Limnology and Freshwater Biology

Assessing the sustainability and management implications of Nile Tilapia, Oreochromis niloticus (Pisces: Cichlidae; Linnaeus 1758)

Bousseba M., Ferraj L., Droussi M., Hasnaoui M. — Fri, 31 Oct 2025 00:00:00 +0800

The Nile tilapia (Oreochromis niloticus) was introduced into the Al Massira Dam Lake with the aim of developing profitable fisheries, as a commercially significant species for local populations. However, the stocks of this species have declined in this reservoir due to overfishing pressure, ecological variations, and the effect of other invasive fish species. An assessment of the population dynamics of this species is necessary to develop appropriate fisheries management. This research aimed to determine the state of Nile tilapia stock applying length-based data-limited methods. The von Bertalanffy growth model served to evaluate the sustainability of fisheries. The annual total mortality (Z) and natural mortality (M) rates were determined as respectively 1.79 year^-1 and 0.67 year^-1. The calculated fishing mortality rate (F = 1.12 year^-1) exceeds of the maximum sustainable yield (F_max = 1.06 year^-1), representing the fishing mortality rate ensuring maximum production of equilibrium recruits. Additionally, the current exploitation rate is 0.63, above the reference level (E > 0.5), implying that the Nile tilapia stock in the Al Massira Dam Lake is subject to fishing pressure. Therefore, the stock is overexploited, exceeding optimal levels, and is currently in decline. The findings of this study provide valuable new information about the Oreochromis niloticus stock, enabling better management efforts. To ensure sustainable exploitation of this species, considered vulnerable in Al Massira Reservoir, it is imperative to reduce unregulated fishing and more effectively control overfishing practices.

Assessment of freshwater snails in relation to physicochemical parameters in a rural river, Southeast Nigeria

Anyanwu E.D., Davis I.C., Adetunji O.G., Dickson A.F. — Fri, 31 Oct 2025 00:00:00 +0800

Freshwater snails play a major role in the transmission of some waterborne parasites as well as in understanding ecosystem health. Freshwater snails of a rural river were assessed in relation to some physicochemical parameters. The study was carried out between January and June 2022 in three stations. The water samples were collected and analysed using standard methods while the snails were collected by sweeping the sediments and aquatic macrophytes with a hand net and picking snails off the sediments and macrophytes with hand. One-way Analysis of Variance was used to ascertain significant differences in the physicochemical parameters among the stations while correlation analysis was used to relate the snails’ abundance to the physicochemical parameters. 531 snails of medical and veterinary importance from three species were recorded. The order of abundance was Melanoides tuberculata >Lanistes varicus > Lymnaea natalensis. Most of the snails were recorded in station 3 despite the intensity of human activities and between March and April 2022. The composition, abundance, and distribution of the freshwater snails were influenced by a combination of environmental and anthropogenic factors which in turn were influenced by season. This study showed that Ikwu River is a potential flashpoint for a myriad of water-borne diseases. Knowledge of the distribution of freshwater snails and the factors regulating them will help in their control and the water-borne parasites associated with them.

Growth, mortality, and exploitation rate of Glossogobius giuris (Hamilton, 1822) in Lake Danao Natural Park, Ormoc City (Philippines)

Gaut M.N.P.C., Romero J.B., Dañal R.M.S., Cabansag J.B.P., la Cruz J.O. — Fri, 31 Oct 2025 00:00:00 +0800

The tank goby (Glossogobius giuris) is the most abundant and economically important fish species in Lake Danao, Ormoc City, Philippines. However, it is an introduced, non-native species in the lake. Despite its economic and ecological importance, there has been a lack of studies on its biology and population parameters in the lake. This study aimed to provide baseline information on the population biology of G. giuris by analyzing 867 individuals collected bi-monthly from March 2021 to February 2022. Using the von Bertalanffy growth model, the estimated asymptotic length (L_∞), growth coefficient (K), longevity (t_max), and growth performance (Ø) were found to be 25.31 cm, 1.30 yr^-1, 2.31 years, and 2.91, respectively. Due to its high growth rates, the species has become abundant, potentially adversely impacting native fish populations. On the other hand, the fishing (F), natural (M), and total mortalities (Z) were 3.56 yr^-1, 2.15 yr^-1, and 5.71 yr^-1, respectively. The exploitation rate (E) was 0.62, which is above the optimal rate of 0.5. Recruitment peaks occurred in September and October, with high values for relative yield-per-recruit and biomass-per-recruit: E_max=0.421, E_0.1=0.355, and E_0.5=0.278. The high fishing mortality and exploitation rate indicate overfishing of the species, which could benefit native species in the lake. However, caution is still needed as continuous intensive subsistence fishing practices may deplete the fish stock in the lake and negatively affect the local fishing community.

