The meromictic coastal water bodies have a specific vertical structure consisting of the layers with different hydrological, hydrochemical parameters and plankton communities that have been poorly investigated, especially the protist component. Here, we report the first data on the species richness and taxonomic composition of protists in the Arctic coastal meromictic Lake Trekhtzvetnoe (Kandalaksha Bay, White Sea). This lake is the second meromictic water body on the coast of the White Sea, where the protist communities have been comprehensively studied using light, transmission and scanning electron microscopy. A total of 189 species and forms of heterotrophic, mixotrophic and phototrophic protists assigned to different taxonomic groups have been recorded, including 50 species new for the White Sea region. The supergroup TSAR was the richest, with Stramenopliles and Alveolata showing the largest number of species. The taxonomic list and micrographs of cells for observed protists are provided. Most species of protists are characterized by a wide geographical distribution. Half of the found species are freshwater. Taxonomic composition of the protist communities varied in the main hydrological zones (myxolimnion, chemocline and monimolimnion) according to the vertical stratification of hydrological and hydrochemical parameters. The similarity and differences of protist communities have been compared with those of another Arctic meromictic Lake, Kislo-Sladkoe.

В прибрежных водоемах, образовавшихся путем отделения от Белого моря в ходе послеледникового под­нятия берега, в хемоклине – градиентной зоне между аэробной и сероводородной зонами – часто наблюдается окрашенная прослойка воды с массовым развитием фототрофных микроорганизмов. Спектры пропускания солнечного света, измеренные на разных горизонтах трех стратифицированных водоемов и в прибрежной морской акватории с помощью погружаемого волоконно-оптического зонда, сопоставлены со спектрами поглощения света водой на этих же горизонтах. Для слоев с массовым развитием аноксигенных фототрофов по спектрам поглощения определены концентрации бактериохлорофиллов. Согласно полученным данным диапазоны проходящего солнечного спектра в водной толще («цветовые экологические ниши») в значительной степени зависят от растворенных в воде гуминовых веществ. Их концентрация возрастает по мере изоляции водоема от моря, из-за чего фотическая зона сужается по глубине, хемоклин оказывается ближе к поверхности и наблюдается сдвиг спектра попадающего в хемоклин света в более длинноволновую сторону. В морской бухте до хемоклина доходит в основном свет с длиной волны 524–593 нм, в морской стратифицированной лагуне преобладают длины волн 526–597 нм, в водоемах с пресным поверхностным слоем воды солнечный спектр сдвинут в красную область (573–727 нм). Показано, что «цветовые экологические ниши» в различных водоемах заняты организмами, светособирающие антенны которых адаптированы к поглощению квантов света соответствующего спектрального диапазона. Этот факт может быть использован для предсказания таксономического состава различных слоев стратифицированного водоема. In coastal water bodies formed by separation from the White Sea during the post-glacial uplift of the coast, in the chemocline (the gradient zone between the aerobic and hydrogen sulfide zones), a colored layer of water with a development of phototrophic microorganisms is often observed. The solar light transmission spectra measured at different horizons under water in three stratified water bodies in the coastal marine area using a submersible fiber optic probe are compared with the absorption spectra of light by water from the same horizons. In the layers with massive development of anoxygenic phototrophs, the concentrations of bacteriochlorophylls were determined from the absorption spectra. According to the data obtained, the ranges of the transmitted solar spectrum in the water column (“color ecological niches”) are largely determined by humic substances dissolved in water. Their concentration increases as the water body is isolated from the sea, due to which the photic zone narrows with depth, the chemocline becomes closer to the surface, and a shift towards longer wavelengths appears in the spectrum of light entering the chemocline. In the marine bay, the 520– 600 nm part of the spectrum reaches the chemocline, in the marine stratified lagoon, wavelengths of 510–6700 nm predominate, in reservoirs with a fresh surface layer of water, the solar spectrum is shifted to the red region (520–7200 nm). It is shown that "color ecological niches" in various water bodies are occupied by organisms whose light-collecting antennas are adapted to absorb light quanta of the corresponding spectral range.

