Uzarzewskie Lake is a small, postglacial lake, located in western Poland. The lake is under restoration treatment since 2006. At first, iron treatment was done for 2 years. In the second stage, spring water was directed into the hypolimnion in order to improve water oxygenation near the bottom sediments. The purpose of our research was to determine changes in the contribution of functional groups to the total number of taxa and total biomass of phytoplankton due to changes in the physical and chemical characteristics of the restored lake. Phytoplankton composition was analyzed in three periods: (1) before restoration; (2) during the first method of restoration; and (3) when the second method was implemented in the lake. Epilimnetic phytoplankton was sampled every year monthly from March to November. The relationship between phytoplankton groups and environmental factors (water temperature, ammonium nitrogen, nitrate nitrogen, dissolved phosphorus, conductivity and pH) was examined, using the canonical analyses. The redundancy analysis indicated that the temperature, dissolved phosphates concentration, ammonium nitrogen and pH were the main determining factors of the phytoplankton community dynamics. During the study, 13 coda dominated the phytoplankton biomass. Cyanobacteria of the codon H1 with such species as Aphanizomenon gracile, Dolichospermum planctonicum, D. viguieri dominated the phytoplankton community before restoration. S1 group consisting of Planktolyngbya limnetica, Limnothrix redekei and Planktothrix agardhii mostly dominated during the period in which the first method was used. Improvement of water quality due to restoration efforts in the third period caused dominance of other groups, especially J (Actinastrum hantzschii and other Chlorococcales), C (Asterionella formosa and other diatoms), Y (Cryptomonas marssonii and other cryptophytes), Lo (Peridiniopsis cunningtonii and other dinophytes) and X2 (Rhodomonas lacustris).

Swarzędzkie Lake, directly polluted for many years with municipal wastewater and heavily loaded with nutrient compounds from the catchment area, has become degraded and strongly eutrophicated. Strong cyanobacterial blooms have contributed, among others, to the cessation of recreational use of this urban lake. Its sustainable restoration was started in autumn 2011. These treatments were a combination of three complementary methods: aeration with a pulverizing aerator, phosphorus inactivation with small doses of magnesium chloride and iron sulphate (<15 kg ha−1) and biomanipulation. These treatments were carried out for three years (2012–2014), and in the next two (2015–2016), treatments were limited from three to one method—aeration. The obtained effects (a decrease in the number of cyanobacteria in phytoplankton and at the same time an increase in its biodiversity, decrease in chlorophyll a concentration and improvement of transparency) were lost due to the cessation of phosphorus inactivation and biomanipulation. The biological balance was upset, which resulted in an increase in chlorophyll a concentration, the return of cyanobacteria dominance in the phytoplankton and a deterioration of water quality. Leaving only a pulverizing aerator active, to maintain low oxygen concentrations near the bottom zone was not sufficient to ensure a gradual improvement of water quality with quite a significant external load of nutrients.

To fill the knowledge gap about the functioning of the lake–river system subjected to restoration treatments, two tributaries, a shallow, restored lake and its outflow, were examined. The quality of water inflows, lake and outflow was compared before (BR), during sustainable (SR, deep water aeration, phosphorus inactivation and biomanipulation for 3 years) and limited lake restoration (LR, only aeration for 2 years). Physico-chemical parameters were analysed monthly at five stations. The nutrient concentrations at the inflows decreased over the years due to the improvement of water and sewage management in the catchment (in Mielcuch from 18.0 to 8.0 mgN L−1 and 1.0 to 0.6 mgP L−1). The decline at the outflow was the result of a better quality of water at the tributaries and SR in the lake. During LR, decrease of phosphorus concentration still occurred (0.11 mgP L−1), but nitrogen concentration slightly increased (3.9 mgN L−1). Although the outflowing waters still transported a high content of chlorophyll a and suspended solids during SR, their amount was lower (34.5 μg L−1 and 17 mg L−1, respectively) than that during BR and LR. During restoration, it is significant to monitor the water quality not only in the lake but also at the outflow. The slow deterioration of water quality at the outflow indicated that introducing changes in the applied restoration methods must be done carefully because the previously achieved effect may be lost. Hence, restoration of the upstream lake and good quality of its tributaries are of great importance for water bodies located downstream.

Sustainable restoration treatments were implemented with the simultaneous application of pro-ecological methods that complement each other to improve water quality in the shallow and heavily polluted Raczyńskie Lake. Phosphorus inactivation with magnesium chloride and Phoslock® was introduced along with biomanipulation. Physico-chemical and biological parameters were studied in 2015 (before restoration) and throughout 2018 and 2019 (during restoration). Water quality improved in the first year of treatment. An increase in water transparency, oxygen concentration above the bottom, a decrease of chlorophyll-a concentration and a reduction in cyanobacteria were observed. In the second year of treatment, a slight deterioration of water quality was recorded, probably caused by fewer phosphorus inactivation treatments and a shortened period of application. However, the deterioration of conditions is also characteristic of sustainable restoration at the beginning of the restoration process. The obtained results showed that sustainable restoration requires more than two years, and its scope and intensity should strictly depend on the pace of changes, determined on the basis of monitoring. In addition, improved water quality will have a positive impact on the recreational use of this reservoir.

Successful management of lake ecosystems used for recreation requires firstly an identification of nutrient sources. It is necessary to identify the factors causing the deterioration of water quality and to plan measures for their mitigation. Analyses of the external and internal nutrient loading were carried out for the hypereutrophic Raczyńskie Lake. The study included flows from lake tributaries, stormwater runoff from impermeable areas and direct catchment impact as external sources of nitrogen and phosphorus, as well as bottom sediments as an internal source of phosphorus. In the case of external sources, the largest load (about 80% of N and 67% of P) is supplied from croplands via the shoreline. Both external and internal loading was characterized by distinct seasonal variability. The loads from watercourses supplying the lake played the most significant role in spring, whereas the release of phosphorus from bottom sediments (accounting for 81.4% of the total P load) was responsible for cyanobacterial blooms in summer. In order to improve Raczyńskie Lake water quality it is crucial to implement both in-catchment and in-lake measures by means of diversion of stormwater runoff, reduction of nutrient content in some of tributaries at their inflow to the lake (gabions filled with dolomite surrounded by macrophytes) and restoration treatments aiming at the inactivation of phosphorus in the water column and reduction of its release from sediments.