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Plastic debris is thought to be widespread in freshwater ecosystems globally1. However, a lack of comprehensive and comparable data makes rigorous assessment of its distribution challenging2,3. Here we present a standardized cross-national survey that assesses the abundance and type of plastic debris (>250 μm) in freshwater ecosystems. We sample surface waters of 38 lakes and reservoirs, distributed across gradients of geographical position and limnological attributes, with the aim to identify factors associated with an increased observation of plastics. We find plastic debris in all studied lakes and reservoirs, suggesting that these ecosystems play a key role in the plastic-pollution cycle. Our results indicate that two types of lakes are particularly vulnerable to plastic contamination: lakes and reservoirs in densely populated and urbanized areas and large lakes and reservoirs with elevated deposition areas, long water-retention times and high levels of anthropogenic influence. Plastic concentrations vary widely among lakes; in the most polluted, concentrations reach or even exceed those reported in the subtropical oceanic gyres, marine areas collecting large amounts of debris4. Our findings highlight the importance of including lakes and reservoirs when addressing plastic pollution, in the context of pollution management and for the continued provision of lake ecosystem services.

Plastic debris is widespread in freshwater ecosystems, but a rigorous assessment of its global distribution has been hindered by a lack of comprehensive and comparable data. We performed the first standardized global survey of lakes to assess the quantity and type of plastics (>250μm). We included 38 lakes located in 23 different countries distributed across five continents, spanning different environmental gradients and varying levels of anthropogenic stress. All samples were collected by horizontal trawling of a plankton net and subsequently treated with hydrogen peroxide. We identified 9425 plastic particles, which were classified based on shape, color, and size. Polymer identification was carried out using Raman micro-spectroscopy. Our results showed that the concentration of plastics spanned four orders of magnitude (10-3-101 particles/m3). Fibers (49%) and fragments (41%) were the most frequently detected particles, suggesting a secondary origin of plastic contamination. The most commonly identified polymers were polyester (30%), polypropylene (20%), and polyethylene (16%), which are widely used in short life-cycle products and account for the majority of global plastic production. Further, we found that urban-related attributes of lakes/watersheds influenced the occurrence and type of plastics in lentic systems and larger and deeper lakes with higher retention times are accumulating plastic debris at higher concentrations. info:eu-repo/semantics/publishedVersion

Calcium (Ca) is an essential element for almost all living organisms. Here, we examined global variation and controls of freshwater Ca concentrations, using 440 599 water samples from 43 184 inland water sites in 57 countries. We found that the global median Ca concentration was 4.0 mg L−1 with 20.7% of the water samples showing Ca concentrations ≤ 1.5 mg L−1 , a threshold considered critical for the survival of many Ca-demanding organisms. Spatially, freshwater Ca concentrations were strongly and proportionally linked to carbonate alkalinity, with the highest Ca and carbonate alkalinity in waters with a pH around 8.0 and decreasing in concentrations towards lower pH. However, on a temporal scale, by analyzing decadal trends in >200 water bodies since the 1980s, we observed a frequent decoupling between carbonate alkalinity and Ca concentrations, which we attributed mainly to the influence of anthropogenic acid deposition. As acid deposition has been ameliorated, in many freshwaters. Financial support was received from the Swedish Research Council (Grant No. 2016-04153), the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 643052 (C-CASCADES project), and from the Knut and Alice Wallenberg Foundation (KAW project). This work is part of the international Global Lake Ecological Observatory Network (GLEON). The work of J.H. is supported by the Deutsche Forschungsgemeinschaft (DFG) under Germany’s Excellence Strategy – EXC 20 2037 ‘Climate, Climatic Change, and Society’ – Project Number: 390683824, contribution to the Center for Earth System Research and Sustainability (CEN) of Universität Hamburg. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. We thank all data providers, listed in Supplementary Table 1. Financial support was received from the Swedish Research Council (Grant No. 2016-04153), the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 643052 (C-CASCADES project), and from the Knut and Alice Wallenberg Foundation (KAW project). This work is part of the international Global Lake Ecological Observatory Network (GLEON). The work of J.H. is supported by the Deutsche Forschungsgemeinschaft (DFG) under Germany’s Excellence Strategy – EXC 20 2037 ‘Climate, Climatic Change, and Society’ – Project Number: 390683824, contribution to the Center for Earth System Research and Sustainability (CEN) of Universität Hamburg. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. We thank all data providers, listed in Supplementary Table 1.

The majority of microplastic research has focused on seawater, with fewer than 4% of microplasticsrelated studies occurring on freshwaters. The limited available information suggests that the abundance of microplastics in freshwaters is often as high or even higher than marine environments. However, comprehensive investigations on occurrence and fate of microplastics in freshwaters are scarce and highly fragmented, partly because detection and identification of microplastic particles is rather complex. In addition, up to now, harmonized and standardized protocols for the sampling and analysis of microplastics in freshwaters do not exist, and studies with different research aims and hypotheses often report unstandardized results, making comparison among studies difficult. In the present study, we performed the first global standardized sampling and analysis effort to investigate the occurrence and distribution of microplastics in surface water of lakes and reservoirs with different anthropogenic impacts. Participants aim to collect water samples of freshwater systems with different features (e.g., area, depth, thermal behavior, watershed), following a common protocol. This 18 establishes the collection of samples by horizontal trawling of a plankton net and, after treatment with hydrogen peroxide, the polymer identification through micro-Raman spectroscopy. This GLEON project will allow obtaining comparable data about microplastic contamination in different freshwater systems around the globe. With this global dataset, our goals are to determine whether a relationship exists between the abundance of microplastics and the waterbody/watershed attributes and understand which factors are likely to influence the occurrence of microplastics in surface water of lentic systems. info:eu-repo/semantics/publishedVersion

Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (Tw-Ta) as a proxy for sensible heat flux (QH). If QH is directed upward, corresponding to positive Tw-Ta, it can enhance CO2 and CH4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative Tw-Ta across small ponds, lakes, streams/rivers and the sea shore (i.e. downward QH), with Tw-Ta becoming increasingly negative with increasing Ta. Further examination of Tw-Ta using high-frequency temperature data from inland waters across the globe confirmed that Tw-Ta is linearly related to Ta. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative Tw-Ta with increasing annual mean Ta since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative Tw-Ta, thereby reducing CO2 and CH4 transfer velocities from inland waters into the atmosphere.

