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  • Kirje
    Long-term data reflect nitrogen pollution in Estonian rivers
    (IWA Publishing, 2022) Nõges, Tiina; Vilbaste, Sirje; McCarthy, Mark J.; Tamm, Marju; Nõges, Peeter
    We analysed long-term (1992–2020) changes in fertiliser use, wastewater treatment, and river water nutrient status in Estonia (N-E Europe) in the context of changing socio-economic situations and legislation. We hypothesised that improved regulation of fertiliser usage and wastewater treatment are reflected as declining riverine nutrient concentrations, with the largest relative improvements occurring in catchments with initially high proportions of point source loading. We analysed nutrient dynamics in 16 rivers differing by catchment land use, population and livestock densities. Data on fertiliser use and wastewater treatment originated from the Statistics Estonia database, and riverine nutrient concentrations from the State Environmental Monitoring Database. We clustered the rivers by their catchment properties and analysed trends in their nutrient status. Point source nutrient loading reductions explained most of the decline in riverine nutrient concentrations, whereas application of mineral fertilisers has increased, hindering efforts to reach water quality and nutrient load targets set by the EU Water Framework Directive and the Baltic Sea Action Plan. Highest nitrogen concentrations and strongest increasing trends were found in rivers within the Nitrate Vulnerable Zone, indicating violation of the EU Nitrates Directive. To comply with these directives, resource managers must address non-point source nutrient loading from river watersheds.
  • Kirje
    Changes in nutrient concentration and water level affect the microbial loop: a 6‑month mesocosm experiment
    (Springer, 2023) Zingel, Priit; Jeppesen, Erik; Nõges, Tiina; Hejzlar, Josef; Tavşanoğlu, Ülkü Nihan; Papastergiadou, Eva; Scharfenberger, Ulrike; Agasild, Helen
    Eutrophication and lake depth are of key importance in structuring lake ecosystems. To eluci- date 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 com- munity dynamics over a 6-month period. The experi- ment 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 bio- masses of bacterial and heterotrophic nanoflagellates (HNF) (RM-ANOVA, p < 0.05). Nutrients affected positively and depth negatively the mean size of indi- vidual 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). Bacte- rivorous ciliates had lowest biomass in shallow and nutrient-rich mesocosms, whilst predaceous ciliates had highest biomass here, influencing trophic interac- tions in the microbial loop. Overall, increased nutri- ent 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.
  • Kirje
    Behavioural responses of invasive (Gmelinoides fasciatus) and native (Gammarus lacustris) amphipods to predators on different bottom substrates
    (Springer Nature, 2023) Teesalu, Paul; Ercoli, Fabio; Tuvikene, Arvo
    The amphipod Gmelinoides fasciatus has invaded and established in numerous large lakes in Eurasia and, in the process, has displaced the native amphipod, Gammarus lacustris. The mechanism behind its invasion success is unclear and remains an important topic for invasion ecology. Three labo- ratory experiments were conducted to determine if superior predator avoidance and different types of bottom substrate could be important factors contrib- uting to the invasion success of G. fasciatus. Our results indicate that, on gravel and sand substrates, G. fasciatus exhibited superior digging behaviour to avoid predation by fishes (perch and common roach), contrary to G. lacustris. On sandy substrate, only 9% of G. fasciatus individuals were consumed, whereas G. lacustris were consumed much more intensively at 74%. In addition, G. fasciatus exhibited a more sub- stantial reduction in activity, where the time spent in motion reduced from 37.6 to 20.8% compared to G. lacustris (from 21.8 to 17.4%) when in the presence of predatory fish kairomones. G. fasciatus movement speed was also reduced in the presence of kairomones (from 57.1 to 39.3 mm/s). However, the presence of kairomones had little effect on digging behaviour of G. fasciatus. G. fasciatus consistently demonstrated superior predator avoidance abilities over G. lacus- tris, suggesting that this mechanism might play an important role in the invasion success of G. fasciatus.
