Volume 21 (2021): Edizione 2 (June 2021)

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 3.4m). 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.

A study was carried out on Lake Mielenko in Kartuzy. It is a small and shallow waterbody (7.8 ha, max. depth 1.8 m), whose morphometric conditions qualify it as a polymictic lake (the first lake in a river - lake system). Before the reclamation began, the lake was characterized by a high concentration of chlorophyll-a (16.4 mg m⁻³ in spring to 33 mg m⁻³ in summer) and low visibility. The purpose of the study was to determine the impact of the first stage of restoration of Lake Mielenko by phosphorus inactivation using a combination of aluminum and iron coagulants (PIX and PAX). After coagulant dosing into the lake, a significant decrease in chlorophyll-a was observed, which resulted in improving water transparency in the lake and decreasing concentrations of both nutrients (nitrogen and phosphorus). The total concentration of nutrients was dominated by organic forms (constituting 71-95%). Maximum nutrient concentrations were 0.273±0.01 mg P L⁻¹ and 5.52±0.1 mg N L-1 (in the summer period before the restoration activities were performed).

Zooplankton is a good indicator of water quality state. Analysing the species composition and abundance, it is possible to assess the condition of the water body and predict the direction of changes. The aim of the study was to analyse the zooplankton in a shallow urban lake, in which restoration was limited to one method, i.e. wind-aeration. The results were compared with the earlier data obtained during sustainable restoration (three methods: wind-aeration, phosphorus inactivation, biomanipulation) and before the restoration period. The zooplankton was sampled monthly in 2015 and 2016 in the deepest place of the lake from the surface to the bottom. The trophic state was determined based on rotifer trophic state index for lakes (TSI_ROT). Although the species composition of zooplankton communities varied very little among the restoration periods (Keratella cochlearis f. tecta mainly dominated), significant changes in the abundance of zooplankton were found in the analysed lake. The maximum of total abundance was noted in 2015, almost 5500 ind. L⁻¹, and in the next year its decrease was almost 3-fold, to ca. 1800 ind. L-1. Based on TSI_ROT, the water was still eutrophic. Leaving only one method of restoration (namely, oxygenation of the bottom waters) proved insufficient to support the development of crucial organisms as cladocerans. The changes in the abundance could have resulted more from seasonal changes than from the effects of aeration. A reduction in species number and maintaining a high proportion of rotifers typical for a high trophic state indicated a return of the ecosystem to its pre-restoration state. High variability in the rotifer abundance indicated a continuous imbalance of the ecosystem. Previous restoration treatments using several methods simultaneously showed better effectiveness. The change of strategy of restoration before obtaining a stable improvement of water quality destroyed previously achieved effects.

Preventive and curative intervention become an imperative to encounter the recent dilemmas of lakes and its ecosystem, eutrophication, external contaminants load and the internal load and hydrological cycle disruption, lakes drought or flood as a result of the global warning are some of these dilemmas which mainly caused anthropogenic activities. At present, mechanical and chemical treatment approaches are the most popular applied techniques for lakes remediation with an escalating pace of biological methods are implemented as biomanipulation; which consider an efficient eco-technique. As experience has proven, desirable reclamation result is achieved by implementing combination of the restoration methods.