Dataset Method: Phytoplankton samples were preserved in Lugol’s (acidified iodine) solution and counted under an inverted microscope (Utermöhl, 1958). 3 ml of preserved sample was settled overnight and counted in random fields or transects. Biovolumes of algal cells, colonies and/or filaments were calculated using assigned geometric shapes dimensions, and converted to biomass assuming the specific density of 1 g cm-3 in accordance with Edler (1979). Approved by CEN on 14 July 2006 “Water quality - Guidance standard on the enumeration of phytoplankton using inverted microscopy (Utermöhl technique)” (CEN 15204, 2006) European Standard EN 15204:2006 Utermöhl, H., 1958. Zur Vervollkommnung der quantitativen Phytoplankton-Methodik. Mitteilungen der Internationale Vereinigung für Theoretische und Angewandte Limnologie 9, 1- 38. Edler, L. (ed.), 1979. Recommendations on methods for marine biological studies in the Baltic Sea. Phytoplankton and chlorophyll. Baltic Marine Biologists WG 9.

Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (Tw-Ta) as a proxy for sensible heat flux (QH). If QH is directed upward, corresponding to positive Tw-Ta, it can enhance CO2 and CH4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative Tw-Ta across small ponds, lakes, streams/rivers and the sea shore (i.e. downward QH), with Tw-Ta becoming increasingly negative with increasing Ta. Further examination of Tw-Ta using high-frequency temperature data from inland waters across the globe confirmed that Tw-Ta is linearly related to Ta. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative Tw-Ta with increasing annual mean Ta since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative Tw-Ta, thereby reducing CO2 and CH4 transfer velocities from inland waters into the atmosphere., Scientific Reports, 7, ISSN:2045-2322

A Thesis for applying for the degree of Doctor of Philosophy in Applied Biology. Väitekiri filosoofiadoktori kraadi taotlemiseks rakendusbioloogia erialal. Human activities and climate change have become the most consequential threats to freshwater ecosystems and their inhabitants, especially fish. The response of the fish population to their environment is not always straightforward because of joint effect of multiple parameters. Fish and fisheries in shallow lakes can be directly affected by changes in air temperature, and also changes can occur via other factors in lakes that are under the influence of climate change. Also, response of each fish species to those parameters can vary depending on their tolerance and adaptation. Therefore, examining complex relationships between fish and their environment plays important role at understanding the dynamic of fish population and fisheries. In this study, we aimed to determine general driving factors, including climate change impact, for fish and fishery in Estonian large lakes. Results show that although winter is one of the most affected seasons by climate change in this region, then the selected fish species and eel fishery in Estonian large lakes were more sensitive to other environmental parameters. However, since the changes in winter surface water temperature and ice formation can affect other parameters, the indirect effect should not be ignored. Nutrients, spring-summer temperature and alkalinity were the most important environmental parameters for the selected fish biomass. We found that the high blue-green algae biomass during restocking period is the strongest negative impact for the eel fishery in Lake Võrtsjärv. The impact of factors can occur subsequently in fish communities and may lead to irreversible consequences, whereas the effects of concurrent climate change and nutrient enrichment can mutually reinforce their symptoms. Inimtegevus ja kliimamuutus on saanud suureks ohuks magevee ökosüsteemidele ja nende elanikele, eriti kaladele. Kalade reaktsioon elukeskkonna muutustele pole mitme teguri koosmõju tõttu alati otsene. Kalad ja kalandus suurtes madalates järvedes nagu Peipsi ja Võrtsjärv on otseselt mõjutatud muutustest õhutemperatuuris ning abiootiliste ja biootiliste tegurite vastastikmõjust, mida omakorda mõjutab kliimamuutus. Kalaliikide reaktsioon elupaiga muutustele võib erineda sõltuvalt nende kohanemisvõimest ja taluvuspiiridest.. Seetõttu on oluline uurida kalade ja nende elukeskkonna vahelisi keerulisi seoseid, et aru saada kalapopulatsioonide ja kalanduse muutuste põhjustest. Antud töös püüdsime selgitada peamiste ökoloogiliste tegurite, sealhulgas kliimamuutuse mõju kaladele ja kalandusele Eesti suurjärvedes. Analüüs näitas, et kuigi talv on kliimamuutustest enim mõjutatud aastaaeg, siis valitud kalaliigid (haug, koha, ahven, kiisk, latikas, nurg) ja angerja asurkond meie suurjärvedes olid tundlikumad teiste keskkonnategurite suhtes. Kuna muutused talvises pinnavee temperatuuris ja jäätekkes mõjutavad ka teisi keskkonnaparameetreid, siis nende kaudset mõju ei tohiks siiski ignoreerida. Selgus, et toitainete sisaldus ja aluselisus vees ning kevad-suvine veetemperatuur olid kõige olulisemad keskkonnategurid valitud kalaliikide biomassile. Sinivetikate kõrgel biomassil asustamise perioodil oli tugevaim negatiivne mõju angerjapopulatsioonile Võrtsjärves. Erinevate faktorite mõju kaladele võib ilmneda hiljem ja viia pöördumatute tagajärgedeni ning kliimamuutuste ja eutrofitseerumise koosmõju võib neid probleeme veelgi süvendada. Publication of this thesis is supported by the Estonian University of Life Sciences.

