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Presentation at the 11th International Shallow Lakes Conference, Estonia 11.-16.06.2023. We acknowledge the Estonian Research Council grants PRG709 and PRG1167, also European Union's Horizon 2020 research and innovation programme (project TREICLAKE) under grant agreement No 951963 for providing financial support for the project. We acknowledge the Estonian Research Council grants PRG709 and PRG1167, also European Union's Horizon 2020 research and innovation programme (project TREICLAKE) under grant agreement No 951963 for providing financial support for the project.

Understanding long-term carbon flows through aquatic food webs is essential to assess impact of climatic and environmental changes on lake ecosystems. We reconstructed temporal changes in carbon sources fuelling chironomid biomass in a small and shallow hemiboreal/boreal lake over the last 11 000 years. Results were compared with temporal dynamics of known potential controlling factors: summer air temperature, human activities, phytoplankton assemblages and organic matter composition. We report an abrupt rise in the contribution of methane-derived carbon to chironomid biomass. However, this sudden increase was not the consequence of drastic changes in the composition of sedimentary organic matter, mean air temperature, lake trophic state or human activities in the catchment area. Our results suggest that shallowing of the lake caused by sedimentary infilling processes was a possible driver of the expansion of anoxic zones, enhancing CH 4 cycling in lake sediments. However, complementary studies are needed to better understand the mechanisms by which lake shallowing could affect carbon flows to aquatic consumers, and then better assess the natural dynamics of past carbon processing in the benthic food web of shallow lakes. Financial support was provided by institutional research grant IUT21-2, IUT1-8 from the Estonian Science Agency and by Estonian University of Life Sciences, project No. 8M160101PKLJ, and ‘Estophilus Grant’ from the Ministry of Culture of Estonia. We are grateful to Christian Hossann (INRA, Nancy, Champenoux) for assistance with the stable isotope analysis of carbon. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01). Financial support was provided by institutional research grant IUT21-2, IUT1-8 from the Estonian Science Agency and by Estonian University of Life Sciences, project No. 8M160101PKLJ, and ‘Estophilus Grant’ from the Ministry of Culture of Estonia. We are grateful to Christian Hossann (INRA, Nancy, Champenoux) for assistance with the stable isotope analysis of carbon. The PTEF facility is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01).

In order to forecast the influence of future climate change on lake ecosystems, the paleolimnological approaches are needed to understand the impact of past climate variability. We reconstruct temporal changes in carbon resources sustaining chironomid biomass over the last 14,000 years, by means of carbon stable isotope analysis of subfossil chironomids (δ13CHC), with the aim of identifying the response of carbon processing in the benthic food web to climate change. We find a negative linear correlation between reconstructed summer mean air temperature and δ13CHC values, revealing that (i) the contribution of allochthonous organic carbon to the chironomid biomass is high during the coldest and low-productive period, (ii) the aquatic organic matter is the main carbon source during intermediate climate conditions, and (iii) a significant part of the chironomid biomass is sustained by methane-derived carbon during the Holocene Thermal Maximum and the Bolling-Allerod. This study confirms that climate change may significantly affect the recycling process of organic carbon in the benthic food web of small lakes. However, deforestation and agricultural practices within the catchment area induced important organic matter inputs into the lake sediments, which seem to disrupt the observed relationship between climate variability and carbon processing in the benthic food web. In this context, complementary studies are needed to better understand the combined effects of the ongoing global warming and human activities on the lake carbon cycling.

