## 4 Discussion  
Freshwater availability is a major issue across the world [39]. Approximately one-third of the required drinking water is obtained from surface sources, such as rivers, lakes or dams and canals. These sources of water also serve as the best receivers for the discharge of stormwater and domestic or industrial wastes. These conditions are very dangerous due to the constant development of agricultural and urbanized areas and the related increase in pollutant loads of different origins introduced into the waters [40; 41]. Among these point sources, untreated municipal wastewater has been identified as the most hazardous to water ecosystems due to its large amount of nutrients and high organic matter content [42]. Raw domestic sewage also includes pathogenic organisms, oxygen-demanding organic substances and inorganic and organic toxic substances [43; 44]. Since 1956, the Kartuzy Lake complex, which includes the Mielenko, Karczemne, Klasztone Male, and Klasztorne Lakes, has been used as a receiver for domestic sewage sources, including that from dairy farms, slaughterhouses, breweries, furniture factories, and municipal hospitals. These lakes receive an excessive load of nutrients. In 2017, the Kartuzy Lakes and Klasztorna Struga River received sewage from 23 runoff collectors. Although some of the outlets were equipped with separators, this does not provide sufficient protection against the inflow of nitrogen and phosphorus, which are the elements responsible for eutrophication. Biogenic substances are mainly removed in their insoluble form together with mineral suspensions (predominantly phosphorus) and organic fractions (mainly nitrogen). Unfortunately, it is the dissolved forms of these substances, such as phosphates, nitrite nitrogen and ammonium ions, that provide an excellent medium for phytoplankton that cannot be entirely eliminated [45; 46]. Considering the proportion of individual sources of contamination in the context of phosphorus supplies, the biggest hazard to water quality in the lakes were as follows: angling in Mielenko Lake (ca. 45%); point sources (ca. 42%) and the Lake Mielenko tributary (ca. 34%) for Karczemne Lake; the Klasztorna Struga Lake tributary (ca. 60.4%) and point sources (ca. 36%) for Klasztorne Male Lake; and the Klasztorne Male Lake tributary (ca. 95%) for Lake Klasztorne Duze. The annual load of phosphorus sourced from the catchment and atmosphere, considering the permissible and dangerous concentrations calculated from [PERSON]'s hydrological model [38], were compared. The analysis revealed that the total phosphorus concentration that was introduced from external sources to the individual lakes in 2017 substantially exceeded the dangerous concentrations that are known to cause accelerated eutrophication. The annual phosphorus concentration that was introduced to Lake Mielenko in 2017 was almost 200% higher than the critical concentration; in Lake Karczemne, it was six times higher (670% in relation to the critical concentration); in Klasztorne Male, it was seven times higher (770% in relation to the critical concentration); and in Lake Klasztorne Duze, it was 1000% higher (nine times higher than the critical load). A strategy for the protection of the Kartuzy Lake complex was developed. The protective measures were implemented to limit the amount of fishing bait introduced into the lakes and to limit the inflow of sewage from rain collectors. In 2018, the Municipal Water and Sewage Company in Kartuzy modernized the city's sewage network. The modernization involved, among other things, the replacement of piping, the construction of reservoirs for excess water, and the reconstruction of the sewage pumping station. These protective measures reduced the external loading of nutrients into the lakes.