Human activities and climate change have become the most consequential threats to freshwater ecosystems and their inhabitants, especially fish. The response of the fish population to their environment is not always straightforward because of joint effect of multiple parameters. Fish and fisheries in shallow lakes can be directly affected by changes in air temperature, and also changes can occur via other factors in lakes that are under the influence of climate change. Also, response of each fish species to those parameters can vary depending on their tolerance and adaptation. Therefore, examining complex relationships between fish and their environment plays important role at understanding the dynamic of fish population and fisheries. In this study, we aimed to determine general driving factors, including climate change impact, for fish and fishery in Estonian large lakes. Results show that although winter is one of the most affected seasons by climate change in this region, then the selected fish species and eel fishery in Estonian large lakes were more sensitive to other environmental parameters. However, since the changes in winter surface water temperature and ice formation can affect other parameters, the indirect effect should not be ignored. Nutrients, spring-summer temperature and alkalinity were the most important environmental parameters for the selected fish biomass. We found that the high blue-green algae biomass during restocking period is the strongest negative impact for the eel fishery in Lake Võrtsjärv. The impact of factors can occur subsequently in fish communities and may lead to irreversible consequences, whereas the effects of concurrent climate change and nutrient enrichment can mutually reinforce their symptoms.

A Thesis for applying for the degree of Doctor of Philosophy in Hydrobiology. Macrophytes play an important role in aquatic ecosystems. Their temporal and spatial distribution is governed by various natural and anthropogenic pressures and disturbances. The aim of the thesis is to study the effect of pressures and disturbances on species composition and richness in Lake Peipsi. The fluctuating water level has been considered to be a natural disturbance factor while eutrophication and reed removal are important anthropogenic influences. During the rapid eutrophication between 1970 and 1980, the frequency of common hygrophytes, helophytes, and amphibious plants increased, as well as the frequency of filamentous green algae. Eutrophication of Peipsi has had a profound effect on macrophytes because it has induced the expansion of reeds, which in turn decreased suitable eulittoral habitats for small-sized species. The expansion of reeds required holidaymakers and local people to manage the lake shores more actively. In general, management supports increase in species richness but the formation of dry ‘terraced’ (elevated) areas should not be the first choice. Among the different management styles the highest species richness was found in the stretches without ‘terrace’ or with a combination of managed and wild habitats. Further, changes in the water level create new ecological niches and a mosaic of habitats. At low water habitats emerge for otherwise declining species such as Alisma gramineum, Ranunculus reptans, Cyperus fuscus. Thus, the water level fluctuation as an intermediate disturbance supports species richness in Peipsi. Remarkably, also the abundance of submerged macrophytes, charophytes and large filamentous algae increased considerably at the low water level. Therefore, the occurrence of indicator species and its influence on the estimation of ecological quality are in synchrony with the natural disturbance factor. Makrofüütidel ehk suurtaimedel on väga oluline roll nii vee keemiliste omaduste mõjutajana kui ka elu- ja varjepaigana ning toidubaasina paljudele teistele liikidele. Seepärast on oluline jälgida suurtaimestikus toimuvaid muutusi ning tuvastada looduslike ja inimtekkeliste survetegurite osa nende kujunemisel. Töö põhineb Peipsi järve pikaajalistel uuringutel. Inimtekkeliste surveteguritena on käsitletud eutrofeerumist ja pilliroo eemaldamist (ühekordsena käsitletav ka häiringuna) ning loodusliku häiringuna veetaseme kõikumist. Kiire eutrofeerumise perioodil (1970–1980) suurenes oluliselt tavaliste hügrofüütide, helofüütide, amfiibsete liikide ning suurte niitrohevetikate esinemissagedus. Tööst selgus, et antropogeensest eutrofeerumisest tingitud rooalade laienemine on vähendanud eulitoraalis väikesekasvulistele liikidele sobivaid elupaiku. Kaldaalade kinnikasvamine on suurendanud kohalike elanike vajadust rannaalade majandamiseks. Üldiselt võib tihedate pillirookogumike eemaldamist pidada taimestiku liigirikkusele positiivseks, kuid vältida tuleks monotoonsete ja kuivade „terrassistatud“ (kõrgendatud) alade kujundamist. Suurim liigirikkus oli iseloomulik rannalõikudele, kus niidetud ja majandamata elupaigad vaheldusid