Естественное или искусственное отделение водоема от материнской акватории ведет к стратификации его водной толщи и развитию вертикальной последовательности планктонных сообществ. Данные по сукцессии водорослевых сообществ единичны. В работе представлено исследование сезонной изменчивости состава и обилия микроводорослей в разных слоях водной толщи полузамкнутой лагуны под названием «Озеро Кисло-Сладкое» в 2021 г. в период устойчивой стратификации с придонной аноксией. Определены видовой состав и углеродная биомасса на разной глубине, интегрированная биомасса фитопланктона (ФП) в водном столбе (Bint). Проведен анализ сходства структуры ФП для разных слоев воды. Выявлено 53 вида и/или рода с преобладанием морских форм и 11 таксонов более высокого ранга. В период ледостава большая часть водной толщи была анаэробной, подо льдом доминировала пресноводная цианобактерия Microcystis ichthyoblabe. После схода льда максимум углеродной биомассы переместился к границе сероводородной зоны. В мае – начале июня наибольшие значения Вint определяла зеленая водоросль Chlamydomonas sp., в июне – июле – диатомовая водоросль Cyclotella choctawhatcheeana. Тип питания ФП с преимущественно автотрофного в начале сезона (март – июль) сменился на преимущественно гетеротрофный (август – октябрь). Наибольшие значения Bint отмечены в июле–сентябре (149,8 мг С/м2) при доминировании хищной динофлагелляты Oxyrrhis marina. Проведено сравнение полученных результатов с данными по водоемам подобного типа.

Natural or artificial separation of a water body from its parent water area leads to stratification of its water column and the development of a vertical sequence of planktonic communities. Data on succession of algal communities are sparse. Seasonal variability in the composition and abundance of microalgae in different water layers of the semienclosed lagoon called Lake Kislo-Sladkoe in 2021 was studied during stable stratification and bottom anoxia. Species composition and carbon biomass at different depths, as well as the integrated phytoplankton biomass in the water column (Bint), were determined. An analysis of the similarity of the phytoplankton structure in different water layers was carried out. A total of 53 species and/or genera with a predominance of marine forms and 11 higher rank taxa were identified. During the ice-covered period, most of the water column was anaerobic, and the freshwater cyanobacterium Microcystis ichthyoblabe dominated under the ice. After ice thaw, the biomass maximum shifted towards the boundary of the sulfide zone (into the chemocline). In May–early June, the highest biomass was determined by green alga Chlamydomonas sp., while that in June-July was by the diatom Cyclotella choctawhatcheeana. The type of nutrition changed from predominantly autotrophic at the beginning of the season (March–July) to predominantly heterotrophic (August–October). The highest biomass values were recorded in July–September (149.8 mg С/m2) due to the dominance of the predatory dinoflagellate Oxyrrhis marina. A comparison was made with water bodies of a similar type.

Measurements of the spectral composition of light at the boundary of the photic zone in seven coastal reservoirs, which are isolated from the sea to varying degrees, as well as in the marine water area and in a freshwater lake showed that in marine and brackish reservoirs, mainly green light reaches it; in lakes with a fresh upper layer it is orange, red and far-red. In meromictic reservoirs, the photic zone was limited by a colored layer of water with a massive development of phototrophic microorganisms. Their pigment composition is in good agreement with the spectral composition of transmitted light. The marine bays and lagoons were dominated by taxa with red pigments: cryptophytic algae with phycoerythrin-545, purple sulfur bacteria with okenone carotenoid, brown green sulfur bacteria with isorenieratene and bacteriochlorophyll e. In lakes with fresh upper water, unicellular algae and green sulfur bacteria with chlorobactene and bacteriochlorophyll d developed. The spectral range can serve as a selective factor that determines the composition of a community of phototrophs with structurally different antennas and similar light absorption spectra.

The article presents the results of hydrochemical studies of waterbodies with varying degrees of development of anaerobic conditions (up to the appearance of meromixia with sulfide anoxia) in the Porya Guba Bay (Kandalaksha Bay, White Sea, Kandalaksha State Nature Reserve). It is shown that these waterbodies, which are at different stages of isolation from the sea, have a unique hydrological and hydrochemical structure, formed at a certain stage of their evolution. The vertical stratification of water, with the development of all phenomena accompanying stagnation, is more pronounced in the most distant parts of their water area from the sea, where elevated contents of hydrogen sulfide (up to 16.5 mg/L), phosphates, silicon, nitrite, and ammonium nitrogen are noted in bottom anaerobic water. However, the concentrations of all major ions are comparable to those in seawater and hardly chance at all vertically. Analysis of the distribution of the hydrochemical parameters of the distant reaches of Ozerki Lagoon made it possible to conclude that, due to changes that have occurred with this waterbody over the past 90 years, it can now most likely be classified as meromictic.

