Total Organic Carbon in the Water of Polish Dam Reservoirs

Total organic carbon (TOC) resources in Polish water reservoirs are presented as an important factor affecting water quality and ecosystem trophic state. The study is based on hydrochemical and biological data from 47 reservoirs from the years 2005–2017 and collected from the archives of the Polish National Monitoring Program, provided by the Chief Inspectorate of Environment Protection. The mean (by weight) TOC concentration in reservoirs is 6.3 mg dm−3, with a range from 2.3 mg C dm−3 in the mountains, the Czorsztyn and Sromowce reservoirs, up to 18 mg C dm−3 in the hypereutrophic, lowland Siemianowka reservoir, varying according to reservoir elevation. Although reservoirs are large and deep, there is a significant negative correlation between mean reservoir depth and TOC. Seasonality and national TOC dynamics were strongly related to the rate of precipitation, with maximal concentrations in late spring and minimal in autumn or winter. The first global warming symptoms of TOC changes in reservoirs are noted, which will manifest as increased TOC and greenhouse gas emissions. Increased water retention time, which promotes water eutrophication, increases TOC resources in most Polish dam reservoirs as well as in flooded areas. Mean TOC concentrations are related to certain biological reservoir water parameters, such as phytoplankton index or diatom index. In future planning, Polish reservoirs should be placed outside lowlands, and their capacity should provide a high water exchange, in less than 2 to 3 months.