ning ei olnud terrassistatud ala. Veetaseme kõikumine Peipsis on käsitletav keskmise häiringuna, mis toetab liigirikkuse säilimist elupaikade suurenenud mosaiiksuse ning liikidele uute niššide kujunemise tõttu. Veetaseme langusega paljanduvad elupaigad, mis on sobivad väheneva levikuga liikidele, nagu näiteks väike konnarohi, kaartulikas ja pruun lõikhein. Madalveeperioodil tõusis märgatavalt liigirikkus ning head seisundit näitavate veesiseste taimede ja mändvetikate ohtrus, aga samas ka kesist seisundit peegeldavate suurte niitrohevetikate ohtrus. Kuna uuring näitas liikide esinemise ning ohtruse väga tugevat sõltuvust veetaseme muutustest, siis on ilmne, et veekogu ökoloogilise seisundi hinnang on sellest looduslikust häiringufaktorist märkimisväärselt mõjutatud. Publication of this thesis is supported by the Estonian University of Life Sciences

Hironomiidid ehk surusääsklased (Diptera: Chironomidae) on Võrtsjärve põhjaloomastikus domineeriv rühm ja tähtis kalatoit, kuid nende hulga aastajalisi ja pikaajalisi muutusi on viimastel aastatel vähe uuritud. Töö eesmärk on selgitada surusääsevatsete arvukuse ja biomassi aastaajalisi ja pikaajalisi muutusi, teha kindlaks seoseid teiste makrozoobentoste rühmadega ja analüüsida ilmastikuga seotud faktorite mõju nende ohtrusele Võrtsjärves. Uuring põhineb järve kahest proovipunkist (profundaali mudapõhjalt alates alates 1964. aastast ja litoraalist alates 1970. aastast) kogutud igakuiste makrozoobentose proovide andmetel. Kasutatud aegread on saadud Eesti Maaülikooli limnoloogiakeskuse arhiivist, andmete statistiliseks analüüsiks kasutati statistikaprogrammi R. Analüüs näitas hironomiidivastsete igakuise keskmise arvukuse ja biomassi suvist langust litoraalis (ilmselt seoses vee kiirema soojenemisega ja valmikute väljalennuga) varem (miinimum juunis) kui profundaalis (miinimum augustis). Hironomiidide keskmine arvukus ja biomass oktoobrist aprillini varieerus aastati märkimisväärselt. Litoraalis täheldati hironomiidide keskmise arvukuse (kuid mitte biomassi) statistiliselt olulist pikaajalist tõusutrendi; profundaalis mõlema näitaja olulisi trende ei ilmnenud. Korrelatsioonid hironomiidide ja teiste makrozoobentose rühmade arvukuste ja biomasside vahel olid enamasti positiivsed ja statistiliselt olulised; seejuures korrelatsioon ühe hironomiidiliigi - Chironomus plumosus - ja kogu makrozoobentose biomassi vahel profundaalis oli väga tugev (0,99; p=10-16). Hironomiidide biomassi profundaalis mõjutasid positiivselt kevadine õhutemperatuur ja suvine veetemperatuur. Litoraalis ilmnes hironomiidide arvukuse positiivne seos aprilli, kevadise ja sügisese vee- ja õhutemperatuuriga ning aprilli ja kevadise veetasemega. Surusääsevastsete seoseid keskkonnaga väga on üsna keeruline uurida, kuna neid ühekorraga mõjutavaid tegureid nii veekogus kui ka väljaspool on väga palju. Paremaks hironomiidide arvukuste ja biomasside muutuste selgitamiseks tuleks uurida hironomiidide suviseid noorjärkke, kes on praeguse metoodikaga määramata. Chironomids (Diptera: Chironomidae) are dominant benthic invertebrates in Lake Võrtsjärv and important fish food, but their seasonal and long term changes have been studied little in previous years. Thesis goal is to identify chironomid larvae abundance and biomass seasonal and long term changes, connection to other macrozoobenthos groups and analyse weather related factors. Study is based on two monitoring stations (mud bottomed profundal from 1964. and inshore littoral from 1970.). Used data series was obtained from Estonian University of Life Sciences Centre of Limnology; data was analysed with statistics program R. Analysis showed chironomids larvae monthly average abundance and biomass summer decrease in littoral (probably faster water temperature rise means faster chironomids growth) was 2 months earlier (minimum in June) than in profundal (minimum in August). Chironomids average abundance and biomass October to April averages had great variability between the years. Littoral showed long term increase in abundance (but not biomass), which was statistically significant, but profundaal failed to show any long term changes. Correlations between chironomids and other macrozoobenthos groups abundance and biomass showed mostly positive correlations, which were statistically significant, notably C. plumosus correlation with macrozoobenthos showed very strong correlation (0,99; p=10-16). Chironomids biomass in profundal was positively influenced by autumn air temperature and summer water temperature. Littoral chironomids abundance was positively affected by April, autumn and spring air and water temperatures and April and autumn water level. Studying chironomids controlling environmental factors is very difficult due to large number of factors inside the lake and outside the lake. For better understanding of chironomids abundance summer young chironomids should be studied as current methodology cannot catch them.