The relic Lake Mogilnoe, separated from the Barents Sea by a sand and pebble dam, is located in the high Arctic on the Kildin island (Murmansk region). This lake is a classic example of a meromictic basin of marine origin. The data obtained during the 2018 expedition showed changes in the hydrochemical regime of the lake that have occurred over the past 20 years. Sulfide concentration in the monimolimnion of the lake was as high as 140 mg/L. A tendency for salinization of the surface waters to 7 g/L has been noted. The Lake Mogilnoe is characterized by a discrepancy between the halocline and thermocline levels. The chemocline zone in the lake is below the halocline level. In a narrow oxygen-containing layer between 3 and 7.5 m, aerobic microflora of the marine type and marine fauna were present. The bacterial plate was formed at the boundary of the sulfide layer at 8 m and mainly consisted of green sulfur bacteria (GSB). Brown-colored GSB species containing bacteriochlorophyll e were predominant. The previously formed concept of anaerobic phototrophic bacteria (APB) biodiversity based on morphological characteristics was modified using metagenomic data obtained by analyzing DNA from two samples of lake water in the chemocline zone, and was also supplemented by identifying new GSB species. Molecular diagnostic data confirmed the absolute dominance of the brackish species of GSB Chlorobium phaeovibrioides. This is the first isolation and identification of brown- and green-colored Prosthecochloris aestuarii morphotypes from Lake Mogilnoe, as well as of bacteriochlorophyll c-containing Prosthecochloris sp. The taxonomic position of Pelodyction phaem, which was constantly present in the Lake Mogilnoe, is discussed in detail. Despite the partial isolation of the ecosystem of Lake Mogilnoe from the Barents Sea, the main properties of the dominant GSB species and Prosthecochlori aestuarii turned out to be similar to those of the phylotypes living in lakes on the White Sea coast.

Lake Kislo-Sladkoe is a stratified water body partly isolated from the White Sea. Perennial meromixis in the lake irregularly alternates with mixing events. Taking into account that the protists of Arctic coastal stratified water bodies are understudied, we evaluated for the first time the vertical structure, species richness, and diversity of protists assigned to different taxonomic groups in Lake Kislo-Sladkoe using light, luminescent, and scanning electron microscopy. To test the research hypothesis that a mixing event affects the vertical stratification and species composition of protists in a stratified lake, we compared the protist communities of Lake Kislo-Sladkoe in two extremely different states: strong meromixis vs. full vertical mixing. A total of 97 morphologically distinct phototrophic, heterotrophic, and mixotrophic protists were revealed with the most diverse supertaxa SAR (59), Obazoa (9), and Excavates (14). The hidden diversity of protists (43 species) was a bit less than the active diversity (54 species). A taxonomic list and micrographs of cells for the observed protists are provided. The majority of species revealed are cosmopolitan or widespread in the northern sea waters. The vertical patterns of protist communities were absolutely different in 2018 and 2021. In July 2018, clearly distinct protist communities inhabited different layers of the lake. Bloom of cryptophyte Rhodomonas cf. baltica was detected in chemocline, whereas the maximum density of its grazers was observed in adjacent layers, mainly dinoflagellates Gymnodinium sp. and Scrippsiella trochoidea, as well as a ciliate Prorodon sp. In 2021 due to the recent mixing of lake and seawater, there were no distinct communities in the water column except the superficial 0–1 m layer of fresh water.

The optical properties of chromophoric dissolved organic matter (CDOM) of natural water have been studied by fluorescence spectroscopy and absorption spectroscopy. Samples of natural water from various depths from the meromictic lakes Trekhtzvetnoe and Elovoe were taken. The dependences of the quantum yield and the maximum of fluorescence emission on the excitation wavelength in a wide range of excitation wavelength variation (250...500 nm) for different horizons of stratified reservoirs are obtained. It is shown that the dependences of the maximum of the fluorescence emission on the excitation wavelength and the dependences of fluorescence quantum yield on the excitation wavelength in both lakes and at all depths studied are similar. On the other hand, the absolute value of the quantum yield differs in different layers of water. For example, the values of the fluorescence quantum yield took values from 1.4% to 2.4% at an excitation wavelength of 340 nm. The similar nature of the dependence of the fluorescence intensity on the excitation wavelength is explained by the common origin of DOM fluorophores in the studied horizons of stratified lakes. The different values of the fluorescence quantum yield are due to the difference in the proportion of aromatic compounds in the composition of the CDOM and are associated with differences in the hydrochemical characteristics of water at different horizons.

