Stable isotope evidence from archived fish scales indicates carbon cycle changes over the four-decade history of the Římov Reservoir (Czechia).

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 (δC) and nitrogen (δN) 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 δC of the fish scales varied greatly throughout the reservoir history. The lowest δC values were observed immediately after the reservoir was filled in 1979, indicating that fish production at that time was likely partially supported by C-depleted CO 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 δC values substantially increased. However, since 1990, the δC 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 δC of fish scales was also used to reconstruct the CO concentration of the surface water. The reconstructed CO 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 δN 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 δC in archived fish scales in reconstructing carbon cycle changes and evaluating human impacts on aquatic ecosystems.