An understanding of the factors that drive the water quality of large lakes is critical to the protection of such waterbodies. In the present study, the contributions made by riverine nutrient loading, sediment resuspension and weather factors to the water quality of large shallow Lake Peipsi were ascertained. High P concentration has been thought to be the main reason for the degradation of the water quality in this lake. It has hitherto been assumed that large shallow lakes are particularly influenced by sediment resuspension. However, there has been a lack of empirical studies that show this, because of the complexity of taking field-measurements. The in-field measurements of sediment resuspension taken for this study are the first ever to carried out in Lake Peipsi. Consequently, these data contribute to the overall knowledge on the unique ecosystems of large and shallow lakes. The Enonselkä basin of the large and shallow Lake Vesijärvi had been suffering from eutrophication, but the lake was successfully restored in the 1990s by reducing the external nutrient loading and also by efficiently removing fish. Since then however, signs of a recurrence in the deterioration of the water quality triggered further investigations of the gross sedimentation and sediment resuspension activities in that lake. The core findings of the studies carried out for this thesis are listed below. 1) Only a small decrease in P loading from the Estonian part of the catchment followed the dramatic reductions in the use of fertilizers, the downturn in livestock production and the improvement in wastewater treatment. Moreover, no significant long-term changes in TP concentrations in Lake Peipsi s.s. occurred, specifically since the 1990s. The annual in-lake TP concentrations were insensitive to the changes in TP loading through the major inflow. The hysteresis in the P dynamics of Lake Peipsi can be due to high P levels in its catchment soils, unchanged external loading from shared parts of its catchment, to internal P loading, and from changes in weather. 2) Sediment resuspension constituted the majority of the gross sedimentation in both Lake Peipsi and in Enonselkä. The spatial differences of the sediment resuspension were determined by the lake’s morphometry. Resuspension-mediated internal loading of TP dominated over the external loading in Lake Peipsi. Therefore, the present study provided the first empirical evidence for the high potential for recycling of P, which was originally brought into the lake by external loading. Sediment resuspension controlled the water quality through its significant influence on the loading of SS, turbidity, the concentrations of TP, and Chl a. Moreover, the results obtained from Enonselkä indicated that sediment fluxes from the shallow areas may govern the water quality of that whole lake. Resuspension, particularly in the shallow areas, triggered the internal loading of SS, which resulted in high water turbidity in the Enonselkä basin. This may be associated with the signs of recurring deterioration of the ecosystem exhibited since the late 1990s following the successful restoration in the 1990s. Macrophyte-based management of the shallow areas could contribute to the restoration of the whole basin. 3) The study of the long-term series of TP measurements revealed the significant contributions to the TP anomalies made by water temperature, water level, photosynthetically active radiation and wind speed. The variability of all these weather factors causes unexpected temporal and geographical deviations of the TP dynamics. Our quantitative estimations of the impacts of different weather variables on TP will help to predict changes in the water quality under different scenarios of climate change. Highly anomalous wind events were the main cause for TP changes. Extensive resuspension measurements in Lake Peipsi demonstrated that the wind affected TP concentration in the water column through the mechanism of sediment resuspension. As a higher frequency of extreme wind events can be expected in the future due to the climate change, severe implications on the lake ecosystems may follow due to the increase of internal P loading. Järvede vee kvaliteet kujuneb paljude looduslike ja inimtekkeliste survetegurite koosmõjul ning nende faktorite mõju selgitamine on esmatähtis keskkonnakaitselisest seisukohast. Käesolevas töös uuriti jõgede reostuskoormuse, setete resuspensiooni ning ilmastiku mõju järvevee kvaliteedi muutlikkusele. Peamiseks uurimisobjektiks oli Peipsi järv, kus tähtsaimaks keskkonnaprobleemiks peetakse vee kõrget fosfori (P) sisaldust. Kvantitatiivselt hinnati P kontsentratsiooni tundlikkust ilmastikutegurite suhtes, mida on siiani vähe uuritud. Esmakordselt uuriti Peipsil katseliselt (settelõksude abil) setete resuspensiooni. Kuigi suuri madalaid järvi peetakse eriti mõjustatuks setete tuuletekkelise resuspensiooni poolt, jäävad empiirilised andmed tihti puudulikeks keeruliste uurimistingimuste tõttu. Võrdluseks uuriti tugevalt eutrofeerunud Vesijärve osa, Enonselkä (Soomes), mille vee kvaliteet paranes 1990. aastatel tänu toiteainete väliskoormuse alandamisele ning kalade väljapüügile. 2000. aastatel täheldati seal taas vee kvaliteedi halvenemist, mis ajendas settimise ja setete resuspensiooni kui selle potentsiaalse põhjuse uurimist. 1) Täheldati vaid vähest Eesti-poolsete jõgede P-koormuse muutust põllumajandustegevuse vähenemise järel 1990. aastate alguses, ning Peipsi Suurjärve vee P kontsentratsioon alates 1990., pikemaajalise perioodi keskmisena, ei muutunud oluliselt. Aastakeskmine järve vee P kontsentratsioon ei reageerinud aastast-aastasse muutuvale P reostuskoormusele. Seoseid võisid mõjutada valgla pinnases ja pinnaveekogudes akumuleerunud P (sisekoormus), muutused ilmastikus ja P sissevoolus uurimata valglalt. 2) Nii Peipsis kui Enonselkäs moodustas setete resuspensioon valdava osa kogu settimisvoost. Uuritud järvedes sõltus setete resuspensiooni (ja settimisvoo) ruumiline varieeruvus peamiselt järve morfomeetriast. Ajaline dünaamika oli seotud põhiliselt tuule mõjuga (tugevate tuulte esinemissagedus; tuule suund). Setete resuspensiooni tekitatud üldfosfori (TP) sisekoormus ületas tunduvalt väliskoormuse väärtust. Seega valglalt pärit P taasringluse potentsiaal Peipsis on väga kõrge. Setete resuspensioon reguleeris Peipsi vee kvaliteeti, mõjutades hõljuvaine (SS), TP ja klorofüll a kontsentratsiooni. Setete resuspensiooni uuringud Enonselkäs näitasid, et järve madalamates piirkondades resuspendeeritud setete ümberpaiknemine võib kontrollida kogu järve vee kvaliteeti. Madalate alade setete resuspensioon oli SS sisevoogude ning suurenenud veehägususe põhjuseks. Antud nähtust võib seostada Enonselkäs ka vee kvaliteedi halvenemisega alates 1990. lõpust. 3) Uuritud ilmastikunäitajad (veetase ja vee temperatuur, päevakeskmine ja maksimaalne tuule kiirus, fotosünteetiliselt aktiivne kiirgus) osutusid olulisteks Peipsi vee kvaliteeti määrajateks. TP kontsentratsiooni muutused olid eriti tundlikud tuule kiiruse muutuste suhtes. Ulatuslikud resuspensiooni mõõtmised, mida tehti Peipsis esmakordselt, näitasid, et tuule mõju vee kvaliteedile avaldub just setete resuspensiooni kaudu. Vastavalt saadud uuringutulemustele võivad kliimamuutuste tagajärjed vee ökosüsteemidele olla dramaatilised. Ekstreemselt kõrge tuulekiiruse juhtude sagenemine, mida prognoositakse seoses kliimamuutustega, võib esile kutsuda P sisekoormuse suurenemist tulevikus. Käesoleva töö tulemused rõhutavad eelkõige väliskoormuse piiramise vajalikkust, mis võimaldaks vähendada primaarproduktsiooni ning teiselt poolt vältida uute toiteainete akumuleerumist setetesse. The doctoral studies and the publication of the current thesis were supported by the Doctoral School of Earth Sciences and Ecology created under the auspices of European Social Fund.