  • Kirje
    Differences on the level of hepatic transcriptome between two flatfish species in response to liver cancer and environmental pollution levels
    (Elsevier, 2023) Sepp, Tuul; Baines, Ciara; Kreitsberg, Randel; Scharsack, Jörn Peter; Nogueira, Pedro; Lang, Thomas; Fort, Jérôme; Sild, Elin; Clarke, John T.; Tuvikene, Arvo; Meitern, Richard
    Environmental factors can cause cancer in both wild animals and humans. In ecological settings, genetic variation and natural selection can sometimes produce resilience to the negative impacts of environmental change. An increase in oncogenic substances in natural habitats has therefore, unintentionally, created opportunities for using polluted habitats to study cancer defence mechanisms. The Baltic and North Sea are among the most contaminated marine areas, with a long history of pollution. Two flatfish species (flounder, Platichthys flesus and dab, Limanda limanda) are used as ecotoxicological indicator species due to pollution-induced liver cancer. Cancer is more prevalent in dab, suggesting species-specific differences in vulnerability and/or defence mechanisms. We conducted gene expression analyses for 30 flatfishes. We characterize between- and within-species patterns in potential cancer-related mechanisms. By comparing cancerous and healthy fishes, and noncancerous fishes from clean and polluted sites, we suggest also genes and related physiological mechanisms that could contribute to a higher resistance to pollution-induced cancer in flounders. We discovered changes in transcriptome related to elevated pollutant metabolism, alongside greater tumour suppression mechanisms in the liver tissue of flounders compared to dabs. This suggests either hormetic upregulation of tumour suppression or a stronger natural selection pressure for higher cancer resistance for flounders in polluted environment. Based on gene expression patterns seen in cancerous and healthy fish, for liver cancer to develop in flounders, genetic defence mechanisms need to be suppressed, while in dabs, analogous process is weak or absent. We conclude that wild species could offer novel insights and ideas for understanding the nature and evolution of natural cancer defence mechanisms.
  • Kirje
    Climate change affects lakes. What can we do?: Policy brief, June 26, 2023
    (Estonian University of Life Sciences, 2023) Tuvikene, Lea; Nõges, Peeter
    ✓ Lake ecosystems provide vital services for humanity, but, due to global climate change, the quality and quantity of these services is decreasing. ✓ Lakes supply us with clean water and fish, they regulate nutrient cycling and modify climate change effects by controlling floods and the amount of CO2, which is the main cause of climate warming. In an urbanizing world, the aesthetic and recreational importance of lakes for human well-being is increasing. ✓ Changes in temperature, precipitation and wind affect lake levels and water availability, alter the distribution of oxygen and nutrients, and impact the survival and abundance of aquatic organisms: algal blooms expand causing massive fish kills. ✓ Understanding these mutual interactions through improved climate models, monitoring of water quality, and assessing ecosystem health is crucial for managing and mitigating the impacts of climate change on lakes. ✓ As the impact of eutrophication and climate change follow the same pathways affecting nutrient availability and cascading effects in the food web, establishing stricter control over anthropogenic nutrient loading is the most efficient adaptive strategy to preserve the ecological integrity of lakes and the services they provide.