The composition and structure of the anoxygenic phototrophic bacterial (APB) community in the water column of meromictic Lake Bol’shie Khruslomeny during winter were investigated. The community developed at the depth of 4.25 m, and its activity in the ice-cover period was very low (6.2 µmol C L–1 day–1). The water in the zone of highest development of phototrophic bacteria was of an unusual lemon-yellow color, probably due to the production and accumulation of polysulfides. The near-bottom water was also of lemon-yellow color and was resistant to oxidation by the air oxygen. In the zone of peak APB development the content of BChl d from green-colored green sulfur bacteria was considerably higher than that of BChl e from brown-colored green sulfur bacteria: 77 and 23

In a saline semi-isolated lagoon on Cape Zeleny (White Sea), the annual dynamics of the vertical hydrological structure and the seasonal dynamics of phytoplankton were traced. Species composition, vertical distribution, abundance, nutrition type, and biomass were analyzed. In total, 293 species and supraspecific taxa of algae and cyanobacteria were found. Most of the identified species are marine, and 38 species are freshwater. Taxonomic composition changed in the lagoon throughout the year. Dinoflagellates dominated in winter and early June; unidentified cocci and flagellates in July; diatoms, dinoflagellates, and unidentified cells in August; dinoflagellates in September; and unidentified cocci and flagellates in October–November. The abundance of algae also changed in the lagoon throughout the year. The integrated biomass in the water column varied from 0.01 g C/m2 in January to 0.78 g C/m2 in early September. According to the environmental parameters, the water column of the lagoon was subdivided into several zones with different environmental conditions and corresponding phytoplankton communities. The similarity between the communities of different horizons was 32–46% in summer and 7% in winter. The chemocline layer was the most populous. It contained a maximum of phytoplankton biomass, 1–2 orders of magnitude higher than that in the overlying horizons. Despite the connection to the sea, the phytoplankton structure in the surface water layer in the lagoon and in the sea differed significantly in composition, quantitative parameters, and seasonal dynamics. The similarity between the communities never exceeded 50%. In terms of biomass dynamics, the lagoon lagged behind the sea until mid-summer, but, starting from August, it outnumbered it, and the phytoplankton development in the lagoon lasted longer, until late autumn. According to sequential tests DistLM, the phytoplankton structure and dynamics in the lagoon and in the sea were related to the daylength, water salinity, oxygen content, and pH by 24.5%. At the same time, the PhP structure did not depend on water temperature, underwater illuminance, or depth. Oxygen content and pH were defined by PhP activity. Salinity serves as a vector of the vertical sequence of ecological niches. The day length seems to be the crucial factor of the seasonal PhP dynamics in the semi-isolated coastal stratified lakes and lagoons.

Chlorosomal bacteriochlorophylls are the main photosynthetic pigments of green sulfur bacteria – anoxygenic phototrophic microorganisms. The spectral properties of chlorophylls of higher plants, algae and cyanobacteria are well studied, however, the spectral-luminescent properties of their related compounds, bacteriochlorophylls, which participate in anoxigenic photosynthesis, are practically not described in the scientific literature. The polarity of the solvent and the environment have a significant effect on the emission spectra (bacterio)chlorophylls, which is expressed in the spectral shift of the absorption and fluorescence maxima, as well as changes in the fluorescence intensity. The spectral characteristics of bacteriochlorophylls d and e were obtained in organic solvents such as acetone, methanol, ethanol and isopropanol, as well as in acetone-ethanol (7:2) and acetone-methanol (7:2) mixtures. These solvents are most often used for the extraction of bacteriochlorophylls from bacterial cells, so the work will be useful for the development of methods for the quantitative determination of chlorosomal bacteriochlorophylls in bacterial cells or in samples of natural water.

