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Lake Rusalka is a shallow, artificial, strongly eutrophic reservoir. Thermal stratification is weak and comprises only about 8% of the bottom surface. In summer, the epilimnion is oversaturated with oxygen due to intensive phytoplankton growth (chlorophyll a up to 80.2 μg l-1), while conditions in the hypolimnion are anaerobic. The high concentration of ammonium nitrogen and low N:P ratio stimulated intense growth of cyanobacteria in the period from June to November. The domination of rotifers in the metazooplankton and low diversity and biomass of benthic macroinvertebrates was the cause of low top-down pressure of these organisms on the phytoplankton. The most advantageous restoration measures were identified to improve water quality and make the recreational use of the lake possible. [ABSTRACT FROM AUTHOR]

Continuous nitrate treatment is an innovative, nature-based and cost-effective restoration method that has been implemented in a small, hypereutrophic lake by redirecting the nitrate-rich waters of the lake's natural tributaries into its hypolimnion. The aim of this treatment is to increase the redox potential at the sediment-water interface in order to provide conditions for efficient phosphorus binding. To assess the effects of this treatment, studies of the physico-chemical and biological parameters of the lake waters were carried out before its application (PreNT – years 2005–2007), during its application (FullNT – 2008–July 2019), and in the period when installation was partly clogged (LimNT- August 2019–2021). The results show effective improvement of oxygenation of the hypolimnion during the treatment followed by a decrease of phosphorus concentration – a proxy of the reduced internal loading. Water quality gradually improved during the treatment. The periods of cyanobacterial blooms shortened in some years, and ceased in other years, and it was also manifested by the increase in the range of Secchi depth and a decrease in chlorophyll-a concentration. The share of eukaryotic algae in the phytoplankton increased. The method showed high resistance to disturbances in its functioning, because during two years of operation of one pipeline, only a slight deterioration of water quality was observed. This enables effective repair of the hypolimnion's supply of nitrates, without a sudden return to the pre-restoration state. Finally, the benefits and limitations of hypolimnetic nitrate treatment (with nitrate from natural sources) were discussed to facilitate the application of this method to other lakes. [Display omitted] • Continuous nitrate treatment method fits in with the idea of a circular economy. • Providing of nitrate-rich waters into the hypolimnion contributes to P reduction. • The use of nitrate treatment contributes to a gradual improvement of water quality. • The most important change is decrease of phosphorus concentration in the hypolimnion. • Cyanobacteria in the epilimnion are limited and transparency is increased. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Highlights • An innovative sustainable lake restoration method by nitrate treatment was proposed. • Nitrate-rich and well-oxidized subsurface waters were delivered into lake hypolimnion to increase the redox potential and reduce internal phosphorus loading. • Phosphorus concentrations in lake waters were diminished as well as chlorophyll-a content in epilimnetic waters. • Cyanobacteria biomass was reduced on the benefit of diatoms and cryptophytes. Abstract The aim of nitrate treatment as a restoration method is to bring about a reduction of the high potential reactivity of sediments by the oxidation of biodegradable organic matter. As a consequence, oxygen uptake by sediments is reduced, sulphate reduction is lowered, hydrogen sulphide formation diminished and iron present in sediments binds phosphorus (P) effectively. This approach was applied in Uzarzewskie Lake, where innovative nitrate treatment of sediments was proposed through the diversion of subsurface waters to the lake hypolimnion. Nitrate-rich and oxygenated water from two tributaries was directly delivered to the hypolimnion through pipelines in order to increase redox potential and eliminate sediments as a P source. Research on selected physico-chemical and biological water characteristics was conducted in the years 2005–2015 to determine the results of restoration measures. P concentrations in lake waters were reduced both in surface water layers and the hypolimnion, hence epilimnetic chlorophyll-a content was diminished as well. Statistically significant changes in phytoplankton community structure were noted as the biomass of cyanobacteria had decreased in 2010 and 2013–2014 in comparison to 2006, while cryptophytes biomass increased in 2011–2013 in relation to 2007. Unfortunately, planktivorous fish domination in fish fauna exert a distinct pressure on zooplankton, thus Rotifera dominated and Cladocera were rarely noted. Additionally, the morphometric lake features (steep slopes, small surface surrounded by forest) create unfavorable conditions for submerged macrophyte development, and hinder the effectiveness of the restoration. Long-term studies revealed that the reconstruction of the phytoplankton community was not permanent, hence further restoration activity is required, with the inclusion of new techniques. [ABSTRACT FROM AUTHOR]