LAUSR

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.