This study investigated fish community dynamics in 26 healthy Latvian lakes to determine the balance between top-down versus bottom-up controls. We employed a multi-methodological approach, combining multi-mesh gillnetting with structured stakeholder interviews and management plan analysis to quantify anthropogenic pressures. We hypothesized that fish community regulation would diverge based on the presence or absence of top predators. Results demonstrated a strong negative correlation between large piscivores and omnivorous fish biomass, validating trophic cascade predictions. While lake area and depth influenced fish length, bottom-up drivers like total phosphorus had limited explanatory capacity. Interestingly, structured stakeholder interviews revealed that fisheries resource management might be an important predictor of ecosystem health. Lakes with perceived low illegal fishing pressures maintained healthy predator proportions, while those under higher pressure had notably fewer large piscivorous fish. We conclude that in otherwise healthy lakes, effective fisheries regulations' enforcement might be more critical than nutrient management for maintaining balanced fish communities.

Chydorus sphaericus is often a dominant cladoceran zooplankton species in water bodies experiencing harmful cyanobacterial blooms. However, its relationship with toxin-producing algae remains largely unexplored. In this study, the feeding behavior of C. sphaericus on colonial cyanobacteria and potentially toxic Microcystis was investigated in a temperate, shallow, eutrophic lake. Liquid chromatographic analyses of phytoplankton marker pigments in C. sphaericus gut content revealed that pigments characteristic of cyanobacteria (identified a zeaxanthin, echinenone, and canthaxanthin) comprised the majority of its diet. Among them, colonial cyanobacteria (marked by the pigment canthaxanthin) were the highly preferred food source despite their minor contribution to phytoplankton biomass. qPCR targeting Microcystis genus-specific mcyE synthase genes, which are involved in microcystin biosynthesis, indicated that potentially toxic strains of Microcystis were present in C. sphaericus gut content throughout its temporal and spatial presence in the lake. The results suggest that the common small cladoceran in eutrophic waters, C. sphaericus, has a close trophic interaction with colonial cyanobacteria (including Microcystis) and may represent an important vector for transferring toxigenic Microcystis to the food web, even under conditions of low Microcystis biomass in the lake water.

Grazing by zooplankton can regulate bloom-forming cyanobacteria but can also transfer toxin-producing cells, as well as toxic metabolites, to the food web. While laboratory investigations have provided extensive knowledge on zooplankton and toxic cyanobacteria interactions, information on zooplankton feeding on toxin-producing cyanobacteria in natural water bodies remains scarce. In this study, we quantified Microcystis-specific mcyE synthase genes from the gut contents of various cladoceran and copepod taxa to assess the in situ crustacean community and taxon-specific ingestion of potentially toxic Microcystis in Lake Peipsi, a large eutrophic lake in Estonia, Northern Europe. Microcystis cells with mcyE genes were found in all crustaceans examined. However, some species, such as the cyclopoid copepod Mesocyclops leuckarti, were more efficient in ingesting potentially toxic Microcystis than other co-occurring cladocerans (Daphnia spp., Bosmina spp., Chydorus sphaericus) and copepods (Eudiaptomus gracilis). The amount of toxigenic Microcystis cells grazed by crustacean population changed temporarily, and copepods were the predominant consumers of toxigenic Microcystis during several months of the 5-month study period. Crustacean ingestion of toxigenic Microcystis was not related to Microcystis biomass or mcyE gene copy numbers in the environment but was instead related to the abundance of major crustacean grazers. Our findings emphasize the close interaction between crustacean zooplankton and toxigenic Microcystis, indicating that some species may play a more significant role in linking toxic cells within the food web than others.

