Suchergebnisse

One of the challenges for predicting global change effects on aquatic ecosystems is the vague understanding of the mechanisms of multiple controlling factors affecting phytoplankton dynamics at different time scales. Here we distinguish between hydrometeorological forcing of phytoplankton dynamics at time scales from days to decades based on a 54-year monthly phytoplankton time series from a large shallow Lake Võrtsjärv (58 160N, 26 020E) in Estonia, combined with daily data on forcing factors— thermal-, wind-, light- and water-level regimes. By using variance partitioning with linear mixed effect modelling (LME), we found a continuum from the large dominant K-selected filamentous cyanobacteria with strongest decadal scale variation (8–30%) to r-selected phytoflagellates with large stochastic variability (80–96%). External forcing revealed strong seasonal variation (up to 80%), while specifically water level and wind speed had a robust decadal variation (8% and 20%, respectively). The effect of external variables was proportionally manifested in the time scales of phytoplankton variation. Temperature, with a clear seasonal variation, had no impact on the dominant cold tolerant filamentous cyanobacteria in Lake Võrtsjärv. We found the LME as a reliable method for resolving the temporal cross-scale problem. It yielded quantitative results that matched our intuitive understanding of the dynamics of different variables. ; Supplementary Information The online version of this article (https://doi.org/10.1007/s10750-021-04594-x) contains supplementary material, which is available to authorised users. ; This study was funded by MANTEL ITN (Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services) through European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 722518 and by the Estonian Research Council grants (PRG1266 and PRG1167). We would also like to thank Estonian Environment Agency for the long-term data used on this study. ; This study was funded by MANTEL ITN (Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services) through European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 722518 and by the Estonian Research Council grants (PRG1266 and PRG1167). We would also like to thank Estonian Environment Agency for the long-term data used on this study.

The post-soviet period in Eastern Europe brought about fast changes in economy, land use, and environmental protection, whereas legacy effects of the previous era of heavy contamination continued emerging in the status of water bodies. In this paper, we analysed the post-soviet (since 1992) changes in catchment nutrient loadings and stoichiometry of nitrogen (N) and phosphorus (P) in two large non-stratified lakes in Estonia – Võrtsjärv and Peipsi. The drastic reduction in the application of P-fertilisers and P discharges with wastewaters since the early 1990s reduced P loadings and increased N/P loading ratio into both lakes. However, it was hard to find clear evidence of reduced in-lake nutrient concentrations and improved water quality. In both lakes, water transparency constantly decreased and phytoplankton biomass increased. Over the years, the difference in N/P ratio between the two lakes became smaller while the large differences in the cyanobacterial community composition remained. Although common thresholds in nutrient ratios favouring N2-fixing species could be revealed in both lakes, the phytoplankton in Võrtsjärv, strongly dominated by Limnothrix spp., remained mostly light-limited and the relationship with N/P stoichiometry was indirect. Random Forest analysis indicated an important role of light limitation in both lakes. Constantly lower levels of N in the deeper Lake Peipsi favoured N2-fixing species, which, as a paradox, became P-limited. As climate warming reinforces eutrophication phenomena in lakes by increasing internal nutrient loading and favouring bloom-forming cyanobacteria, more stringent measures would be needed to further limit nutrient loads (especially that of P) to lakes through improved wastewater treatment and increased efficiency of fertiliser application. ; Main financial support for EMU: European Union's Horizon 2020 research and innovation programme Under the Marie Skłodowska-Curie Action, Innovative Training Networks, European Joint Doctorates. ; Project name, acronym and grant number: Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services, MANTEL, grant agreement No 722518. ; Publication date and, if applicable, length of embargo period: 20.11.2020, no embargo period.