Spatiotemporal variations of water quality parameters in selected young reservoirs in Tigray, Northern Ethiopia

Kalayu M.A., Mekonen T., Tsehaye A.D., Tsegazeabe H.H. — Fri, 31 Oct 2025 00:00:00 +0800

The health status of aquatic ecosystem is depended on the physicochemical and biological characteristics of water which provides significant information about the available resources for supporting life. This study aims to assess spatiotemporal variation of water quality parameters in selected young reservoirs in Tigray. Depth-integrated water samples (surface, middle, and just above bottom) were collected bimonthly (two times during the wet, dry-cold and dry-hot seasons) at each sampling sites with a heart-valve sampler. Variation between the wet, dry-cold and dry-hot seasons was examined using one-way ANOVA but variation between littoral and pelagic sites were analyzed with the independent t-test. We showed a significant (p<0.05) seasonal variation in temperature, pH and transparency in all the reservoirs investigated. Kalema reservoir was recorded with highest mean value of temperature, chlorophyll-a and conductivity. Mean value of pH has a spatially significant difference (p<0.05) in all the study reservoirs except Kalema. None of the physicochemical water quality parameters in Kalema reservoir exhibited spatially significant (p>0.05) variations. Dissolved oxygen and pH showed spatially significant variation (p<0.05) in Mihtsab-Azmati and Seisa reservoirs. A significant correlation (p<0.01) was recorded between temperature with turbidity, chlorophyll-a, transparency and conductivity. The physicochemical parameters of the reservoirs showed distinct temporal and spatial variations. The result obtained in this study is very important as baseline information to know the ecology of reservoirs for future reservoir management. Further detailed studies that include total phosphorus, total nitrogen, etc., are needed to suggest the trophic conditions of the reservoirs.

Primary production and consumption of organic matter by zooplankton in the Curonian Lagoon during the period of water de-eutrophication

Aleksandrov S.V., Semenova A.S. — Fri, 31 Oct 2025 00:00:00 +0800

The Curonian Lagoon is a waterbody with intensive development of freshwater phytoplankton and zooplankton species. The article considers the features of long-term dynamics of chlorophyll, phytoplankton production, structure, abundance, biomass, and ration of zooplankton. Cyanobacteria and diatoms dominate in phytoplankton, while crustaceans (Cladocera by biomass) predominate in zooplankton, among which filter feeders predominate (80-90%). Since 2019, there has been no “hyperblooming” of water, the abundance (chlorophyll) and primary production of phytoplankton have significantly decreased, as a result of which the trophic status of the lagoon has lessen to the eutrophic level. The amount of organic matter formed by phytoplankton was ~1000 kcal/(m³·year). The cessation of water “hyperbloom” and a sharp decrease in phytoplankton abundance led to a comparable decrease in the biomass and ration of zooplankton in 2019. In subsequent years (2020-2022), an increase in the biomass and ration of zooplankton was observed, which may have been the result of adaptation to changes in the structure and productivity of phytoplankton and zooplankton. A significant increase in ration, along with the intensive development of large-sized cladocerans, characterizes the effective use of phytoplankton production (up to 60-90%) after cessation of water “hyperblooms”.

Modelling of melt using simple and enhanced temperature-index models: testing for the Sygyktinsky Glacier, the Kodar Range (northern Transbaikalia, Russia)

Osipov E.Yu., Osipova O.P. — Fri, 31 Oct 2025 00:00:00 +0800

Modelling snow and ice melt in drainage basins can significantly improve the forecasting of hydrological processes at local and regional scales. Melt rates on the surface of the Sygyktinsky Glacier were measured using an ultrasonic ranger and calculated by the physically based surface energy-balance (SEB) model with a 30-minute resolution for the period from 7 July to 23 August 2021. Two temperature-index models, simple (TM) and enhanced (ETM), were tested to calculate melt rates. All models used 30-minute meteorological data from automatic weather station installed in the central part of the glacier at the mean equilibrium line altitude (2561 m a.s.l.). The performance of the temperature-index models was assessed by comparing 30-minute melt rates with the physically based SEB model. The TM model used air temperature as a predictor, explaining up to 30% of the variability of melt rates. Meanwhile, the ETM model, taking into account incoming shortwave radiation and albedo besides air temperature, demonstrated significantly better performance. The model explained up to 90% of variance in the 30-minute melt rates. The advantages of the enhanced model can be used for future studies of spatial variability of melt rates on the Kodar glaciers.