Microplastics (MPs) are frequently detected in the gastrointestinal tracts of aquatic organisms worldwide. We studied the contamination with plastic in an aquatic environment and in the gastrointestinal tracts of fish in two Amazonian rivers waters, Rio Negro and Rio Solimões. The research was conducted in November/December 2023 during the low water season in a year of drastically decreased water level. We evaluated the abundance, shape, type, size as well as polymer type of the MPs and compared the contamination between fish species and feeding types. The amounts of MPs in the water were 0.25 ± 0.05 and 0.78 ± 0.19 pcs/L in the Prato canal and Vila do Janauacá, respectively. In total, 128 fish individuals belonging to different feeding types (piscivorous, planktivorous, herbivorous, invertivorous, detritivorous, omnivorous) were analysed in this study, with 84.0% and 87.4% of fish containing MPs caught from the Prato canal and Vila do Janauacá, respectively. We found no statistical relationship between the length/weight of the fish and the amount of MPs or the sizes of the plastic parts. MPs with dimensions of 0.04-5.0 mm were found in the digestive tracts of fish. There was an average of 3.3 ± 2.9 and 2.8 ± 2.1 plastic particles per fish in the study sites of the Prato canal and Vila do Janauacá, respectively. There was a statistical difference in the size of the MPs between the river sites that was correspondingly reflected in fish. In the digestive tracts of Solimões fish, plastics were on average larger than in the Rio Negro study site. At the Vila do Janauacá study site, predatory fish had the most MPs in their digestive system, while in the Prato canal, plankton-eating fish had the most MPs. Regarding the shape of MPs ingested by fishes, most were filaments, followed by fragments. The majority of the fibres were determined to be polyamide (nylon), polyethylene terephthalate and polypropylene. Our results indicate that even the remote aquatic environments may be impacted with intensive MP infestation. At the same time, with higher rainfall and higher water levels, a higher concentration of MPs is expected and this, in turn, can lead to greater contamination of fish with MPs. It has been observed that when water has a higher plastic content, more MPs end up in fish, especially in visual foragers.

Amazonian floodplain lakes, distinguished by their water types, are crucial ecosystems for fish biodiversity. In these ecosystems, the annual hydrological cycle, known as the "flood pulse," is the primary driving force for fish productivity, providing habitat and feeding opportunities. However, how seasonal water level changes affect fish assemblages and feeding guild responses in floodplains with varying watershed characteristics is still poorly understood. We conducted a comparative stable isotope study in both a white-water and a black-water floodplain lake to investigate fish trophic dynamics during periods of rising and low water levels. Analyses of a broad taxonomic range of fish (seven orders and 27 families, with 73 and 82 taxa in the studied lakes, respectively) revealed that the fish assemblages in the studied white-water and black-water lakes exhibited different trophic dynamics in response to changes induced by the hydrological cycle. In contrast to the white-water lake, a strong, trophic guild-related effect shaped the δ13C and δ15N dynamics in the black-water lake. As a result, the fish assemblage in the black-water lake experienced significant trophic shifts with generally opposite dynamics over the water levels compared to the white-water lake. In the black-water lake, water level changes caused alterations in trophic niche width in almost all analyzed guilds (carnivorous, detritivorous, herbivorous, invertivorous, omnivorous, planktivorous), while this was less consistent in the white-water lake. Our results suggest that the fish assemblage in the nutrient-poor black-water lake is more dependent on the annual flood pulse and associated resources than in the white-water lake. These findings may further imply a relatively higher vulnerability of the black-water lake to alterations in the regular flood pulse compared to the white-water lake, indicating greater stability in feeding conditions and fish trophic dynamics in the latter.

The annual flood pulse is a defining feature of Amazonian floodplain lakes, creating a highly variable environment that influences resource availability, such as food and habitat. These cyclical changes necessitate a high degree of adaptability among fish species, many of which have evolved specialized strategies to cope with the fluctuating conditions. In 2023, the Amazon basin experienced a record-breaking drought event, leading to mass mortality of Amazonian fish and other wildlife. This study examines the effect of this extreme event on fish condition in white-water (Rio Solim & otilde;es basin) and black-water (Rio Negro basin) floodplain lakes. These contrasting environments provide a unique opportunity to study how different water qualities and extreme water-level fluctuations impact fish condition. Research was conducted during the normal low-water period in November 2019 and the drastically decreased water levels in November 2023. The main objective was to understand how extreme water-level fluctuations affect fish health and nutritional status. A total of 585 fishes were analyzed, with 294 from white-water and 291 from black-water, representing different feeding types to provide a comprehensive picture of changes in fish condition. Water-level changes had a statistically significant impact on fish condition in both areas. Comparing low-water and extreme low-water levels, fish condition was consistently higher during the normal low-water period. The linear mixed-effects model revealed that the intensity of the low-water season had a significant effect on fish length-adjusted mass, suggesting that the decrease in water level is associated with an overall decrease in fish length-adjusted mass. When comparing the mean water-level effect (Glass's Delta) between low-water and extreme low-water levels, we found a bigger effect in the black-water system than in the white-water system. This difference may be attributed to the lower nutrient content and higher levels of humic acids and refractory dissolved organic matter in black-water, which can further limit primary productivity and food availability for fishes.