  • Kirje
    Peipsi järve ökosüsteemi seisundi parandamise strateegia: D7.2
    (Estonian University of Life Sciences, 2023) Tuvikene, Lea (koostaja)
    Strateegia Peipsi järve ökoloogilise seisundi parandamiseks sisaldab lühikest kokkuvõtet Euroopa suuruselt neljanda järve, Peipsi järve ökosüsteemi seisundist ja seda mõjutavatest teguritest, annab ülevaate Eesti Vabariigi ja Venemaa Föderatsiooni piirijärve majandamisest ning kavandab ökosüsteemi seisundi paranemist toetavad tegevused. Järve põhiliseks keskkonnaprobleemiks on inimtekkeline eutrofeerumine, mida võimendavad globaalsed kliimamuutused. Selle suure ja madala järve ökoloogilise seisundi peamised looduslikud mõjutajad on veetaseme kõikumine ja lokaalsed ilmastikuolud. Kliima soojenemine toob kaasa sagedasemad madala vee perioodid ja annab sinivetikatele eelise teiste vetikate ees, põhjustades suviti nende massilist vohamist. Vetikaõitsengud põhjustavad kalade suremist ja vee läbipaistvuse vähenemist. Roostikualad laienevad hoogsalt ja võõrliigid ohustavad kohalikke liike. Peipsi seisundi halvenemisest annavad märku vee läbipaistvuse usaldusväärne langustrend järve kõikides osades, fütoplanktoni hulga kasv, sinivetikate osakaalu suurenemine vetikate biomassis, fütoplanktoni liigirikkuse vähenemine, metazooplanktoni hulgas keriloomade osakaalu suurenemine ja võõrliikide kahjulik mõju kohalikule elustikule. Peipsi järve ökosüsteemi seisundi säilitamise ja parandamise strateegia on suunatud kõigile järve hea seisundi eest seisvatele ametiasutustele, mittetulundusühingutele, Peipsi-äärsetele omavalitsustele. Strateegia peaeesmärk on välja tuua tegevused, mida tuleb järjepidevalt jätkata, aga ka kavandada võimalikke järve ökosüsteemi toimimist toetavaid tegevusi. Esmajoones on vaja pidurdada inimtekkelist eutrofeerumist. Sisuliselt tähendab see nii suure järve puhul jätkuvat punkt- ja hajureostuse vähendamist, et veelgi piirata taimtoiteainete lisandumist järve. Klassikaliste järvede tervendamise meetodite (nt setetest puhastamine, setete keemiline stabiliseerimine, veevahetuse muutmine, biomanipulatsioon) rakendamine on nii suure järve puhul ilmselt seotud liiga suurte kulutustega. Kavandatud on seniste tegevuste (iga-aastane riiklik keskkonnaseire, piiriülene koostöö ja teadusuuringud) jätkamine ja täiendamine vastavalt ressursside kättesaadavusele, olulisemad rakendus- ja teadusuuringud, teavitustegevused. Peipsi järve ökosüsteemi seisundi parandamise eesmärgil on keskse tähtsusega rakendusuuringud (paremal juhul kompleksne uuring), mis on suunatud fosfori ja lämmastiku bilansi väljaselgitamisele ja toiteainete koormuse vähendamise meetmete kavandamisele. See tähendab nii väliskoormuse (punkt- ja hajureostuse) kui sisekoormuse ulatuse võimalikult täpset hindamist. Põhjalik saasteainete koormuse kaardistamine Peipsi vesikonnas on tehtud aastal 2015 (Piirimäe et al., 2015), kasutades modelleerimist (PolFlow mudel), kuid aastate jooksul on koormuse vähendamise meetmeid rakendatud nii asulate kui hajaasustuse heitveekäitluses. Samas näitavad uuringud põllumajandusest lähtuva hajureostuskoormuse mõningast suurenemist viimasel ajal (Nõges et al., 2020). Lisaks regulaarsele suurtaimestiku seirele on vaja teostada roostike uuringud, et luua nende majandamise kava, mis arvestaks ka roostike kui elupaigaga. Kaasaegsete meetodite nagu kaugseire ja pidevsagedusmõõtmiste kasutamine annab kiiret ja olulist informatsiooni muutuste kohta, modelleerimine võimaldab prognoosida tulevikustsenaariume ja kavandada ennetavaid samme ökosüsteemi huvides. Sinvetikamürkide regulaarne seire suve- ja sügiskuudel annab olulist teavet järve öksosüsteemi seisundi kohta, aga panustab ka ökosüsteemiteenuste hindamisse puhkemajanduslikust, toiduturvalisuse ja veekasutuse seisukohast. Välja pakutud rakendus- ja teadusuuringud katavad erinevaid Peipsi ökosüsteemi toimimise mõttes olulisi teemasid nagu toiteainete ringe ja toiduahela uuringud, järveressursside (nt vesi, pilliroog, kalad, võõrliigid) kasutamise mõjude selgitamine ökosüsteemile, toksiliste sinivetikatüvede eristamine mittetoksilistest (genotüpiseerimine), ohtlike ainete ning mikro- ja nanoplastide uuringud. Lisamaterjalid annavad ülevaate Eesti Maaülikoolis hiljuti tehtud või käimasolevatest teadus- ja rakendusprojektidest, mis on otseselt või kaudsemalt seotud Peipsi ökosüsteemiga, ning olulisematest teemakohastest teaduspublikatsioonidest. Strateegia valmimist rahastas Horisont 2020, Euroopa Liidu teadusuuringute ja innovatsiooni raamprogramm grandiga nr. 951963, TREICLAKE ning Eesti Teadusagentuur grantidega PRG709 ja PRG1167.