Fiard coasts are common on the periphery of the areas previously covered by ice sheets. The conditions in such areas are favorable for separation of bays and straits from the sea. As a result of the glacial isostatic adjustment of the area the fiards would be transformed into coastal lakes. To discover the regularities of evolution of such water bodies, we studied topography, sea and lake sediments, hydrology and diatom associations of meromictic Lake Kislo-Sladkoe at the Karelian Coast of the Kandalaksha Bay, the White Sea, Russia (66°32′54″N, 33°08′05″E). Detailed geomorphological, geodetic, bathymetric, and aerial imagery field works have been completed on the coastal area. We built a Digital Elevation Model (DEM) of the coast and seabed. Radioisotope dating (210Pb, 14С), grain size distribution, loss on ignition, Corg/Norg ratio and diatom analysis have been completed for the entire 1.5-m sequence of the Lake's sediments. Currently, the Lake Kislo-Sladkoe communicates with the sea through a rising sill. We have established that the water body is meromictic due to its upper water layer being of variable salinity, middle layer comprising of aerated salty water, and the near-bottom water layer being anaerobic. However, several-year-long periods of water stratification alternate there with occasional late-autumn or early-winter flushes. The water body evolution falls into four stages: (1) a strait with an active hydrodynamic environment (prior to 1500–1560s), (2) a strait with a variable hydrodynamic environment (1500–1560s to 1850–1890s), (3) a semi-isolated lagoon with a quiet hydrodynamic environment (1850–1890s to early 1950s), and (4) a meromictic lake at an early stage of separation from the sea (early 1950s to present). We show how coastal processes on fiard coasts change the mode and duration of isolation of coastal water bodies from the sea. Based on the assessment of the rate of the post-glacial rebound and hydrological conditions of the coastal area we propose an approach to estimating the duration of transitional phase between a marine bay or strait and freshwater lake, including its meromictic stage. We predict that the meromictic stage of the lake will be completed no earlier than c. 100–200 years after the sill rises above the tidal zone.

Along the shoreline of the White Sea, due to the post-glacial uplift of the coast, some water bodies with stable stratification have been formed. They have been classified as meromictic as they are at different stages of isolation from the Sea. As separation progresses, significant changes occur in the water column, including the composition of chromophoric dissolved organic matter (CDOM) and the structure of the aquatic microbial community. In this work, we searched for optical proxies of euxinia (anoxic conditions with accumulated hydrogen sulfide) in the water column of the meromictic lagoon on Zeleny Cape. The lagoon is separated from the White Sea basin by a shallow threshold that completely isolates the lagoon during low tide, but marine water enters the lagoon during high tide. The ecosystem in the lagoon is characterized by the marine salinity of water and a high organic matter content in the bottom water and sediments. In this study, spectral methods were used to obtain the depth distribution of CDOM, chlorophyll, and bacteriochlorophyll in the lagoon with strong water stratification and euxinic conditions in the bottom water. The measured optical CDOM characteristics were compared with hydrochemical data (water salinity, Eh, pH, dissolved oxygen), phytoplankton (oxygenic phototrophs), and green sulfur bacteria (anoxygenic phototrophs) distribution along the water column. The spectroscopic methods showed to have the advantages of not requiring water sample pre-treatment and allowing rapid sensing of CDOM and photosynthetic pigments at each horizon.

The article presents data on the composition and abundance of algal flora in the coastal stratified Kislo-Sladkoe Lake (the Karelian coast of the White Sea) after its flushing with seawater in autumn. The seasonal dynamics and vertical structure of the phytoplankton were monitored from March 17 to October 3, 2019. We also studied ice flora on March 17 and April 1. The largest carbon biomass of ice algae (0.79 g C/m(3)) dominated by Nitzschia frigida was recorded in April. The integrated biomass of phytoplankton in the water column varied from 0.01 g C/m(2) in March to 5.83 g C/m(2) during spring bloom in late May (when it was dominated by Chaetoceros invisibilis) and remained at the level of 0.07 to 0.29 g C/m(2) until October; heterotrophic forms replaced autotrophic algae during this period. A regression analysis has shown that the structure of the phytoplankton was influenced by length of daylight, water temperature, dissolved oxygen level, depth, pH, salinity, and illuminance.