Sõnastik sisaldab ligi 300 märksõnakirjet ehk terminiartiklit, mida erinevalt traditsioonilisest alfabeetilisest järjestusest käsitletakse teemadesse jaotatuna. Sõnastiku materjalina on kasutatud peaasjalikult kolme teost:  „Keskkonnasõnaraamat EnDic2004“ (koos veebirakendusega; avaldatud 2004);  „Ökoloogialeksikon“ (avaldatud 1992);  10-köiteline „Eesti entsüklopeedia“ (avaldatud 1985–1998). Sõnastiku ülesehitusel on järgitud iseseletuvuspõhimõtet: terminiartiklis sisalduv info peab olema arusaadav, ilma et tuleks näiteks lühendite selgitusi otsida sõnastiku vastavast peatükist. Enamik eestikeelseid termineid on varustatud ingliskeelsete vastetega. Sõnastiku edasine arendamine peab toimuma terminibaasis, mistõttu esimene samm arendamise suunal ongi sõnastiku sisu viimine mõnda internetis saadavasse terminibaasi. Terminibaasi kujul saab sõnastikule igaüks hõlpsasti ligi ja moodsad terminibaasid lubavad baasi haldajale anda vajaduse korral ka kiiret tagasisidet. The title of the master’s thesis is Small explanatory dictionary of freshwater ecology. The main goals of the dictionary are the following:  to make a small contribution to the Estonian terminology in natural sciences;  to create a dictionary which is understandable and usable at first glance, i.e. without reading any instructions as to how to use it;  to provide study materials for students studying any discipline related to freshwater ecology. In addition, almost all the Estonian terms are provided with their English equivalents. The dictionary consists of around 300 concepts which are divided into topics. A large part of the dictionary’s contents is taken from the following three books:  Environmental Dictionary EnDic2004 (with web support; published in 2004);  Ökoloogialeksikon (Lexicon of Ecology) (published in 1992);  the 10-volume Eesti entsüklopeedia (Estonian Encyclopedia) (published in 1985–1998). The present dictionary can never be completed and therefore further developments are urgently needed. The first challenge, in the author’s opinion, is to integrate the present dictionary into termbase. In termbase, one can access it easily, share with the community, and be able to give quick feedback if necessary.

Phyto- and zooplankton, the targets of the study, are the most important links of a water body’s food web. The stability of water body’s ecosystem is largely influenced by the interactions of phyto- and zooplankton. These both are largely influenced by the trophic state of the water body and, at the same time, also by weather conditions. About 20 years ago, the ecosystem of Lake Peipsi was in rather good state, but from the second half of the 1990s, the significant changes took place in the trophic level of the lake: algal blooms in summers became more intense and more frequent. The aims of the study were: to ascertain how phyto- and zooplankton communities and their interactions respond to changes in the trophic level of L. Peipsi; to test in time and space the indicative value of zooplankton to phytoplankton biomass ratio (BZp/BPhyt) in assessing the interrelations between zoo- and phytoplankton and the trophic state of L. Peipsi; to find out how the ice cover duration of L. Peipsi influences the dynamics of nutrients and phyto- and zooplankton in spring. The conclusions of the study were: 1. The continual anthropogenic phosphorus input and decreasing N:P ratio have offered favourable conditions for the development of cyanobacteria. The whole lake, especially L. Pihkva, was characterized by massive cyanobacterial blooms (primarily the genera Microcystis and Aphanizomenon) in summer months. It seems that the ecosystem of the L. Pihkva has lost its resilience to eutrophication. 2. The analyzed data revealed that the biomass of zooplankton decreased. As the pressure of fish on zooplankton has declined, we conclude that cyanobacterial blooms and presence of cyanotoxins are the main reasons for the significant decrease in the amount of zooplankton. 3. The analyzed BZp/BPhyt ratio showed a sharp decrease in parallel with the increasing trophy of the lake and indicated the eutrophic state of L. Peipsi s.s. and the hypertrophic state of L. Pihkva and L. Lämmijärv. Since 1997, the mean ratio for the vegetation period has decreased two times, which indicates a deterioration of the state of L. Peipsi. 