  • Kirje
    Spatio-temporal variations in sediment phosphorus dynamics in a large shallow lake: Mechanisms and impacts of redox-related internal phosphorus loading
    (Elsevier, 2023) Tammeorg, Olga; Nürnberg, Gertrud K.; Tõnno, Ilmar; Toom, Lauri; Nõges, Peeter
    The role of redox-related sediment phosphorus (P) release in shallow polymictic lakes remains poorly understood. Our previous studies in large and shallow Lake Peipsi suggested the importance of the redox-related P release in internal P loading. In the current study, we explored the validity of this hypothesis by also considering organic sediment P (Org-P). We analysed spatio-temporal changes in diffusive P flux and sediment P forms determined by P fractionation and nuclear magnetic resonance (NMR) spectroscopy in summer 2021. Using 1997–2021 data, we computed internal P load (IL) by two methods and studied their relationships with several water quality variables. Anoxia of sediment surfaces and P release progressed with an increase in water tem- perature during summer. In the long-term, IL estimates by two methods were similar (mean values: 315 and 346 mg/m2/year) and correlated with the predicted anoxia of sediment surfaces (AApred). A contribution of sediment iron-bound P (Fe-P) to P release was indicated by the positive correlation of Fe-P with orthophosphate (NMR) in the short-term studies. No similar evidence was found for Org-P, which contradicts the common tendency to attribute internal P loads largely to Org-P in eutrophic lakes. Nevertheless, organic matter seemed to support reductive dissolution, because seasonal changes in sediment Org-P correlated with those in Fe-P, and organic matter content and diffusive P flux were negatively correlated over different sites. Complex bottom morphology and hydrology affected spatial distribution of the sediment P forms and masked the relationships between sediment P variables and P release. Finally, the importance of redox-related release was reflected in significant relationships between AApred and associated IL with Secchi depth transparency, chlorophyll a concentration, and the biomass of phytoplankton and cyanobacteria. To our knowledge, this is the first time when such direct ev- idence was provided for a large polymictic lake.
  • Kirje
    Sustainable lake restoration: From challenges to solutions
    (Wiley, 2023) Tammeorg, Olga; Chorus, Ingrid; Spears, Bryan; Nõges, Peeter; Nürnberg, Gertrud K.; Tammeorg, Priit; Søndergaard, Martin; Jeppesen, Erik; Paerl, Hans; Huser, Brian; Horppila, Jukka; Jilbert, Tom; Budzyńska, Agnieszka; Dondajewska-Pielka, Renata; Gołdyn, Ryszard; Haasler, Sina; Hellsten, Seppo; Härkönen, Laura H.; Kiani, Mina; Kozak, Anna; Kotamäki, Niina; Kowalczewska-Madura, Katarzyna; Newell, Silvia; Nurminen, Leena; Nõges, Tiina; Reitzel, Kasper; Rosińska, Joanna; Ruuhijärvi, Jukka; Silvonen, Soila; Skov, Christian; Važić, Tamara; Ventelä, Anne-Mari; Waajen, Guido; Lürling, Miquel
    Sustainable management of lakes requires us to overcome ecological, economic, and social challenges. These challenges can be addressed by focus- ing on achieving ecological improvement within a multifaceted, co-beneficial context. In-lake restoration measures may promote more rapid ecosystem responses than is feasible with catchment measures alone, even if multiple interventions are needed. In particular, we identify restoration methods that support the overarching societal target of a circular economy through the use of nutrients, sediments, or biomass that are removed from a lake, in agricul- ture, as food, or for biogas production. In this emerging field of sustainable restoration techniques, we show examples, discuss benefits and pitfalls, and flag areas for further research and development. Each lake should be assessed individually to ensure that restoration approaches will effectively address lake-specific problems, do not harm the target lake or downstream ecosystems, are cost-effective, promote delivery of valuable ecosystem ser- vices, minimize conflicts in public interests, and eliminate the necessity for repeated interventions. Achieving optimal, sustainable results from lake res- toration relies on multidisciplinary research and close interactions between environmental, social, political, and economic sectors.