Despite surprisingly a small number of protein-coding gene in mammalian genomes, a large variety of different RNAs is being produced. These RNAs are amazingly different in their number, size, cell localization, and mechanism of actions. Although new classes of short RNAs (sRNAs) are being continuously discovered, it is not yet obvious how many of the sRNAs are originated. Altogether, the research in the recent few years has identified an unexpectedly rich variety of mechanisms by which noncoding RNAs act, suggesting that we have identified probably only few of the many potential functional mechanism and more investigation will be needed to comprehensively understand the complex nature and biology of mammalian RNAome. Here, we focus on various aspects of the diversity of the biological role of these nonprotein-coding RNAs (ncRNAs), with emphasis, on functional mechanisms recently elucidated.

An important characteristic of a natural water body is the distribution of pigment composition of different groups of photosynthesizing organisms over depth and its variations in different seasons. Optical methods are widely used to monitor the concentrations of chlorophylls in algae or cyanobacteria in aqueous ecosystems, but they have very limited application in monitoring the bacteriochlorophyll (BChl) concentration in phototrophic bacteria. The paper describes comprehensively three new methods of quantitative analysis of the photosynthetic pigment of BChl d of green sulfur bacteria using spectral measurements in both initial samples of natural water (absorption spectra), and in pigment extracts in organic solvents (absorption and fluorescence spectra). A comparative analysis of the results from different methods is carried out using, as an example, natural water from the chemocline region in Lake Trekhtzvetnoe, one of the meromictic water bodies in the Kandalaksha Bay in the White Sea. Conclusions are drawn about the applicability conditions of different methods, their advantages and limitations. All three methods suggested have been proven to be efficient using, as an example, samples of natural water with BChl concentrations differing by orders of magnitude, from 5 to 5000 mg/m(3).

The composition of microbial communities and the rates of microbial processes in the water column of a meromictic trough in the Biofilter Bay, Kandalaksha Bay, White Sea were investigated in September 2020 and March 2021. The chemocline zone was located at the depths from 8 to 9 m. Sulfide was present in the monimolimnion, its concentration at the bottom reaching 25‒38 mg L–1 (0.7‒1.1 mmol). Conditions of the redox zone favored development of both anaerobic anoxygenic phototrophs and aerobic sulfur-oxidizing chemoautotrophs. A pinkish-brown bacterial plate was formed in this zone. Sequencing of the 16S rRNA gene fragments revealed predominance of green sulfur bacteria (GSB) Chlorobium phaeovibrioides. Aerobic sulfur-oxidizing bacteria Sulfurimonas sp. were the second most abundant group. Plating of chemocline samples on selective media resulted in massive formation of brown-colored Chlorobi colonies, while green GSB colonies were few. Both morphotypes were identified as Chlorobium phaeovibrioides. At the time of our study, purple sulfur bacteria (PSB) did not form a bacterial plate in the redox zone, although few PSB colonies were revealed. The PSB isolate was identified as Thiocapsa rosea. Members of this species are able to switch from photosynthesis to aerobic chemosynthesis and may compete for sulfide with sulfur-oxidizing chemo-autotrophs. We suggest that due to its openness the Biofilter Bay meromictic trough may act as a source for the propagation of anoxygenic phototrophic bacteria in the Kandalaksha Bay.

Phytoplankton of the stratified Kislo-Sladkoe Lake (White Sea coast) was studied using chlorophyll fluorescence methods in 2018. A layer of red water was observed with a chlorophyll a concentration of 1.7 µg/L at a depth of 3.3 m in the chemocline (at the aerobic and anaerobic zone interface). Cryptomonad algae (Rhodomonas sp.) were dominant in the layer; high functional activity of the photosynthetic apparatus was recorded in the algae based on the PIABS fluorescence parameter (high values of the quantum yield of the primary photochemical reaction in photosystem 2 (PS2) (FV/FM)); the proportion of active reaction centers (RCs) (RC/ABS); and the quantum yield of electron transport in PS2 ( φ_E_O ). The high functional activity of the photosynthetic apparatus of cryptomonad algae in the chemocline points toward their stability in the presence of hydrogen sulfide. By comparison, the phytoplankton in the surface water layer showed lower photosynthetic activity. Experiments with photoinhibition showed that the chemocline phytoplankton community is characterized by a greater reparative capacity after photooxidative stress despite the increased photosensitivity. It is proposed to employ chlorophyll fluorescence methods to study a state of phytoplankton in stratified meromictic water bodies.