As part of a global phenomenon, a 30% decrease in average wind speed since 1996 in southern Estonia together with more frequent easterly winds resulted in 47% decrease in bottom shear stress in the large (270 km2), shallow (mean depth 2.8 m), and eutrophic Lake Võrtsjärv. Following a peak in eutrophication pressure in the 1970s–80s, the concentrations of total nutrients were declining. Nonmetric Multidimensional Scaling (NMDS) ordination of a 54-year phytoplankton community composition time-series (1964–2017) revealed three distinct periods with breaking points coinciding with changes in wind and/or water level. Contrary to expectations, we detected no decrease in optically active substances that could be related to wind stilling, whereas phytoplankton biomass showed an increasing trend despite reduced nutrient levels. Here we show how opening of the "light niche," caused by declining amount of suspended sediments, was capitalized and filled by the light-limited phytoplankton community. We suggest that wind stilling is another global factor, complementary to climate warming that counteracts eutrophication mitigation in lakes and may provide a challenge to assessment of the lake ecological status. ; Main financial support for EMU: European Union's Horizon 2020 research and innovation programme Under the Marie Skłodowska-Curie Action, Innovative Training Networks, European Joint Doctorates. ; Project name, acronym and grant number: Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services, MANTEL, grant agreement No 722518. ; Publication date and, if applicable, length of embargo period: Published as Early View on 07.10.2019, no embargo period. ; Main financial support for EMU: European Union's Horizon 2020 research and innovation programme Under the Marie Skłodowska-Curie Action, Innovative Training Networks, European Joint Doctorates

Received 11 January 2022; Received in revised form 6 March 2022; Accepted 10 March 2022; Available online 17 March 2022; Editor: Jay Gan ; Sediment phosphorus (P) recycling is one of the key issues in lake water quality management. We studied sediment P mobility in Võrtsjärv, 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 decline 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 fromthe 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 Võrtsjärv, 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. ; 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.

Large lakes are important because of their size and ecological distinctiveness as well as their economic and cultural value. Optimal management of them requires a proper understanding of anthropogenic impacts both on the lake ecosystems as such and on the services they provide for society. The specific structural and functional properties of large lakes, e.g. morphology, hydrography, biogeochemical cycles and food-web structure, are all directly related to their size. Although large lakes are among the best-studied ecosystems in the world, the application to them of environmental regulations such as the European Water Framework Directive is a challenging task and requires that several natural and management aspects specific to these water bodies are adequately considered. These vulnerable ecosystems often suffer from accelerated eutrophication, over-fishing, toxic contamination and invasive species. Large lakes offer socio-economic benefits and can be used in many ways, and are often areas in which economic, cultural and political interests overlap. In this book the problems regarding the present status of European large lakes and the directions of change are discussed. Threats caused by direct human impact and by climate change, protection needs and restoration measures are considered.

In many regions across the globe, extreme weather events such as storms have increased in frequency, intensity, and duration due to climate change. Ecological theory predicts that such extreme events should have large impacts on ecosystem structure and function. High winds and precipitation associated with storms can affect lakes via short-term runoff events from watersheds and physical mixing of the water column. In addition, lakes connected to rivers and streams will also experience flushing due to high flow rates. Although we have a well-developed understanding of how wind and precipitation events can alter lake physical processes and some aspects of biogeochemical cycling, our mechanistic understanding of the emergent responses of phytoplankton communities is poor. Here we provide a comprehensive synthesis that identifies how storms interact with lake and watershed attributes and their antecedent conditions to generate changes in lake physical and chemical environments. Such changes can restructure phytoplankton communities and their dynamics, as well as result in altered ecological function (e.g., carbon, nutrient and energy cycling) in the short- and long-term. We summarize the current understanding of storm-induced phytoplankton dynamics, identify knowledge gaps with a systematic review of the literature, and suggest future research directions across a gradient of lake types and environmental conditions. ; Main financial support for EMU: European Union's Horizon 2020 research and innovation programme Under the Marie Skłodowska-Curie Action, Innovative Training Networks, European Joint Doctorates. ; Project name, acronym and grant number: Management of climatic extreme events in lakes and reservoirs for the protection of ecosystem services, MANTEL, grant agreement No 722518. ; Publication date and, if applicable, length of embargo period: 22.04.2020, no embargo period. ; Main financial support for EMU: European Union's Horizon 2020 research and innovation programme Under the Marie Skłodowska-Curie Action, Innovative Training Networks, European Joint Doctorates