Sirvi Autor "Cremona, Fabien (advisor)" järgi
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Kirje Hydrometeorological and climatic control over lake phytoplankton : the importance of time scales(Eesti Maaülikool, 2021) Janatian, Nasime; Nõges, Peeter (advisor); Obrador, Biel (advisor); Cremona, Fabien (advisor); Laas, Alo (advisor); Institute of Agricultural and Environmental Sciences; Uusi-Heikkilä, Silvia (opponent)Phytoplankton reflects changes in the environment and plays a vital role in biogeochemical cycles and the climate system. The thesis attempts to link the phytoplankton dynamics with the timing, intensity, and duration of the local forcing factors at different time scales. We highlight the influence of two extremes of the wind gradient – storms and atmospheric stilling, on lake environments and phytoplankton dynamics over short and long periods, several aspects of which are poorly understood. Until recently, atmospheric stilling as a climatic phenomenon has been largely overlooked in lakes studies. To fill this research gap, we focussed on a large shallow polymictic lake (Võrtsjärv, Estonia), that was affected by a 30% decrease in average wind speed since 1996, and for which a long-term (54 years) phytoplankton and hydrometeorological database was available. Further, a contradiction between the continuous decrease in the lake’s nutrient loading and an increasing trend in phytoplankton biomass emerged as a topic of interest for this thesis. We summarise how storms interact with and alter the dynamic of phytoplankton communities. Further, we highlight to what extent this impact can change the ecological processes (e.g., nutrient, carbon, and energy cycling) within lakes and their environmental conditions in the short and long term. Using Nonmetric Multidimensional Scaling ordination of phytoplankton community composition for the years 1964–2017, we revealed three distinct periods with breaking points coinciding with abrupt changes in the wind and/or water level. We introduced a concept of "light niche," a newly discovered mechanism of meteorological control over phytoplankton in light-limited shallow lakes. Combining the monthly phytoplankton data with daily data on hydrometeorological forcing factors — thermal, light, wind, and water-level regimes and using variance partitioning with linear mixed effect modelling (LME), we found that (i) the external forcing factors relevant for each phytoplankton variable could be individualised by having a similar variance partitioning among time scales as the particular phytoplankton variable; (ii) with the largest seasonal variation component, the dominant shade-tolerant filamentous cyanobacteria were most affected by seasonal factors such as solar irradiance and water level; (iii) the LME was proven appropriate for resolving the temporal cross-scale issues.Kirje Impact of climate change and other ecological factors on selected fish populations and fishery in Estonian large lakes(Eesti Maaülikool, 2021) Öğlü, Burak; Kaart, Tanel (advisor); Kangur, Külli (advisor); Cremona, Fabien (advisor); Kuparinen, Anna (opponent); Olli, Kalle (pre-opponent)Human activities and climate change have become the most consequential threats to freshwater ecosystems and their inhabitants, especially fish. The response of the fish population to their environment is not always straightforward because of joint effect of multiple parameters. Fish and fisheries in shallow lakes can be directly affected by changes in air temperature, and also changes can occur via other factors in lakes that are under the influence of climate change. Also, response of each fish species to those parameters can vary depending on their tolerance and adaptation. Therefore, examining complex relationships between fish and their environment plays important role at understanding the dynamic of fish population and fisheries. In this study, we aimed to determine general driving factors, including climate change impact, for fish and fishery in Estonian large lakes. Results show that although winter is one of the most affected seasons by climate change in this region, then the selected fish species and eel fishery in Estonian large lakes were more sensitive to other environmental parameters. However, since the changes in winter surface water temperature and ice formation can affect other parameters, the indirect effect should not be ignored. Nutrients, spring-summer temperature and alkalinity were the most important environmental parameters for the selected fish biomass. We found that the high blue-green algae biomass during restocking period is the strongest negative impact for the eel fishery in Lake Võrtsjärv. The impact of factors can occur subsequently in fish communities and may lead to irreversible consequences, whereas the effects of concurrent climate change and nutrient enrichment can mutually reinforce their symptoms.Kirje Modelling of large shallow lake food web based on long term monitoring data(Estonian University of Life Sciences, 2024) Bhele, Upendra; Cremona, Fabien (advisor); Institute of Agricultural and Environmental Sciences; Lobry, Jérémy (opponent)ABSTRACT. Climate change poses a serious threat to freshwater ecosystems. With population growth, the demand for food production increases, potentially leading to excessive use of natural resources, high nutrient loads, eutrophication, and overfishing in water bodies. Modelling the food web is a valuable solution for understanding the key processes and ecological issues in a lake ecosystem, as it avoids the need for expensive experimental studies. Ecological modelling allows the exploration of the interconnected effects of environmental conditions, nutrient loading causing eutrophication, and fishing activities on the aquatic ecosystem. The Ecopath model, along with its modules Ecosim and Ecospace (EwE), is a widely used tool for investigating the structure of aquatic food webs, encompassing both top-down and bottom-up control mechanisms from producers to predatory fish to detritivores. This doctoral thesis analyses the potential impacts of climate change and associated fluctuations in water levels on the biodiversity of Lake Võrtsjärv. The modelling process considers the biomasses of all major functional groups in the lake. Using the spatial module Ecospace within the EwE modeling suite, scenarios based on realistic water level fluctuations were examined. The models also simulated top-down and bottom-up processes based on predator-prey relationships and commercial fishing data. These models were employed to understand the process-based dynamics of phytoplankton, invertebrates, fish groups, and other key functional groups in the ecosystem. The outcome of the research shows that modelling indicated that zooplankton is not efficient in controlling the biomass of cyanobacteria, leading to detritus accumulation. Global warming and eutrophication are expected to increase cyanobacterial production and the frequency of algal blooms, especially in shallow lakes. Consequently, lakes may become more dependent on detrital organic matter in the near future. Reducing factors causing eutrophication would decrease cyanobacterial dominance, thereby improving the overall feeding capacity of zooplankton. Ecospace modelling to elucidate the effects of water level changes on functional group biomass and spatial distribution in the lake showed that prolonged water level decline could trigger a trophic shift, increasing the number of predatory groups at the expense of plankton and smaller fish. A stable water level would be more beneficial for Lake Võrtsjärv biodiversity than significant water level fluctuations. The third objective of this study was to investigate simulating biomass changes in several functional groups, whether trophic cascading was “bottom-up” or “top-down” in shallow lakes. The study simulated biomass changes in multiple functional groups. The "Keystoneness" analysis revealed that the macrozoobenthos as a lower consumer group and adult pikeperch as a top predator on the food web is equally important. Sensitivity analysis showed that changes in the biomass of phytoplankton, zooplankton, and macrozoobenthos affect the biomass of all major functional groups more than changes in the fish population. The models demonstrated that a sudden change in the food web (plankton, macroinvertebrates) due to eutrophication or climate change poses a threat to the entire functioning of the food web, as environmental tolerance limits have already been reached through maximum carrying capacity.