenotypic traits of ecological and evolutionary importance.