Phytoplankton is the most important primary producer in the aquatic ecosystems and account for half of all photosynthetic activity on earth, thus creating the base for most aquatic food chains. It also plays key role in the oxygen- carbon dioxide balance of the atmosphere and helping to ameliorate green-house gases, by removing nearly a third of the anthropogenic carbon released to the atmosphere. In Europe, the EU has implemented the Water Framework Directive, which is focused on assessing water quality in all waterbodies spanning from ground water to coastal marine waters. The directive requires that undisturbed or reference conditions should be defined for these aquatic environments. Monitoring records rarely go back more than a few decades and in order to define reference conditions other approaches are needed. Phytoplankton pigments that capture and transform light energy are important source of historical information, because if buried in sediments they preserve for centuries and can give information about algal communities in the past. As opposed to deep lakes, the use of phytoplankton pigments in palaeolimnological studies is complicated, because constant resuspension of sediments disturbs the formation of clear sediment archives. This dissertation is focused on phytoplankton pigment analysis of two interconnected large shallow lakes Võrtsjärv and Peipsi. The first issue that is covered is how the pigments in the upper sediment layer of the lake Võrtsjärv follow the annual dynamics of phytoplankton pigments in the water column. Thereafter eutrophication history and palaeolimnological aspects of these two lakes on the 20th century are compared in a multi-proxy studies. As a result of contemporary pigment flux and multi-proxy palaeolimnological studies of lakes Peipsi and Võrtsjärv we can conclude that according to fossil pigments and other palaeoindicators three periods in the evolution of lakes Peipsi and Võrtsjärv can be distinguished on the 20th century: from 1900 to 1950s, from 1950s to 1990s and from 1990s to 2000. Palaeoindicators show that eutrophication of both lakes started to increase rapidly in 1950s. This is also supported by the monitoring data that is available. Although some palaeoindicators peaked in 1970s no significant recovery to more natural state was observed. Results do show, that phytoplankton pigments can be used in multi-proxy palaeolimnological studies to track changes in phytoplankton communities and work out reference conditions even in large shallow lakes. Fütoplankton on tähtsaim orgaanilise aine esmastootja vee-keskkonnas ning hõlmab umbes pool Maal toimuvast fotosünteesist, pannes aluse enamikele vees olevatele toiduahelatele ning omades olulist rolli atmosfääris olevate kasvuhoonegaaside tasakaaluprotsessides. Euroopa Liidus jõustunud veepoliitika raamdirektiivi kohaselt tuleb kehtestada veekogude hea seisundi kriteeriumid. Loodusliku ja inimtekkelise eutrofeerumise eristamine on tihti võimalik vaid mitmekülgsete paleoökoloogiliste uuringutega, sest seireandmed ulatuvad enamasti vaid mõnede aastakümnete taha, kui veekogud olid juba tugeva inimmõju all. Vetikate valgusenergiat püüdvad ning edasikandvad pigmendid on olulised ajaloolise informatsiooni allikad, kuna säilivad settesse mattununa tuhandeid aastaid ning võimaldavad hinnata järvedes toimunud vetikate biomassi ja taksonite vahelisi muutusi. Erinevalt sügavatest järvedest, on pigmentide kasutamine suurte ja madalate järvede paleolimnoloogilistes uurimustes raskendatud, kuna selget ajaloolist settejälge segab vee intensiivse liikumise poolt põhjustatud setete resuspensioon. Käesoleva töö fookuses on suurtes ja madalates järvedes, Võrtsjärves ja Peipsi järves, toimuvate fütoplanktoni pigmentide voogude ning lähiajaloo uurimine mitmekülgsete paleolimnoloogiliste näitajate kaasabil. Esmalt selgitati Võrtsjärve veesamba ja sette ülemise kihi pigmendivoogude seaduspärasusi ning edasi rekonstrueeriti järvede eutrofeerumise ajalugu 20. sajandil kasutades selleks kromatograafiliselt mõõdetud fütoplanktoni üldist ohtrust näitavaid ja taksonispetsiifilisi pigmente ning teisi eutrofeerumist iseloomustavaid parameetreid, nagu sette poorivee lahustunud orgaanilise aine omadused, karbonaatide ja fosforisisaldus, ränivetikate ohtrus ja taksonoomiline koosseis. Töö tulemusena selgus, et fütoplanktoni pigmentide ja teiste paleoindikaatorite põhjal võib mõlema järve arengus eristada kolme ajaperioodi: 1900 kuni 1950-ndad, 1950-ndad kuni 1990-ndad ning sajandi viimane kümnend. Eutrofeerumine kiirenes mõlemas järves 1950-ndatel, mida toetab ka olemasolev seireandmestik. Kuigi mõned eutrofeerumise indikaatorid saavutasid oma maksimumi 1970-ndatel ei ole märkimisväärset järvede seisundi paranemist sajandi viimasel kümnendil näha. Ajaloolised muutused on paremini talletunud Peipsi järve settes, kuna Võrtsjärve madal veetase ning intensiivne vee liikumine segab settearhiivi moodustumist. Eestis esmakordselt on käesolevas töös kasutatud fütoplanktoni pigmente suurte madalate järvede paleolimnoloogilistes uurimustes ning näidatud nende kasutamisvõimalusi seisundi hindamise kriteeriumite väljatöötamisel.