  • Kirje
    The diet of Eurasian perch larvae in lakes with different zooplankton assemblages
    (2023) Zingel, Priit; Agasild, Helen; Zagars, Matiss; Feldmann, Tõnu; Tuvikene, Arvo; Zingel, Tiina; Puncule, Linda; Karus, Katrit; Centre for Limnology. Institute of Agricultural and Environmental Sciences. Chair of Hydrobiology and Fishery. Estonian University of Life Sciences
    Eurasian perch has a great ecological importance in freshwaters as it is often 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.
  • Kirje
    Generalist invasion in a complex lake food web
    (Wiley, 2023) Kuparinen, Anna; Uusi-Heikkilä, Silva; Perälä, Tommi; Ercoli, Fabio; Eloranta, Antti P.; Cremona, Fabien; Nõges, Peeter; Laas, Alo; Nõges, Tiina; Chair of Hydrobiology and Fishery
    Invasive species constitute a threat not only to native populations but also to the structure and functioning of entire food webs. Despite being considered as a global problem, only a small number of studies have quantitatively predicted the food web-level consequences of invasions. Here, we use an allometric trophic network model parameterized using empirical data on species body masses and feeding interactions to predict the effects of a possible invasion of Amur sleeper (Perccottus glenii), on a well-studied lake ecosystem. We show that the modeled establishment of Amur sleeper decreased the biomasses o ftop predator fishes by about 10%–19%. These reductions were largely explained by increased larval competition for food and Amur sleeper predation on fish larvae. In contrast, biomasses of less valued fish of lower trophic positions increased by about 0.4%–9% owing to reduced predation pressure by top piscivores. The predicted impact of Amur sleeper establishment on the biomasses of native fish species vastly exceeded the impacts of current-dayfishing pressures.
  • Kirje
    Strong temporal variation of consumer δ13C value in an oligotrophic reservoir is related to water level fluctuation
    (Nature Portfolio, 2023) Veselý, Lukáš; Ercoli, Fabio; Ruokonen, Timo; Bláha, Martin; Duras, Jindřich; Haubrock, Phillip J.; Kainz, Martin; Hämäläinen, Heikki; Buřič, Miloš; Kouba, Antonín; Chair of Hydrobiology and Fishery
    Using stable carbon and nitrogen isotope analysis (δ 13 C and δ 15 N) to assess trophic interactions in freshwater ecosystems is a well established method, providing insight into ecosystem functioning. However, the spatial and temporal variability of isotope values, driven by environmental fluctuation is poorly understood and can complicate interpretations. We investigated how the temporal variation of stable isotopes in consumers (fish, crayfish and macrozoobenthos) of a canyon-shaped oligotrophic reservoir is associated with environmental factors such as water temperature, transparency, flooded area, and water quality measures. Consumers and their putative food sources were sampled and analyzed for carbon and nitrogen stable isotopes annually, and environmental parameters were measured monthly from 2014 to 2016. Results revealed significant differences in δ 13 C and δ 15 N values in each consumer among studied years. Over the years, fish and crayfish expressed differences in δ 13 C between 3 and 5‰, whereas in zoobenthos differences were 12‰. Variability in δ 15 N was similar across all consumers (2–4‰). Moreover, results suggest that the flooded area of the reservoir was a major driver of δ 13 C stable isotope values variation in consumers, while variation in δ 15 N was not linked to any of the studied environmental factors. Bayesian mixing models further showed significant changes in the origin of detritivorous zoobenthos carbon sources (reversal shift from terrestrial detritus to algae origin) between years with low water level to years with the standard water level. Other species showed only slight differences in food source utilization among years. Our study highlights the importance of environmental factors as sources of variation in consumer’s stable isotope values which should be considered especially when studied ecosystem strongly fluctuate in some environmental factor.
