The Influence of the Modernization of the City Sewage System on the External Load and Trophic State of the Kartuzy Lake Complex
A study was carried out in the Kartuzy lake complex, which has been a receiver of raw domestic sewage since the 1950s. In 2018, the city’s sewage system of Kartuzy was modernized. An analysis of the water quality prior to the modernization of the sewage system revealed that the total phosphorus (TP) load that was introduced to the individual lakes from external sources substantially exceeded the dangerous load concentration (defined by Vollenweider) that causes accelerated eutrophication. The annual TP load introduced to the analyzed lakes in 2017 exceeded the critical load by 200% (Mielenko) to 1000% (Klasztorne Duże). Protective measures reduced the external loading of nutrients. In the case of Mielenko Lake, a 37% decrease in the external TP load was noted, and also a 32% decrease in the external TP load in Karczemne Lake, a 66% decrease in Klasztorne Małe Lake and a 54% decrease in Klasztorne Duże Lake was noted. The protective measures resulted in a slight decrease in the concentrations of phosphorus and nitrogen in the water. However, these changes did not improve the environmental conditions in the lakes. In a situation where the internal fertilization process in the lakes has started, the improvement of water quality will only be possible through restoration efforts with methods adjusted to the individual characteristics of each lake.