Phosphorus in the Shallow, Urban Lake Subjected to Restoration—Case Study of Lake Domowe Duże in Szczytno

The paper presents the results of the research on the restoration of the shallow Lake Domowe Duże in Szczytno (area 0.6 km², average depth 3.4 m). Restoration treatments were carried out in the years 2010–2012 and consisted in the inactivation of phosphorus using polyaluminium chloride. Technical restoration was supported by biomanipulation treatments consisting in stocking the lake with predatory fish. Water quality study were carried out before remediation and annually for a decade (2010–2019) during and after the application of coagulant. It was shown that the efficiency of removing excess phosphorus from water column was high (decrease from 0.23 mgP L⁻¹ to 0.05 mgP L⁻¹). The effects achieved during the coagulant application were maintained for 5 years after the end of the project. Currently, the phosphorus pool is still twice as low as before the treatments, but it is gradually increasing. The main external factor limiting permanent improvement of water quality is the open hydrological system and transport of biogenic matter by surface inflows draining anthropogenically transformed areas. Maintaining permanent improvement of water quality will require continuation of restoration measures.