The aim of this study was a recognition of the carbon cycle in a small anthropogenic reservoir located in Wrocław (SW Poland). The research investigated the geochemical processes and isotopic interactions in the water column as well as those between the water column and the sediment. Moreover, an attempt was made to identify the sources of carbon in the studied water body. Observations of temporal and spatial (vertical and horizontal) geochemical trends were the subject of this research. Chemical and isotopic analyses were the main tool used in this study. A total of 49 samples of water, sediment and plants were prepared, with sampling carried out in the period from October 2015 to May 2017. Two periods: autumn and spring were chosen for the study. During the autumn cycle, smaller variations in the values of δ13C in DIC were found compared to the spring cycle. The enrichment of dissolved inorganic carbon (DIC) in the heavy isotope during the spring period was caused by the dynamic growth of microorganisms. The process of assimilative reduction of dissolved inorganic carbon by aquatic organisms, which use inorganic carbon in biochemical reactions, occurred in the reservoir. This process led to an enrichment of DIC in the 13C isotope. The analysis of the sedimentary organic carbon revealed a greater enrichment in the heavy isotope of carbon (by about 3‰) in April compared to May. This is due to the growth of microorganisms responsible for degradation of sedimentary organic matter and plant detritus. The sediment and the water column were shown to interact through the exchange of carbon.
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