Spatial and temporary differences in the development of Ulnaria danica, Ulnaria acus, and Fragilaria radians in Lake Baikal

Petrova D.P., Galachyants Yu.P., Firsova A.D. — Fri, 31 Oct 2025 00:00:00 +0800

The assessment of phytoplankton diversity in freshwater bodies strongly depends on the methods used for taxonomic identification. Araphid diatoms of the genera Ulnaria (Kützing) Compère and Fragilaria Kützing are often dominate the phytoplankton composition of freshwater ecosystems. To use of an integrated approach, including metabarcoding using analysis of fragments of the V3-V4 region of 18S rRNA, light and scanning electron microscopy, made it possible to assess the differences of these species. It was shown that in 2017, a gradual change of the studied species occurred in the Southern and Middle parts of Lake Baikal, which is due to the fact that the high development of Fragilaria radians is early then ones of Ulnaria acus. Thus, during the hydrological spring in the pelagic zone of the lake in these areas, and in the Chivyrkuisky and Barguzinsky Bays, U. acus dominated among diatoms. The population development in the Northern part of the lake did not correspond to the revealed trends, according to which a high number of both species should be expected here in the late spring, but it did not exceed 3% of the total number. In the summer, the number of U. acus decreased significantly and reached its minimum in the autumn. Ulnaria danica was present in the phytoplankton of large bays throughout the entire study period.

Biodiversity and structure of zooplankton communities in the Yantarnyi water-flooded quarry (Kaliningrad region, Russia)

Moiseenko V.V., Semenova A.S. — Fri, 31 Oct 2025 00:00:00 +0800

To study the biodiversity and quantitative development of zooplankton, in vestigations were conducted in the Yantarnyi water-flooded quarry in Kaliningrad region in 2011 and 2023–2024. Due to its quarry origin, this water body has a relatively large surface area and high depth. The study included hydrochemical analysis of the water, determination of the taxonomic composition of zooplankton communities, their quantitative indicators, and calculation of the Shannon diversity index for areas with macrophytes and for open water. It was established that Cladocera dominate the quarry in terms of the number of species found, abundance, and biomass. Despite the small projective cover of aquatic vegetation, species diversity is higher in macrophyte stands due to a greater number of Rotifera species and species characteristic of these biotopes. Low mortality rates of zooplankton were recorded in the Yantarnyi water-flooded quarry, indicating favorable conditions for planktonic communities. This is confirmed by the results of hydrochemical analysis, which classify this water body as oligosaprobic.

The effect of sample volume and number of reads on the sequencing result of a microeukaryotic community from an oligotrophic lake

Bashenkhaeva M.V. — Fri, 31 Oct 2025 00:00:00 +0800

Microeukaryotes are an important component in aquatic ecosystems and can be used as an indicator of environmental conditions. Changes in their abundance or diversity can indicate changes in the habitat. In the last decade to addition to light microscopy method, DNA metabarcoding is used a monitoring of microeukaryote community. In order to optimise the metabarcoding method, it is important to determine the necessary sample volume and sequencing depth. This study compared samples with different filtered water volumes (50, 10 and 1.2 L) and sequencing depths to determine their impact on the diversity and taxonomic composition of the unicellular microeukaryotic community in Lake Baikal. ANOSIM analysis revealed similarity among samples with different filtered volumes. This suggests that a small filtered sample volume (1.2 L) is sufficient to reveal a high diversity of taxa in an oligotrophic reservoir, comparable to that obtained with a larger filter volume (50 L). Ciliophora species were detected the highest number of reads by metabarcoding at the large volume of filtered sample (50 L), while diatoms and green algae were better read in smaller volumes (10 and 1.2 L). It is seems that samples volumes in 10 and 1.2 L are sufficient for microalgae monitoring. Samples with different sequencing depths produced similar results in the characterisation of dominant species. Differences were observed in minor taxa, some of which were only detected with a higher number of reads. These results are important for improving the monitoring of oligotrophic bodies using metabarcoding.

Organic matter and nutrients in Lake Glubokoe (Moscow region, Russia)

Sokolov D.I., Tereshina M.A., Erina O.N., Labutin T.A. — Fri, 31 Oct 2025 00:00:00 +0800

According to monitoring data from 2019-2025, the current regime of organic matter and nutrient elements in the water of Lake Glubokoe is assessed. The lake is located in a nature reserve within the Moscow region and can be considered as a natural (background) water body for the region (the maximum nitrogen and phosphorus content is several times lower than in the Mozhaysk Reservoir and dozens of times lower than in Lake Beloe in Moscow). Significant interannual variability in the ecological state of the lake was observed: in years with incomplete spring circulation, intensity of summer anoxia is the strongest, and approximately twice as much mineral nitrogen, phosphorus, and colored iron compounds accumulate at the bottom than in other years. At the same time, no monotonous trend is observed in the regime of nutrient elements throughout the history of research, and no long-term changes occur in the organic matter content since the 1960s, when its content in the lake decreased by about half as a result of reclamation work in the catchment area. This calls into question the existing hypothesis of gradual eutrophication of Lake Glubokoe.