# Is It Possible to Restore a Heavily Polluted, Shallow, Urban Lake?
## 1 Introduction  
Strong anthropoppressure in the catchment areas of lakes (urbanization, industrialization, deforestation and intensive livestock), which stimulates an increased supply of nutrients to waters, causes the acceleration of eutrophication [1; 2; 3]. Accelerated eutrophication is exemplified by strong cyanobacterial blooms, species depletion at all trophic levels, the disappearance of valuable fish species and the deterioration of the taste and smell of the water, which leads to the cessation of the reservoir being used for municipal and recreational purposes [4; 5; 6]. A sign of excessive eutrophy of the lake is an increase in nutrient concentrations (nitrogen and phosphorus) in the water and an uneven distribution of the amount of oxygen in the water column [7; 8; 9]. The surface layers of the reservoir are usually saturated with oxygen, while at the bottom, anaerobic conditions prevail, promoting the release of nutrients from bottom sediments. In oligotrophic and mesotrophic lakes, bottom sediments are a \"trap\" that bind excess nutrients, especially phosphorus. In degraded lakes, in which the bottom water layers are deoxygenated and therefore accompanied by a reduction in redox potential, the reverse process takes place. The stored substances, such as nutrients, are released into the near-bottom water layers [10; 11; 12]. This phenomenon is called internal loading. The amount of nutrients in the bottom sediment of the lake is extremely high. The sediments of degraded lakes arethe main, inexhaustible source of biogenic compounds. Considering the content of the most important nutrient--phosphorus--in the particular parts of lake ecosystems, it was found that approximately 90% of the total phosphorus was contained in the upper 10-cm layer of the bottom sediments [13]. This result clearly indicates the need to limit this \"source\". The inhibition of internal loading is the main goal of all lake restoration methods, including artificial aeration, phosphorus inactivation, capping and the Ripl method [14; 15; 16]. The removal of bottom sediment is widely regarded as a radical but highly effective restoration method for shallow, heavily degraded reservoirs [17]. The complete removal of bottom sediment to the parent rock floor guarantees radical renovation of the lake and an increase in its volume, which often has previously been decreased due to the inflow of excessive pollution. Considering the thickness of sediments in postglacial lakes, often up to several meters or more, this method should be considered unrealistic. Innovative lake restoration projects using the dredge method should be preceded by detailed monitoring of water and bottom sediment. Therefore, lake monitoring should be started with an examination of the spatial composition of bottom sediments to precisely determine the thickness of the most contaminated layer that is supposed to be removed. In addition, uncovered deposits that remain in the lake should be poor in nutrients, while phosphorus, as the main element responsible for eutrophication, must be stored in a biologically inactive form, i.e., as residual phosphorus or calcium-bound phosphorus [18; 19; 20]. This method of removing bottom deposits is very rarely used due to its complexity, cost and implementation difficulties. However, although the name of the method refers to bottom sediment removal, such a project must also take into account the output development and processing/utilization of the solid and liquid fractions [15; 16; 21]. The processing, management and utilization of deposits is often the greatest logistical and economic challenge. An example of a very inept attempt at lake restoration by the bottom sediment removal method was the case of Mogilerskie Lake in Poland. The sediment and interstitial water removed from the lake were not properly managed, and before they were processed, the effluents returned to the lake water, destroying the initial effects and causing an ecological catastrophe [22].