We present a 57-year time series and relationship with environmental parameters of a zooplankton species Chydorus sphaericus, a common small-sized cladoceran species in eutrophic lakes, in a large, shallow and eutrophic Lake Võrtsjärv (Estonia). We show that over the course of more than five decades, the abundance, mean weight and the proportions of C. sphaericus among metazooplankton biomass in this lake have markedly changed. Planktivorous fish (bleak Alburnus alburnus in particular) and total phosphorus were the most influential variables explaining the interannual changes of C. sphaericus individual weight. Abundance and the C. sphaericus proportion among metazooplankton biomass were dependent of a complex set of antagonistic interactions between water temperature, cyanobacteria biomass and pH. Cyanobacterial biomass stood out as the most influential factor for C. sphaericus at the monthly scale. Chydorus sphaericus metrics were negatively correlated with cyanobacteria, positively when cyanobacteria biomass was paired with pH or water temperature, and negatively again when all three variables were present at the same time. Our results confirm that C. sphaericus occupies an important position in a eutrophic lake food web and has been able to thrive in the recent decades through adaptative interactions with its environment.

Net ecosystem production (NEP) is an important indicator of lake ecosystem function and integrity. An earlier study, restricted to one geographical region, indicated that oxygen saturation levels (DO

Eutrophication and lake depth are of key importance in structuring lake ecosystems. To elucidate the effect of contrasting nutrient concentrations and water levels on the microbial community in fully mixed shallow lakes, we manipulated water depth and nutrients in a lake mesocosm experiment in north temperate Estonia and followed the microbial community dynamics over a 6-month period. The experiment was carried out in Lake Võrtsjärv—a large, shallow eutrophic lake. We used two nutrient levels crossed with two water depths, each represented by four replicates. We found treatment effects on the microbial food web structure, with nutrients having a positive and water depth a negative effect on the biomasses of bacterial and heterotrophic nanoflagellates (HNF) (RM-ANOVA, p < 0.05). Nutrients affected positively and depth negatively the mean size of individual HNF and ciliate cells (RM-ANOVA; p < 0.05). The interactions of depth and nutrients affected positively the biomass of bacterivorous and bacteri-herbivorous ciliates and negatively the biomass of predaceous ciliates (RM-ANOVA; p < 0.05). Bacterivorous ciliates had lowest biomass in shallow and nutrient-rich mesocosms, whilst predaceous ciliates had highest biomass here, influencing trophic interactions in the microbial loop. Overall, increased nutrient concentrations and decreased water level resulted in an enhanced bacterial biomass and a decrease in their main grazers. These differences appeared to reflect distinctive regulation mechanisms inside the protozoan community and in the trophic interactions in the microbial loop community.

We studied the relationships between the planktonic food base and feeding patterns of juvenile mid-summer/early autumn Eurasian perch Perca fluviatilis L., a common predatory freshwater fish in large parts of Europe and Asia. The feeding of 0+ perch was studied during summer and autumn in littoral habitats of seven lakes with different environmental conditions - four Latvian (Auciema, Riebinu, Varzgunes, Laukezers) and three Estonian (Kaiavere, Prossa and Akste) lakes. Simultaneously, the abundance, biomass and structure of zooplankton communities were examined. We focused on the littoral areas because many studies in lakes suggest that littoral habitats are particularly important for 0+ fish growth and survival. We were interested in the question: can the diet and growth of 0+ perch be explained by zooplankton community structure? We also presumed that if the amount of zooplankton is low, more benthic invertebrates will be consumed by 0+ perch. Opposite to expectations, we found that zooplankton always counted for over 90% of diet biomass in perch. There were also clear correlations between the zooplankton biomass in a given lake, the zooplankton biomass in 0+ perch stomachs, and the fish growth rate. The study also suggested that nutrient enrichment can positively impact the 0+ perch feeding conditions in lakes.

