## 1 Introduction  
In recent years, more and more attention has been paid to water quality deterioration worldwide. Anthropoppressure is becoming more visible due to the continuous increase in the human population. [PERSON] et al. [1] defined so-called planetary boundaries for nine major categories. One of the most important planetary boundaries, which has unfortunately been exceeded beyond the safe range for maintaining Holocene-like conditions, is the change in the biogeochemical cycles of two major biogenic elements, nitrogen and phosphorus, which was confirmed by numerous studies, e.g., [2, 3]. Both elements are emitted to surface water from various sources, the most important of which are spatial sources (agriculture--plant and animal production) or point sources associated with the presence of human settlements and industrial facilities in catchments [2, 3].  
The deterioration of water quality is especially noted in water reservoirs located in urban agglomerations. Anthropogenic eutrophication causes numerous changes in the ecology of aquatic ecosystems, which impede the use of strategic water resources, such as surface waters. Water is a valuable resource, the protection of which is regulated by law (the superior act in Europe is Directive 61/EC/2000) [4]. Therefore, there is an obligation to maintain the quality of this resource. Lake restoration methods also serve this purpose [5].  
The purpose of restoration treatments used in lakes is to reverse or reduce the avalanche process of eutrophication. Usually, treatments aim to change the biogeochemical circulation of both main nutrients, phosphorus and nitrogen, but above all, phosphorus [6, 7, 8].  
Physical and chemical adsorption are one of the main mechanisms of phosphorus binding in lake bottom sediments. Recognition of P adsorption possibilities is, therefore, an important issue when considering the impact of sediments on a lake ecosystem, especially in lakes undergoing remediation treatments [9; 10]  
The hypolimnetic withdrawal method is the oldest lake restoration technique, which has been implemented on many lakes worldwide [11; 12; 13; 14; 15; 16]. The installation on Lake Kortowskie is the oldest in the world (operating since 1956). The pipeline and small dam on the Kortowka outflow were designed by professor [PERSON] in the early 1950s. In 1976, the original wooden pipeline was replaced by a pipeline made of glass fibers and polyester resin, and this installation is still operating. Since its installation, the pipeline has been operating with different water flows (50-250 dm\({}^{3}\) s\({}^{-1}\)) in various years [11; 12; 17; 18; 19; 20]. Previous research mainly focused on the hypolimnetic withdrawal effect on water column nutrient balance in Lake Kortowskie. Only a few works, [PERSON] [21] and [PERSON] [20], have given information concerning the chemical composition of the bottom sediment in Lake Kortowskie.  
Our main study goal was to analyze the water-sediment interface chemistry and assess the sediment P adsorption characteristics of Lake Kortowskie, restored by hypolimnetic withdrawal methods for several decades. We also attempted to find whether the hypolimnetic withdrawal method created differences in the chemistry of the water-sediment interface and P adsorption characteristics.