  • Kirje
    Changes in nutrient concentration and water level affect the microbial loop: a multi-seasonal mesocosm experiment : [submitted version]
    (Springer, 2023) Zingel, Priit; Jeppesen, Erik; Nõges, Tiina; Hejzlar, Josef; Tavşanoğlu, Ülkü Nihan; Papastergiadou, Eva; Scharfenberger, Ulrike; Agasild, Helen; Centre for Limnology
    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, we manipulated water depth and nutrients in a mesocosm experiment in north temperate Estonia and followed the microbial community dynamics during a 6-month period. 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 bacterial biomass, heterotrophic nanoflagellates (HNF) and metazooplankton biomass. Nutrients and water depth had both positive impacts on phytoplankton biomass. 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.
  • Kirje
    Behavioural responses of invasive (Gmelinoides fasciatus) and native (Gammarus lacustris) amphipods to predators on different bottom substrates : [submitted version]
    (Springer, 2023) Teesalu, Paul; Ercoli, Fabio; Tuvikene, Arvo; Chair of Hydrobiology and Fishery. Institute of Agricultural and Environmental Sciences
    The amphipod Gmelinoides fasciatus has invaded and established in numerous large lakes in Eurasia and, in the process, has displaced the native amphipod, Gammarus lacustris. The mechanism behind its invasion success is unclear and remains an important topic for invasion ecology. Three labo- ratory experiments were conducted to determine if superior predator avoidance and different types of bottom substrate could be important factors contrib- uting to the invasion success of G. fasciatus. Our results indicate that, on gravel and sand substrates, G. fasciatus exhibited superior digging behaviour to avoid predation by fishes (perch and common roach), contrary to G. lacustris. On sandy substrate, only 9% of G. fasciatus individuals were consumed, whereas G. lacustris were consumed much more intensively at 74%. In addition, G. fasciatus exhibited a more sub- stantial reduction in activity, where the time spent in motion reduced from 37.6 to 20.8% compared to G. lacustris (from 21.8 to 17.4%) when in the presence of predatory fish kairomones. G. fasciatus movement speed was also reduced in the presence of kairomones (from 57.1 to 39.3 mm/s). However, the presence of kairomones had little effect on digging behaviour of G. fasciatus. G. fasciatus consistently demonstrated superior predator avoidance abilities over G. lacus- tris, suggesting that this mechanism might play an important role in the invasion success of G. fasciatus.
  • Kirje
    Bimodality and alternative equilibria do not help explain long-term patterns in shallow lake chlorophyll-a
    (Springer Nature, 2023) Davidson, Thomas A.; Sayer, Carl D.; Jeppesen, Erik; Søndergaard, Martin; Lauridsen, Torben; Johansson, Liselotte S.; Baker, Ambroise; Graeber, Daniel
    Since its inception, the theory of alternative equilibria in shallow lakes has evolved and been applied to an ever wider range of ecological and socioecological systems. The theory posits the existence of two alternative stable states or equilibria, which in shallow lakes are characterised by either clear water with abundant plants or turbid water where phytoplankton dominate. Here, we used data simulations and real-world data sets from Denmark and north-eastern USA (902 lakes in total) to examine the relationship between shallow lake phytoplankton biomass (chlorophyll-a) and nutrient concentrations across a range of timescales. The data simulations demonstrated that three diagnostic tests could reliably identify the presence or absence of alternative equilibria. The real-world data accorded with data simulations where alternative equilibria were absent. Crucially, it was only as the temporal scale of observation increased (>3 years) that a predictable linear relationship between nutrient concentration and chlorophyll-a was evident. Thus, when a longer term perspective is taken, the notion of alternative equilibria is not required to explain the response of chlorophyll-a to nutrient enrichment which questions the utility of the theory for explaining shallow lake response to, and recovery from, eutrophication.
  • Kirje
    Effects of filtration timing and pore size on measured nutrient concentrations in environmental water samples
    (Wiley, 2022) Reed, Megan H.; Strope, Erica K.; Cremona, Fabien; Myers, Justin A.; Newell, Silvia E.; McCarthy, Mark J.