Eurasian perch has a great ecological importance in freshwaters as it is orten a dominating predatory fish in ecosystems. The knowledge of perch feeding patterns, strategies, and adaptivity to food environment in their early life stages is essential to understand its population development and dynamics. It has been demonstrated that there exists a positive relationships between preferred prey availability and larval feeding success. We examined the diet of larval perch in their natural habitats of different zooplankton assemblages in three small lakes with varying ecological status. In each lake, both pelagial and littoral were studied. We found significant relation between zooplankton biomass in the lake and zooplankton biomass consumed by fish larvae. The most important food objects were crus-taceans (Bosmina longirostris, Chydorus sphaericus). Ciliates were consumed only in one lake. Our results show that perch larvae are flexible in their choice of food and can adapt to different food environments.

As a result of natural and anthropogenic eutrophication, shallow lakes ultimately become wetlands. Several aquatic ecosystem values diminish, but some biotic communities may benefit. Lake Lahepera is a very shallow lake filled with sediments and overgrown with macrophytes. It is a former bay and an important spawning ground for fishes of Lake Peipsi, the fourth largest lake in Europe. The main question is, how to reconcile the goals of nature conservation and circular economy - restore and maintain good functioning of the lake ecosystem, preserve habitats for wetland communities, make economic use of sapropel, and renew spawning conditions for fish. The lake has been investigated since the 1950s. Resulting from strong human pressure, especially in the 1970s and 1980s, the accumulated organic sediments and macrophyte overgrowth have diminished the habitat diversity of the lake. Irregular flushing of the lake with Lake Peipsi waters can wash away large amounts of phosphorus. According to the investigations in 2014-15, phosphorus in- and outflow are in balance, but the internal loading is high. A set of possible restoration options with sediment and macrophyte removal methods is proposed and their outcome assessed using the ecosystem service concept. A comparison of possible future scenarios, based ecosystem service values shows that with a balanced combination of different habitat restoration methods it is possible to achieve stable ecological status of the lake. Species diversity, especially that of floating leaved macrophytes, will increase in the lake. At the same time, wetland habitats will retain their values.

We live in the information age, but we still do not fully understand how the surrounding nature exchanges, collects, and manages information—both at the ecosystem and cellular levels. Still, it is crucial to possess a dynamic and developing comprehension of the functioning of nature in order to understand the phenomena occurring in our ever-changing world. Sometimes we encounter an attitude that there is no need to investigate anything further, as everything has already been researched. This is a very dangerous attitude. Without information, we should not actually talk about the age of information, and it is worth reviewing some knowledge that is deeply ingrained in us from time to time. Occasionally, it is worth taking a step back and considering whether we ourselves may have become trapped in dogmas that may hinder our understanding. This presentation focuses on various aspects of understanding nature, starting from single-celled organisms and ending with ecosystems. The topics that will be discussed among others are (1) infochemicals (how do they actually affect the biota?); (2) the remarkable process of cellular computation (does this imply that cellular materials can show a primitive intelligence?); and (3) the ability of plants to influence wind patterns and bring in more moisture from the ocean (what will happen to us when we excessively deforest?).

