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Presentation at the 11th International Shallow Lakes Conference, Estonia 11.-16.06.2023. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963, by Estonian Ministry of the Environment through the state monitoring programme, and also from the Estonian Research Council grant PRG1167. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963, by Estonian Ministry of the Environment through the state monitoring programme, and also from the Estonian Research Council grant PRG1167.

Presentation at the BIOGEOMON 2022, 10th International Symposium on Ecosystem Behavior, June 26–30, 2022, Tartu, Estonia. We are grateful to Tartu Environmental Research Ltd (Estonia) for water chemistry data and to the Estonian Environment Board for providing long-term air temperature data and supporting lake monitoring. This research was financed by Estonian Research Council Grant PRG709, PRG1167, and institutional research funding P210160PKKH of the Estonian Ministry of Education and Research. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963. Data collection within the frames of the state monitoring programme were supported by the Estonian Ministry of the Environment.

Space-for-Time-Substitution surveys (SFTS) are commonly used to describe zooplankton community dynamics and to determine lake ecosystem health. SFTS surveys typically combine single point observations from many lakes to evaluate the response of zooplankton community structure and dynamics (e.g., species abundance and biomass, diversity, demographics and modeled rate processes) to spatial gradients in hypothesized environmental drivers (e.g., temperature, nutrients, predation), in lieu of tracking such responses over long time scales. However, the reliability and reproducibility of SFTS zooplankton surveys have not yet been comprehensively tested against empirically-based community dynamics from longterm monitoring efforts distributed worldwide. We use a recently compiled global data set of more than 100 lake zooplankton time series to test whether SFTS surveys can accurately capture zooplankton diversity, and the hypothesized relationship with temperature, using simulated SFTS surveys of the time series data. Specifically, we asked: (1) to what degree can SFTS surveys capture observed biodiversity dynamics; (2) how does timing and duration of sampling affect detected biodiversity patterns; (3) does biodiversity ubiquitously increase with temperature across lakes, or vary by climate zone or lake type; and (4) do results from SFTS surveys produce comparable biodiversity-temperature relationship(s) to empirical data within and among lakes? Testing biodiversity-ecosystem function (BEF) relationships, and the drivers of such relationships, requires a solid data basis. Our work provides a global perspective on the design and usefulness of (long-term) zooplankton monitoring programs and how much confidence we can place in the zooplankton biodiversity patterns observed from SFTS surveys. info:eu-repo/semantics/publishedVersion

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.