    Nutrient monitoring is important for informing management decisions to mitigate eutrophication in aquaticsystems. Many nutrient monitoring programs usefilter pore sizes that allow microorganisms to pass into sam-ples and/or wait extended times between sample collection andfiltration/preservation, allowing microbial pro-cesses to alter nutrient concentrations. Here, 34 sites were sampled to determine howfilter pore size andfiltration timing affected measured ammonium (NH4+) and orthophosphate (ortho-P) concentrations. Threefil-ter pore sizes (0.22, 0.45, and 0.70μm) were used tofilter water immediately upon collection and after 5 and22 h in a bottle. NH4+and ortho-P concentrations varied relative to“baseline”measurements (i.e., 0.22μm,field-filtered samples), both over time and with differentfilter pore sizes, and showed no predictable direction ofchange based on ambient nutrient concentration or trophic status. As expected, larger relative changes occurredwith lower ambient concentrations; however, for the entire dataset, samples with > 1μmol L 1ortho-P and>3μmol L 1NH4+were lower by 11 and 33%, respectively, which would result in reported nutrient concentra-tions that were not representative of in situ conditions. Whole-water samplesfiltered after 22 h varied up to3070% for NH4+and 480% for ortho-P from baseline concentrations. Filtering water samples with a 0.22filter(or 0.45μm, at worst), immediately upon collection, should be adopted as standard practice to ensure thatreported nutrient concentrations represent the most accurate measurement possible. Inconsistent and/or insuffi-cient sampling and sample handling procedures can lead to poorly calibrated models and misinformed manage-ment and legislative decisions.
  • Kirje
    The comparison of the feeding of European perch Perca fluviatilis L. larvae in littoral and pelagic habitats of northern temperate lakes
    (Estonian Academy Publishers, 2022) Karus, Katrit; Zagars, Matiss; Agasild, Helen; Feldmann, Tõnu; Tuvikene, Arvo; Puncule, Linda; Zingel, Priit; Chair of Hydrobiology and Fishery; Centre for Limnology
    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 Müller) 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.
  • Kirje
    Effects of environmentally relevant concentrations of microplastics on amphipods
    (Elsevier, 2022) Rani-Borges, Barbara; Meitern, Richard; Teesalu, Paul; Raudna-Kristoffersen, Merilin; Kreitsberg, Randel; Heinlaan, Margit; Tuvikene, Arvo; Ivask, Angela
    Lack of microplastics (MP) toxicity studies involving environmentally relevant concentrations and exposure times is concerning. Here we analyzed the potential adverse effects of low density polyethylene (LDPE) MP at environmentally relevant concentration in sub-chronic exposure to two amphipods Gmelinoides fasciatus and Gammarus lacustris, species that naturally compete with each other for their habitats. 14-day exposure to 2 μg/L (8 particles/L corresponding to low exposure) and 2 mg/L (~8400 particles/L, corresponding to high exposure) of 53–100 μm LDPE MP were used to assess ingestion and egestion of MP, evaluate its effects on amphipod mortality, swimming ability and oxidative stress level. Both amphipod species were effectively ingesting and egesting LDPE MP. On the average, 0.8 and 2.5 MP particles were identified in the intestines of each amphipod exposed to 2 μg/L and 2 mg/L LDPE MP, respectively. Therefore, intestinal MP after 14-day exposure did not fully reflect the differences in LDPE MP exposure concentrations. Increased mortality of both amphipods was observed at 2 mg/L LDPE MP and in case of G. lacustris also at 2 μg/L exposure. The effect of LDPE on swimming activity was observed only in case of G. fasciatus. Oxidative stress marker enzymes SOD, GPx and reduced glutathione GSH varied according to amphipod species and LDPE MP concentration. In general G. lacustris was more sensitive towards LDPE MP induced oxidative stress. Overall, the results suggested that in MP polluted environments, G. lacustris may lose its already naturally low competitiveness and become overcompeted by other more resistant species. The fact that in the sub-chronic foodborne exposure to environmentally relevant and higher LDPE MP concentrations all the observed toxicological endpoints were affected refers to the potential of MP to affect and disrupt aquatic communities in the longer perspective.