Aquatic macrophyte taxonomic composition, species abundance and cover determine the physical structure, complexity and heterogeneity of aquatic habitats - the structuring role of macrophytes. These traits influence richness, distribution, feeding and strength of the relationships between food web communities in lakes. The aim of this study was to determine how lakes with different dominating macrophyte ecological groups affect planktonic food web components, emphasising the influence on young of year (YOY) fish and large (>= 1 +) fish community. We hypothesised that different dominating macrophyte ecological groups have different structural effects on food web components and YOY fish growth, abundance and feeding. Studied lakes categorised into three different macrophyte ecological groups - lakes dominated by emergent, floating+floating-leaved or submerged vegetation. We found that all dominating ecological groups had a strong influence on plankton communities (except heterotrophic bacterioplankton and nanoflagellates), YOY fish and large fish. Floating-leaved plant dominance was positively related to planktonic food web structure and YOY fish weight, length, abundance and the consumption of zooplankton as a prey of all major species of YOY fishes. Larger fish tended to favour the presence of emergent vegetation. This conclusion has important implications for local managers and conservationists in respect to the maintenance and protection of littoral habitats and fish resources.

Ecosystem models that measure the impact of quantitative interactions between trophic levels are widely used tools in ecosystem studies and fishery management. We constructed a mass-balance trophic model using an Ecopath with Ecosim (EwE) modelling suite for large shallow Lake Võrtsjärv, Estonia. The model was calibrated for 36 years (1983–2018) and included 23 functional groups. We examined trophic relationships, functional group interactions, energy fluxes, and keystone groups having a high impact on the ecosystem relative to their biomass. We tested 6 hypothetical scenarios based on future biomass changes for the major functional groups (phytoplankton, zooplankton, macrozoobenthos, piscivorous fish, and bream) for 20 years. The output of the predictive scenarios showed that the biomass changes of planktonic groups would affect the whole food web. Among consumers, macrozoobenthos was crucial for the food web balance because a reduction of their biomass would also reduce the biomass of the fish community. Changes in fish catches would cause minimal biomass difference in other groups. While increased fishing pressure on large piscivores would have a marked effect on the rest of the food web, the reduction of nonpiscivorous fish like bream would have little effect. The results suggested a positive relationship between the biomass of small phytoplankton and fish, alluding to the prevalence of bottom-up trophic processes. These outcomes could be helpful for assessing trophic dynamics in shallow lakes and important aspects for fisheries and ecosystem management.

Photoautotrophic picoplankton (0.2–2 μm) can be a major contributor to primary production and play a significant part in the ecosystem carbon flow. However, the understanding about the dynamics of both eukaryotic and prokaryotic components of picoplankton in shallow eutrophic freshwater environments is still poor. Very few studies in these ecosystems reveal the taxonomic composition of picoeukaryotes. The main objective of this study was to investigate the seasonal dynamics of phototrophic picoplankton with the emphasis on the eukaryote community composition in a large shallow, eutrophic lake of the northern temperate zone (Lake Võrtsjärv). Phytoplankton pigments were employed to determine the taxonomic composition of photoautotrophic picoplankton. We found out that photoautotrophic picoplankton constitutes an important part of the phytoplankton community in Lake Võrtsjärv and its contribution can be highly variable (from ~9.3% to ~39%) in different years. The eukaryotic photoautotrophic picoplankton was dominated by diatoms followed by chrysophytes and other minor groups. Picoeukaryotes were prevailing in low-light conditions and low temperatures as their predominance in the picoplankton community was tightly linked to the presence or absence of ice cover. Ice cover strongly suppressed the growth of picocyanobacteria. Total phosphorus, turbidity and metazooplankton abundance had a clear relationship with photoautotrophic picoplankton chlorophyll a.

We studied the feeding of European perch Perca fluviatilis L. larvae in littoral and pelagic habitats of four different lakes - one Latvian (Auciema) and three Estonian (Akste, Kaiavere, and Prossa). Altogether, 162 perch larvae (81 from both habitats) were collected to estimate the diet composition of gathered larval specimens in spring (2019) using gut content analysis via epifluorescence microscopy. Attention was paid particularly to the question how does the larval perch food composition differ in pelagic and littoral habitats. We hypothesized that the consumption of zooplankton is higher and the larval condition is better in littoral habitats. We assessed the feeding on both protozoo- (ciliates) and metazooplankton and applied multiple indices (Hurlbert's standardized niche breadth, Ivlev's selectivity and relative importance index) to evaluate, respectively, the larval fish prey importance, feeding homogeneity and strategies. The results showed that larval length and weight were slightly higher and body condition was slightly better in the lakes' littoral habitats. The feeding niche of perch larvae was narrower in the littoral, which can indicate more favourable feeding conditions in littoral than lake pelagic habitats. While the small cladocerans (Bosmina longirostris Muller) were generally the preferred and important food objects, ciliates were avoided and consumed only when their share in the total zooplankton biomass was >40%. However, in shortage of cladocerans, ciliates could be vitally important food objects for perch larvae.

