Looduslike ja inimtekkeliste survetegurite toime vee kvaliteedi muutlikkusele suurjärvedes

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

Recycling lake sediment to agriculture : Effects on plant growth, nutrient availability, and leaching

Sediment removal from eutrophicated shallow lakes may not only be an effective method for lake restoration but also provides the potential for recycling nutrients from sediments to crop production. However, finding a suitable strategy for sustainably reusing the sediment remains a challenge. Therefore, current study focused on the best practices in applying the sediment from a shallow eutrophicated lake to the soil in terms of grass yield, nutrient uptake, and nutrient leaching. During a nine-month lysimeter experiment, 100-cm high columns were filled with six combinations of soil, sediment, and biochar, with or without meat bone meal organic fertilizer. Aboveground biomass, root mass distribution in soil, nutrient concentration, phosphorus (P) uptake of perennial ryegrass (Lolium perenne L.) along with easily soluble nutrients in the growing medium, and leached mineral nitrogen (N) and P levels were measured. Plant growth conditions were improved by sediment additions, as the yield and P uptake of ryegrass nearly doubled in treatments containing sediment compared to the control soil. While the sediment was richer in macro and micronutrients (e.g. P and N) compared to the soil, the leached N and P levels from both treatments were almost equivalent (N < 830 mg m−2 and P < 3 mg m−2). In addition, applying a 2-cm layer of biochar between the sediment and soil reduced P and N leaching by 50%. According to the results, applying a 75-cm thick layer of sediments on agricultural sandy loam soils surrounding the lake seems a promising practice for improving plant yield and soil nutrient status without increasing of P and N leaching from soil.Peer reviewe

Internal phosphorus loading in a small shallow Lake: Response after sediment removal

Mankind is taking advantage of numerous services by small shallow lakes such as drinking water supply, irrigation, and recreational function; however, many of these lakes suffer from eutrophication. Given the key role of phosphorus (P) in eutrophication process, one of the effective restoration methods especially for small shallow lakes is removal of sediments enriched with nutrients. In our study, we used interannual, seasonal, and spatial data to examine the changes in sediment P mobility after removal of sediments in 2016 from a 1-ha highly eutrophic lake. We measured the sediment redox potential, analyzed soluble reactive P (SRP) in the pore water and P fractional composition of the surface sediments, and calculated the P diffusive flux in three locations in two continuous years (2017 and 2018) after the excavation. Similar measurements were done before sediment removal at central site of the lake in 2015. Removing nutrient-rich sediment also removed 6400 kg of P, and thus the potential for release of P from sediments decreased on a long-term scale. However, a large pool of releasable P was rebuilt soon after the sediment removal due to high external P loading, resulting in extensive anoxia of sediment surface and associated internal P loading as high as 1450 mg m−2 summer−1. Moreover, the Fe-P and labile P fractions were the most important sources of P release, as evidenced by their considerable seasonal and interannual changes after the sediment removal. The sediment total Fe negatively correlated with sediment P diffusive flux, pore water SRP, and near-bottom water total P and SRP concentrations which indicated a strong linkage between sediment P dynamics and Fe after the restoration. Sediment removal could be a beneficial restoration approach, but the effects on lake water quality remain only short-term unless there is an adequate control on external loading to the lake.Peer reviewe

Leping nr 3-2_3/8693-5/2018

Inimtegevuse tagajärjel rikastuvad veekogud toiteainetega, mis toob endaga kaasa primaarproduktsiooni tõusu ning vee kvaliteedi halvenemise. Eutrofeerunud järvede vee kvaliteedi parandamine on praegusel ajal veekaitse üks peamisi ülesandeid kogu maailmas. Järvede tervendamine on kriitilise tähtsusega veeressursi säästliku majandamise seisukohalt. Tervendamiseks peetakse tegevust, mille tagajärjel saab veekogu hea seisundi. Seda eristatakse tavaliselt taastamisest (põhjalik järvesängi muutmine), korrastamisest (välisilme muutmine, mis ei pruugi parandada vee kvaliteeti), rehabiliteerimisest (Ott ja Timm, 2020). Järvi on vaja tervendada ka selleks, et vastata kehtivate veekaitseliste regulatsioonide (nt EL Veepoliitika raamdirektiiv) nõuetele. Värskeimate, 2020. a., andmete põhjal pole ligi 78% seirega kaetud Eesti seisuveekogumitest heas seisundis (Keskkonnaagentuur, 2021). Eriti murettekitav on aastail 2012-2020 jälgitav tendents, mille järgi heast kehvemas seisundis olevate järvede osakaalu on koguni suurenenud. Säärane statistika rõhutab vajadust üle vaadata seniseid järvede tervendamise praktikaid. Käesolevas töös pakutakse võimaliku lahendusena tervendamist jätkusuutlikul moel. Käesolevad juhised on välja töötatud 2020. a. augustis KIK toel valminud projekti „Jätkusuutlik väikejärve tervendamine sette eemaldamise ja selle toiteainete taaskasutuse teel“ jätkuprojekti raames. Meetodid on suunatud peamiselt väikejärvede tervendamiseks ning on toeks veemajandajatele.Keskkonnainvesteeringute Kesku

The role of humic substances in sediment phosphorus release in northern lakes

phosphorus (P) release are largely unexplored. Here we elucidated the factors behind experimentally-derived sediment release rates of P by diffusion (DF) in four Finnish lakes with a range of colour. Next, we extended our analysis to a larger set of northern lakes for further insights regarding possible implications of organic substances on sediment P release. The significant correlation between pore-water soluble reactive P and dissolved iron, and a positive effect of ironbound sedimentary P (Fe-P) on DF supports the classic paradigm of redox-dependent P release in the four Finnish lakes studied. Nevertheless, the P release from Fe-P may be inhibited by humic substances, as we observed lower Fe-P and negative DF in two humic rich lakes (high DOC). The analysis of a larger set of northern lakes supported the negative effect of humic substances on P release rate (RR) determined by in situ P increases. In this dataset, DOC correlated positively with water colour and negatively with RR. Furthermore, multiple stepwise regression analysis selected sediment total P and organic matter content in sediments (LOI) as the best predictors of RR, similar to a previously published model by Nurnberg (1988). While the model predictions (RRpred) were correlated to RR in the present study, they tended to overestimate RR that was determined in closed experimental systems. The inhibiting effects of humic substances on RR may be manifested in both internal P loading and primary production.Peer reviewe

* Corresponding author at: . E-mail address: (O. Tammeorg).

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 com-bining 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 exam-ple, 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 under -or 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.(c) 2022 The Authors. Published by Elsevier B.V. on behalf of International Association for Great Lakes Research. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/).Peer reviewe

Redox-related release of phosphorus from sediments in large and shallow Lake Peipsi: evidence from sediment studies and long-term water monitoring data.

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Weather conditions influencing phosphorus concentration in the growing period in the large shallow Lake Peipsi (Estonia/Russia)

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Spatio-temporal variations in sediment phosphorus dynamics in a large shallow lake: Mechanisms and impacts of redox-related internal phosphorus loading

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.This study was funded by the Estonian Research Council grant PRG1167 and Estonian Ministry of Environment 4-1/21/76. Also, this project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963.This study was funded by the Estonian Research Council grant PRG1167 and Estonian Ministry of Environment 4-1/21/76. Also, this project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 951963

Lake catchment characteristics and external P load - cultivated area/lake area ratio as a tool for evaluating the risk of eutrophication from land use information

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