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Using archived fish scale samples together with long-term monitoring data, this study investigates the potential of fish scales to record historical changes in the aquatic environment. We analysed stable carbon (δ 13 C) and nitrogen (δ 15 N) isotopes in the scales of two planktivorous cyprinid species collected from the meso-eutrophic Římov Reservoir, Czechia, over its entire four-decade history (1979-2016). The δ 13 C of the fish scales varied greatly throughout the reservoir history. The lowest δ 13 C values were observed immediately after the reservoir was filled in 1979, indicating that fish production at that time was likely partially supported by 13 C-depleted CO 2 released from the inundated soil. During the 1980s, due to the high levels of phytoplankton production stimulated by high phosphorus inputs from the catchment, the δ 13 C values substantially increased. However, since 1990, the δ 13 C values have generally decreased, reflecting a gradual reduction in reservoir primary production caused by the decreasing input of phosphorus and increasing input of dissolved organic carbon from the catchment. The δ 13 C of fish scales was also used to reconstruct the CO 2 concentration of the surface water. The reconstructed CO 2 varied significantly during the four-decade history, but it was always below the air-equilibrium concentration, suggesting that the surface water of the reservoir has consistently absorbed atmospheric carbon. The fish-scale δ 15 N values remained relatively stable, while slightly increasing within three years after impoundment, likely because the nitrogen supply was high throughout the studied period. Our study contributes to the growing body of literature demonstrating that stable isotope analysis of archived biological samples is a promising approach for understanding historical trends in the biogeochemistry of aquatic environments. In particular, our results highlight the potential of δ 13 C in archived fish scales in reconstructing carbon cycle changes and evaluating human impacts on aquatic ecosystems., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Environmental changes can exert strong pressure on freshwater biota and lead to unwanted alterations of local communities and deterioration of ecosystem services. Disentangling the links between environmental and community changes is, therefore, essential to understand and predict the impact of human activities on freshwater ecosystems. This is particularly relevant for man-made freshwater reservoirs that represent a nexus between anthropogenic, environmental, and biotic effects. Reservoir food webs depend strongly on phytoplankton dynamics, which are affected by abiotic conditions, nutrient availability and grazing pressure by zooplankton. We studied the effects of relevant environmental drivers (hydrochemistry, hydrodynamics and zooplankton) on the composition, diversity and community stability of main morpho-functional phytoplankton groups over 32 years in the Římov Reservoir (Czech Republic). Environmental conditions in the reservoir are characterised by three distinct periods (1983-89, 1990-99, and 2000-14) defined by shifts and breakpoints in temporal trends in reservoir hydrochemistry and hydraulic conditions, and we examined if and how phytoplankton responded to these abrupt changes. We found significant differences in phytoplankton composition among the three periods. Phytoplankton underwent a substantial compositional shift towards a dominance of pennate diatoms. Time-lag analysis of dissimilarity in phytoplankton composition revealed higher and stochastic annual variations until 1999, followed by a lower variability and divergence in phytoplankton composition in subsequent years. Changes in overall phytoplankton assemblage and most abundant morpho-functional phytoplankton groups were driven mainly by hydrochemical (total nitrogen) and hydrodynamic variables (inflow rate, surface level and mixing depth) and less by zooplankton dynamics. These results suggest that phytoplankton are driven primarily by nutrient input and water regime, both of which can be appropriately managed to support valuable ecosystem services provided by phytoplankton in freshwater reservoirs., Competing Interests: Declaration of competing interest The authors declare they have no conflict of interest., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Man-made reservoirs are common across the world and provide a wide range of ecological services. Environmental conditions in riverine reservoirs are affected by the changing climate, catchment-wide processes and manipulations with the water level, and water abstraction from the reservoir. Long-term trends of environmental conditions in reservoirs thus reflect a wider range of drivers in comparison to lakes, which makes the understanding of reservoir dynamics more challenging. We analysed a 32-year time series of 36 environmental variables characterising weather, land use in the catchment, reservoir hydrochemistry, hydrology and light availability in the small, canyon-shaped Římov Reservoir in the Czech Republic to detect underlying trends, trend reversals and regime shifts. To do so, we fitted linear and piecewise linear regression and a regime shift model to the time series of mean annual values of each variable and to principal components produced by Principal Component Analysis. Models were weighted and ranked using Akaike information criterion and the model selection approach. Most environmental variables exhibited temporal changes that included time-varying trends and trend reversals. For instance, dissolved organic carbon showed a linear increasing trend while nitrate concentration or conductivity exemplified trend reversal. All trend reversals and cessations of temporal trends in reservoir hydrochemistry (except total phosphorus concentrations) occurred in the late 1980s and during 1990s as a consequence of dramatic socioeconomic changes. After a series of heavy rains in the late 1990s, an administrative decision to increase the flood-retention volume of the reservoir resulted in a significant regime shift in reservoir hydraulic conditions in 1999. Our analyses also highlight the utility of the model selection framework, based on relatively simple extensions of linear regression, to describe temporal trends in reservoir characteristics. This approach can provide a solid basis for a better understanding of processes in freshwater reservoirs., (Copyright © 2017 Elsevier B.V. All rights reserved.)