We aimed to investigate the influence of environmental factors and predict zooplankton biomass and abundance in shallow eutrophic lakes. We employed time series of zooplankton and environmental parameters that were measured monthly during 38 years in a large, shallow eutrophic lake in Estonia to build estimates of zooplankton community metrics (cladocerans, copepods, rotifers, ciliates). The analysis of historical time series revealed that air temperature was by far the most important variable for explaining zooplankton biomass and abundance, followed, in decreasing order of importance, by pH, phytoplankton biomass and nitrate concentration. Models constructed with the best predicting variables explained up to 71% of zooplankton biomass variance. Most of the predictive variables had opposing or antagonistic interactions, often mitigating the effect of temperature. In the second part of the study, three future climate scenarios were developed following different Intergovernmental Panel on Climate Change (IPCC) temperature projections and entered into an empirical model. Simulation results showed that only a scenario in which air temperature stabilizes would curb total metazooplankton biomass and abundance. In other scenarios, metazooplankton biomass and abundance would likely exceed historical ranges whereas ciliates would not expand. Within the metazooplankton community, copepods would increase in biomass and abundance, whereas cladocerans would lose in biomass but not in abundance. These changes in the zooplankton community will have important consequences for lake trophic structure and ecosystem functioning.

We numerically explored the effects of long-term water level changes on biotic biomass and spatial distribution of fish in a large shallow lake. We calibrated Ecospace model (Ecopath with Ecosim modelling suite) with data from various functional groups (ranging from phytoplankton to piscivorous fish), and considered 14 different habitats. Two scenarios representing, respectively, a long-term water-level increase and decrease by 1 m were constructed and run for a period of thirty eight years (1979-2016). The results showed a very uneven spatial distribution of fish biomass in the lake, with the highest concentration in the southern basin. The 1 m decrease scenario caused a diminution in the biomass of all groups but piscivorous fish. The 1 m increase scenario saw a weak decrease in most species biomass. Consequently, in both scenarios, long-term water level changes would be generally detrimental to the lake biota. In the context of more frequent climate-induced hydrological fluctuations, we encourage the use of these simulations as effective tools for future prediction and assessment of ecosystem-based fisheries management and ecological status maintenance of shallow lakes. (C) 2020 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

Important drivers of gross primary production (GPP) and ecosystem respiration (ER) in lakes are temperature, nutrients, and light availability, which are predicted to be affected by climate change. Little is known about how these three factors jointly influence shallow lakes metabolism and metabolic status as net heterotrophic or autotrophic. We conducted a pan-European standardized mesocosm experiment covering a temperature gradient from Sweden to Greece to test the differential temperature sensitivity of GPP and ER at two nutrient levels (mesotrophic or eutrophic) crossed with two water levels (1 m and 2 m) to simulate different light regimes. The findings from our experiment were compared with predictions made according the metabolic theory of ecology (MTE). GPP and ER were significantly higher in eutrophic mesocosms than in mesotrophic ones, and in shallow mesocosms compared to deep ones, while nutrient status and depth did not interact. The estimated temperature gains for ER of similar to 0.62 eV were comparable with those predicted by MTE. Temperature sensitivity for GPP was slightly higher than expected similar to 0.54 eV, but when corrected for daylight length, it was more consistent with predictions from MTE similar to 0.31 eV. The threshold temperature for the switch from autotrophy to heterotrophy was lower under mesotrophic (similar to 11 degrees C) than eutrophic conditions (similar to 20 degrees C). Therefore, despite a lack of significant temperature-treatment interactions in driving metabolism, the mesocosm's nutrient level proved to be crucial for how much warming a system can tolerate before it switches from net autotrophy to net heterotrophy.