  • Kirje
    Opportunities for combining data of Estonian and Russian monitoring to reflect on water quality in large transboundary Lake Peipsi
    (Elsevier, 2022) Tammeorg, Olga; Tuvikene, Lea; Kondratyev, Sergey; Golosov, Sergey; Zverev, Ilya; Zadonskaya, Olga; Nõges, Peeter
    Lake Peipsi, one of the world’s largest lakes, is shared between Estonia and Russia. The water quality in different parts of the lake has so far been assessed independently. Here we explore opportunities for combining data of Estonian and Russian monitoring. For that, we 1) analysed the compatibility of data for some water quality variables; 2) estimated the potential effects of the differences in sampling frequency; 3) provided a few regression models to calculate the missing data for months not sampled by the Russian side. Data of the concurrent Estonian and Russian sampling indicated a good compatibility. Estonian data analysis suggested that water quality assessment results are sensitive to sampling frequency. For example, total phosphorus (TP) in the largest basin showed a long-term decreasing trend in three month data that disappeared when data for other months were added. Disregarding some months may lead to underor overestimation of certain factors with no consistency in the response of different basins. Hence, data of the whole ice-free period are recommended for an adequate water quality assessment. Furthermore, we demonstrated that monthly values of the water quality variables of the same year are autocorrelated. Based on this, we filled the gaps in the long-term data and compiled a dataset for the whole lake that enables its most comprehensive use in water quality assessment and management. Long-term data revealed no water quality improvement of Lake Peipsi. Further reduction of the external nutrient load is needed. Eutrophication is sustained by high internal phosphorus load.
  • Kirje
    Summer greenhouse gas fluxes in different types of hemiboreal lakes
    (Elsevier, 2022) Rõõm, Eva-Ingrid; Lauringson, Velda; Laas, Alo; Kangro, Kersti; Viik, Malle; Meinson, Pille; Cremona, Fabien; Nõges, Peeter; Nõges, Tiina
    Lakes are considered important regulators of atmospheric greenhouse gases (GHG). We estimated late summer open water GHG fluxes in nine hemiboreal lakes in Estonia classified under different lake types according to the European Water Framework Directive (WFD). We also used the WFD typology to provide an improved estimate of the total GHG emission from all Estonian lakes with a gross surface area of 2204 km2 representing 45,227 km2 of hemiboreal landscapes (the territory of Estonia). The results demonstrate largely variable CO2 fluxes among the lake types with most active emissions from Alkalitrophic (Alk), Stratified Alkalitrophic (StratAlk), Dark Soft and with predominant binding in Coastal, Very Large, and Light Soft lakes. The CO2 fluxes correlated strongly with dissolved CO2 saturation (DCO2) values at the surface. Highest CH4 emissions were measured from the Coastal lake type, followed by Light Soft, StratAlk, and Alk types; Coastal, Light Soft, and StratAlk were emitting CH4 partly as bubbles. The only emitter of N2O was the Alk type. We measured weak binding of N2O in Dark Soft and Coastal lakes, while in all other studied lake types, the N2O fluxes were too small to be quantified. Diversely from the common viewpoint of lakes as net sources of both CO2 and CH4, it turns out from our results that at least in late summer, Estonian lakes are net sinks of both CO2 alone and the sum of CO2 and CH4. This is mainly caused by the predominant CO2 sink function of Lake Peipsi forming ¾ of the total lake area and showing negative net emissions even after considering the Global Warming Potential (GWP) of other GHGs. Still, by converting CH4 data into CO2 equivalents, the combined emission of all Estonian lakes (8 T C day−1 ) is turned strongly positive: 2720 T CO2 equivalents per day.
  • Kirje
    Prey selection and growth in 0+ Eurasian perch Perca fluviatilis L. in littoral zones of seven temperate lakes
    (Wiley, 2022) Karus, Katrit; Zagars, Matiss; Agasild, Helen; Feldmann, Tõnu; Tuvikene, Arvo; Puncule, Linda; Zingel, Priit
    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, Vārzgūnes, 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.