4. The ice cover period of L. Peipsi shortened in the last decade. The nutrients’ concentration in lake water depended significantly on the severity of winter: mild winters affected nitrogen and silica concentrations positively and phosphorus concentration negatively. 5. The duration of the ice cover period determines the phytoplankton succession pattern in spring: the growth of all algal groups occurred earlier in springs after warm winters. The development of centric diatoms was most considerable. 6. The short ice duration had a positive impact on the biomass of zooplankton in spring: the biomass was twice as high as after the severe winters. After the warm winters, the densities of thermophobic rotifers were low, while those of more eurythermic rotifers increased. Concurrently, the larger species of copepods and cladocerans benefited from a shorter ice cover period, which raised the mean zooplankter weight and BZp/BPhyt ratio. Doktoritöö käsitleb Peipsi järve avavee planktoni ökoloogiat viimasel aastakümnel. Füto- ja zooplankton on järve ökosüsteemi toiduvõrgustiku olulised koostisosad. Nende omavahelistest suhetest sõltub suuresti vee ökosüsteemi stabiilsus. Füto- ja zooplankton on mõjutatud ühtaegu nii veekogu troofsustasemest kui ka ilmastikutingimustest. Nende liigiline koostis, dünaamika ning nende omavahelised suhted on keskkonnamõjurite integraalne tulem. Veel 20 aastat tagasi oli Peipsi järve ökosüsteem võrdlemisi heas tasakaalus, kuid alates 1990ndate teisest poolest toimusid järve troofsustasemes olulised muudatused: sagenesid suvised massilised veeõitsengud ning nendega kaasnevad kalade suremised. Töö eesmärkideks olid: välja selgitada, kuidas füto- ja zooplanktoni kooslused on vastanud muutustele Peipsi järve troofsustasemes; hinnata zoo- ja fütoplanktoni biomassi suhte (BZp/BPhyt) indikatiivset väärtust Peipsi järve ökoloogilise seisundi hindamisel; uurida jääkatte kestuse mõju järve kevadisele füto- ja zooplanktonile ning vee biogeenide dünaamikale. Töös jõuti järgmistele tulemustele: 1) Järve jätkuv eutrofeerumine lõi soodsad tingimused sinivetikate arenguks. Kogu järve, eriti kõrgema troofsusega Pihkva järve iseloomustavad suvised massilised sinivetikate (peamiselt perekonnad Microcystis ja Aphanizomenon) õitsengud. Pihkva järv on kaotanud oma ökoloogilise tasakaalu ning vastupanuvõime jätkuvale eutrofeerumisele. 2) Uuritud perioodil vähenes oluliselt zooplanktoni hulk. Kuna kalade surve zooplanktonile on vähenenud, peame peamiseks zooplanktoni hulga kahanemise põhjuseks suviseid massilisi vetikaõitsenguid ning vetikamürke. 3) Analüüsitud zoo- ning fütoplanktoni biomasside suhte (BZp/BPhyt) väärtus langes järve troofsuse tõusuga ning näitas Peipsi Suurjärve eutroofset ning Lämmi- ja Pihkva järve hüpertroofset seisundit. Alates 1997. aastast on vegetatsiooniperioodi keskmine BZp/BPhyt väärtus vähenenud kaks korda, mis viitab järve seisundi halvenemisele. 4) Uuritud perioodi andmed näitavad, et Peipsi järve jääkatte kestus on muutunud lühemaks, mis avaldas mõju vee biogeenide sisaldusele: lühike jääkatte periood mõjutas positiivselt vee lämmastiku ja räni sisaldust ning negatiivselt fosfori sisaldust. 5) Peipsi järve jääkatte kestus avaldas mõju ka füto- ja zooplanktoni koosseisule ning dünaamikale. Kõik fütoplanktoni rühmad ilmusid vette varem kevadel, millele eelnes soe talv lühikese jääkattega (eriti silmatorkav oli väikeste ränivetikate biomassi tõus) 6) Lühike jääkate avaldas positiivset mõju kevadisele zooplanktoni biomassile, mis mais ja juunis oli kuni kaks korda suurem kui pikema jääkattega aastatel. Pärast sooja talve oli külmalembeste keriloomade hulk madal, samal ajal kui eurütermsete keriloomade biomass tõusis. Kalatoiduks sobivate suurte aerjalgsete ja vesikirbuliste arengule mõjus soe talv soodsalt, tõstes zooplankteri keskmist kaalu ning BZp/BPhyt suhet. Perekond Daphnia liikide biomass oli juunis pärast lühikese jääkattega talvesid neli kuni kümme korda suurem kui pikema jääkattega aastatel. The doctoral studies and the publication of the current thesis were supported by the Doctoral School of Ecology and Environmental Sciences created under the auspices of European Union Social Fund. Funding for this research was provided through Estonian target financed project SF0170006s08 and by Estonian Science Foundation grants 6820, 7392 and 7643.

