Effective protection of lakes against degradation requires a detailed recognition of the factors leading to their eutrophication. This also pertains to small lakes, which constitute an important element of various ecosystems and are crucial for maintaining biodiversity. Therefore, the aim of the study is to determine the trophic state as well as the susceptibility to degradation of three small lakes in north-western Poland: Maszewo, Starzyca, and Nowogardzkie. This study analyzes the morphometric characteristics of the lakes, the impact of the catchment, their actual trophic state, and phosphorus levels. The analysis showed varying regeneration potentials of the lakes. Maszewo Lake is eutrophic-hypertrophic, nonresilient, strongly exposed to influence from the catchment, with phosphorus levels within the norm. Starzyca Lake is a eutrophic lake, not very resilient to the influence of the catchment, with excessive phosphorus levels. Nowogardzkie Lake is a eutrophic lake with moderate resilience to degradation, but with excessive phosphorus levels, which endangers the functioning of this aquatic ecosystem. In all the lakes, phosphorus supply was predominantly internal. Restoration of these lakes could be performed by completely cutting off the inflow of nutrients, as well as reclamation involving the deactivation of phosphorus (e.g., by precipitation) as well as the removal of the bottom sediments responsible for the internal supply of phosphorus.

High-performance biofilters for water purification in recirculating aquaculture systems (RAS) ensure the safety of cultures of highly nutritious fish. As the most critical step in the functioning of biofilters is their activation, the objective of this study was to evaluate the suitability of commercial artificial media, namely RK Plast (BR-1), Mutag-BioChip30 (BR-2), and LevaPor (BR-3), for the passive activation of biofilters used in rainbow trout farming. Changes in NH₄⁺-N, NO₂⁻-N, NO₃⁻ -N, phosphorus, and carbon concentrations were analyzed. In the first period, an increase in NH₄⁺-N concentration was recorded, before an increase in NO₂⁻-N concentration (maximum concentrations ranged 0.728–1.290 and 0.982–5.198 mg N dm⁻³, respectively), followed by a reduction and stabilization to a level safe for the fish (both below 0.100 mg N dm⁻³). Concurrently, a steady increase in NO₃⁻-N concentration was noted, with a maximum concentration between 6.521 and 7.326 mg N dm⁻³. Total phosphorus and total carbon ranged from 0.423 to 0.548 mg P dm⁻³, and from 43.8 to 45.2 mg C dm⁻³. The study confirmed the feasibility of using the tested artificial biofilter media for rainbow trout farming in RAS with passive biofilter activation. Biofilter activation efficiency was highest for the media with the highest specific surface area (BR-2 and BR-3). The removal of ammonium nitrogen and nitrite nitrogen was above 90%. Nitrogen biotransformation was not limited by phosphorus or carbon concentrations.

The effective assessment and improvement of water quality require analysis not only of the main river flowing into the sea but also of its tributaries, which may contribute to significant pollution. This study aimed to evaluate the physicochemical conditions of water in nine streams flowing into the Rega River between 2018 and 2022. It also sought to determine whether the water quality in these tributaries meets the standards defined by EU regulations for inland waters that serve as habitats for fish. The parameters analyzed included water temperature, dissolved oxygen (DO), pH, total suspended solids (TSSs), electrical conductivity (EC), alkalinity, total hardness (TH), biochemical oxygen demand (BOD₅), nitrite nitrogen (NO₂⁻-N), ammonium nitrogen (NH₄⁺-N), and total phosphorus (TP). The results indicated that most indicators met the requirements for waters suitable for salmonid species. Elevated concentrations of NO₂⁻-N observed across all sites were still within acceptable limits for cyprinid species. Among the parameters studied, TSSs was identified as the main factor that downgraded water quality over the study period. Principal component analysis (PCA) showed that the dominant variables influencing water chemistry were NH₄⁺-N, NO₂⁻-N, TP, EC, and chloride (Cl⁻), all of which are associated with anthropogenic sources.