Sediment phosphorus mobility in Vortsjarv, a large shallow lake : Insights from phosphorus sorption experiments and long-term monitoring

Sediment phosphorus (P) recycling is one of the key issues in lake water quality management. We studied sediment P mobility in Vortsjarv, a large shallow lake in Estonia using both sorption experiments and long-term (1985-2020) monitoring data of the lake. Over the years studied, the lake has undergone a dedine in external phosphorus loading (EL), while no improvement in phytoplankton indicators was observed. The results of the sorption experiments revealed that it may be successfully used as a tool to determine P forms involved in P retention, as up to 100% of the P from the water column was detected in sediments. Incubation of wet sediment is preferred to dry because of the sensitivity of organic P to desiccation. In the sediments of Vortsjarv, the labile P (Lab-P) and iron bound (Fe-P) fractions are the major forms of the mobile pool that supply internal P load as sediment released P. The internal P load calculated from summer total P (TP) increases (ILin situ) in the water column was on average 42%, but could reach 240% of EL at extreme environmental conditions. ILin situ was correlated with the active area, which resembles the area involved in redox-related P release in polymictic lakes, and with the mean bottom shear stress in summer. ILin situ showed a similar decreasing pattern as the external P load over the years 1985-2020, and was likely driven by the decrease of the pool of releasable P. Similarly, the decreases in sediment loading by P retention in our P sorption experiment were associated with decreases in the concentration of the potentially mobile P forms (mainly lab-P and Fe-P). These results show that changes in external P loading can successfully control internal P loading and are useful in water quality management of large lakes.Peer reviewe

Towards understanding the abundance of non-pollen palynomorphs : A comparison of fossil algae, algal pigments and sedaDNA from temperate lake sediments

Given the increased interest in non-pollen palynomorphs (microscopic objects other than pollen identified from pollen slides) in palaeoecological studies, it is necessary to seek a deeper understanding of the reliability of these results. We combined quantitative information of algal pigments and sedimentary ancient DNA (sedaDNA) of phylotaxonomical resolution to validate the richness and abundance of fossil algae in the sediment of a small temperate lake. For the first time, fossil and sedaDNA algae data were combined in a composite data-set and used to reconstruct algae turnover rates over the last 14,500 years. This comparison serves as both an example of how fossil algae can be used to answer various research questions and as a method of revealing to what extent we can rely on palaeoecological interpretations based solely on the fossil algae record from pollen slides. (C) 2017 Elsevier B.V. All rights reserved.Peer reviewe

Both climate trends and short-term fluctuations affected algae-zooplankton interactions in a boreal lake during the late Holocene

Most studies aiming to explore the response of algae and zooplankton trophic interactions to climate variability have only been focused on unidirectional and very short-term trends in temperature changes. As a result, the non-stationary aspect of climate change (warming and cooling periods, frequencies) remains completely unstudied. We studied elemental and stable isotope composition of sedimentary organic matter, photosynthetic pigments, and carbon stable isotope composition of Cladocera resting eggs in a sediment record covering the last c. 2,600 years. We examined how past climate change acting at different timescales affected algal biomass and community composition, and carbon assimilation by zooplankton in a boreal lake. Our study revealed major effects of both long-term climate trends and shorter-term fluctuations on algae-zooplankton interactions in a boreal lake. We found the main climate trends, in particular the Little Ice Age, induced algal biomass and community composition changes and drastic changes in carbon assimilation by zooplankton. Interestingly, we found that temperature fluctuations could also contribute to regulating algae-zooplankton interactions. Specifically, we observed drastic changes in sedimentary markers and stable isotope composition of zooplankton remains during the most recent period, suggesting a strong influence of ongoing anthropogenic change on algae-zooplankton interactions. Our study confirms previous findings showing close long-term linkage between the temporal dynamics of zooplankton diet and planktonic algae, and that both climate trends and short-term fluctuations are key in regulating consumer-resource trophic interactions. Novel approaches that combine high temporal resolution paleolimnological reconstructions and contemporary monitoring studies are needed to better understand climate change effects on algae-zooplankton interactions and lake food webs

Environmental drivers and abrupt changes of phytoplankton community in temperate lake Lielais Svetinu, Eastern Latvia, over the last Post-Glacial period from 14.5 kyr

Understanding the long-term dynamics of ecological communities on the centuries-to-millennia scale is important for explaining the emergence of present-day biodiversity patterns and for predicting possible future scenarios. Fossil pigments and ancient DNA present in various sedimentary deposits can be analysed to study long-term changes in ecological communities. We analysed recent compilations of data, including fossil pigments, microfossils, and molecular inventories from the sedimentary archives, to understand the impact of gradual versus abrupt climate changes on the ecosystem status of a regional model lake over the last similar to 14.5 kyr. Such long and complete paleo-archives are scarce in North-Eastern Europe. The study site lies in a sensitive area, both climatically and in respect to vegetation. Namely the maritime-continentality line runs west to east in the central Baltic area to NE Europe and its south-north transect lies within the gradual decay of the nemoral forest into a boreal environment. Therefore, the selected location is an ideal sampling point to decipher long term environmental changes in the temperate climate zone. The main objective of the present study was to find out external factors influencing phototroph dynamics at temperate Lake Lielais Svetinu over the post-glacial period (similar to 14.5 kyr). We were able to model climate change together with vegetation change and the appearance of anthropogenic forcing, either as a gradual change or as abrupt events that influenced the phototrophs, which are keystone groups within the lacustrine ecosystem. Most interestingly, the gradual increase of species richness of phototrophs was linked to similar increase in fungal parasites of the same group - phototrophs. Abrupt climate change in the Late Glacial period caused abrupt events in the ecosystem but equally abrupt events were caused by gradual changes during the stable period of the Holocene Thermal Maximum (HTM). In addition, we highlight the increased frequency and degree of perturbation in pristine lakes due to low impact human activity over a larger region. Both observations demonstrate an impaired relationship between gradual external drivers and ecosystem response and apply to future scenarios of climate warming and increased human impact in north-eastern Europe. (C) 2021 The Authors. Published by Elsevier Ltd