Pikeperch, Sander lucioperca (L.), is a pelagic predator, which plays a key role in many inland water bodies of the temperate zone by structuring populations at lower trophic levels (Frankiewicz et al., 1997; Dörner et al., 2007). Pikeperch is an important predator also in Estonian lakes Peipsi and Võrtsjärv. In lakes Peipsi and Võrtsjärv the nutrient concentration (P and N) have risen (Kangur & Möls, 2008; Nõges et al., 2010) and the water temperature increased (Ahas & Aasa, 2006; Nõges et al., 2010). As pikeperch is acknowledged to prefer warm and eutrophic conditions (Lehtonen, 1996; Keskinen & Marjomäki, 2003) yields of pikeperch have risen in Estonian large lakes (Järvalt et al., 2004; Kangur et al., 2008a), nonetheless, decrease in recent years. Smelt populations have collapsed in both lakes (Järvalt et al., 2004; Kangur et al., 2008a), but smelt is considered to be the most suitable first prey fish for young pikeperch when it moves from zooplanktovorous stage to fish diet (van Densen, 1985; Sutela & Hyvärinen, 2002; Kangur & Kangur, 1998). Although, the size and structure of any pikeperch population are related with the success progressing between the feeding stages (van Densen, 1985; Persson & Brönmark, 2002; 2008), the most recent investigation on the diet of juvenile pikeperch in lakes Peipsi and Võrtsjärv was carried out in the 1950s (Erm, 1961), thus, new investigations were needed in changed conditions. This investigation provided new data on the factors affecting the success of pikeperch population in large shallow lakes: 1. 0+ pikeperch diet and growth varied in lakes Peipsi and Võrtsjärv - lakes with different trophic status and food resources. 2. In the course of eutrophication of L. Võrtsjärv the diet of 0+ pikeperch has changed. 3. The diet shift to piscivory of 0+ pikeperch is postponed as smelt is absent and other fish fries are too big. 4. Despite the postponed diet shift, juvenile pikeperch are able to survive winters in changed environmental conditions (milder winters). 5. The inferior small fishes can be considered more successful than 0+ pikeperch in the current conditions and thus the potential prey is approximately in the same length than predator. 6. Summer heat wave can affect 0+ pikeperch and thus the whole pikeperch population. 7. The top-down regulation of zooplankton by pikeperch may be increased and the top-down regulation of fish by pikeperch may be decreased. 8. Our studies allow concluding that eutrophication and climate change have affected the food supplies and therefore the diet and growth of juvenile pikeperch, hence, also linkages between trophic levels. Koha (Sander lucioperca (L.)) on peamine avavees elutsev röövkala Euroopa eutrofeerunud veekogudes ning on olulisel kohal ka Eesti suurjärvedes, Peipsis ja Võrtsjärves. Koha on tippkiskja, kes kontrollides toiduahelat ülevalt alla võib oluliselt mõjutada vee kvaliteeti (Frankiewicz et al., 1997; Dörner et al., 2007). Peipsi ja Võrtsjärve on vee toiteainete (fosfori ja lämmastiku) sisaldus aja jooksul suurenenud (Kangur & Möls, 2008; Nõges et al., 2010) ja temperatuur järjest tõusnud (Ahas & Aasa, 2006; Nõges et al., 2010). Kuna koha peetakse eutrofeerunud ja sooja vee liigiks (Lehtonen, 1996; Keskinen & Marjomäki, 2003) on viimastel aastakümnetel märgata tõusu ka Eesti suurjärvede kohasaakides (Järvalt et al., 2004; Kangur et al., 2008a). Siiski, on kohasaakides täheldatud langust viimastel aastatel. Peipsis ja Võrtsjärves on tindi arvukus langenud ajaloolise miinimumini (Järvalt et al., 2004; Kangur et al., 2008a), kuid just tinti peetakse oma saleda keha tõttu kohamaimudele sobivaks saak-kalaks ajal kui vahetab zooplanktonitoidu kalatoidu vastu (van Densen, 1985; Sutela & Hyvärinen, 2002; Kangur & Kangur, 1998). Kuigi kohapopulatsiooni suurus ja tugevus on mõjutatud kohamaimude edust liikuda zooplanktonitoidult kalatoidule, viidi viimased uuringud koha noorjärkude toitumise kohta läbi 1950ndatel (Erm, 1961). Seega oli vajadus uute uuringute järgi, kuna järvedes on olnud pidevas muutuses. Käesolev uuring andis uut informatsiooni seostest erinevate mõjufaktorite ja kohapopulatsiooniga suurtes madalates järvedes: 1. 0+ koha toitumine ja kasv erines Peipsis ja Võrtsjärves – järvedes millel erinev troofsus ja toiduressursid. 2. Võrtsjärve eutrofeerumis käigus on oluliselt muutunud ka 0+ koha toitumine. 3. Toitumisnihe kalatoidule on edasi nihkunud, see ei toimu esimesel elusuvel, kuna tinti pole ja teised noorkalad on liiga suured. 4. Vaatamata edasilükkunud toitumisnihkele suudab 0+ koha muutunud tingimustes (pehmemad talved) talve üle elada. 5. Kiisk ja särg on zooplanktoni nišis edukamad kui noorkohad ning seega on 0+ koha potentsiaalsed saakkalad sama pikad kui koha ise. 6. Suvised kuumalained võivad mõjutada 0+ kohasid ning seega ka kogu koha populatsiooni. 7. Ülalt alla zooplanktoni regulatsioon võib olla antud tingimustes suurenenud, kuid ülalt alla regulatsioon kaladele hoopiski nõrgenenud. 8. Meie uuringud lubavad järeldada, et eutrofeerumine ja kliima muutused on põhjustanud muutusi toiduressurssides ja seeläbi mõjutanud noorkohade toitumist ja kasvu, seega on võimalikud muutused ka troofiliste tasemete suhetes.