Limnol. Rev. (2016) 16, 2: 85–94 DOI 10.1515/limre-2016-0009 Is unstable reliable? Oxygen conditions in the ecological status assessment of lakes Hanna Soszka*, Agnieszka Pasztaleniec, Agnieszka Kolada Institute of Environmental Protection – National Research Institute, Kolektorska 4, 01-692 Warsaw, Poland, e-mail: hasoszka@ios.edu.pl (*corresponding author) Abstract: Te Water Framework Directive introduces the requirement for ecological status assessment based primarily on aquatic organisms and supporting physico-chemical elements, including oxygen conditions. Te criteria for assessing oxygen conditions applied in routine lake monitoring in Poland are based on the mean hypolimnetic saturation with oxygen at the peak of summer stagnation in stratifed lakes, and on the oxygen content at the bottom in the summer in polimictic lakes. Te currently used oxygen criteria usually cause dismay, particularly if biological elements indicate at least good ecological status and the oxygen conditions are the only reason for lowering status class. Te aim of the article is to present the scale of variability of oxygen conditions in selected lakes within the last two decades and to analyse the limitations and conditions of applying this indicator in routine monitoring, assessment and classifcation of Polish lakes. Data from the years 1999–2015 on oxygen and thermal conditions in a set of nine benchmark lakes in Poland were examined and the reasons for changeability in water oxygenation from year to year were discussed. Alternative oxygen indicators were considered as well as oxygen standards based on fsh protection requirements that have been adopted in some European countries. It was suggested that when assessing lake ecological status, data on the oxygen conditions from a number of years must be taken into consideration, e.g., for the analysis of trends in lake trophy changes or for documenting the efects of remedial actions in lake catchment areas in long time frames. Key words: oxygen conditions, lake assessment, ecological status, hypolimnion oxygenation Introduction Measurements of temperature and oxygen con- tent in water are by far the most frequent measure- ments in limnological studies. Oxygen content in lake waters is closely related to the intensity of primary production and the decomposition of organic matter. Hyperoxygenation of surface waters and reduced oxy- genation of hypolimnetic waters are secondary efects of eutrophication, and oxygen content in the hypolim- nion and the pace of oxygen depletion in this layer may be early symptoms of changes in lake trophy (Gibson et al. 2000). A thorough analysis of changes in oxygen concentration in Polish lakes over the past few decades (practically the second half of the 20th century) was presented by Marszelewski (2005), who found consid- erable reduction of oxygen concentration in most Pol- ish lakes, especially in the hypolimnion. In the 1960s, oxygen was usually present in the entire vertical profle of lakes, including stratifed ones, although its concen- tration at the bottom varied. At the end of the 20th cen- tury, in most dimictic lakes oxygen was absent from the bottom waters in the summer, not only at the deepest point. Currently, summer oxygen defcits ofen occur even at a depth of several metres. Tus, oxygen condi- tions well document the progress of eutrophication of Polish lakes, and may play a signifcant role in the as- sessment of changes in the quality of their waters. Te monitoring of lakes in Poland, carried out since the 1970s, has always involved the investigation of thermal and oxygen conditions. Until 2006, the strategy of monitoring had been compliant with the Lake Qual- ity Evaluation System (LQES; Kudelska et al. 1997) and provided for two measurements a year (in the spring and at the peak of the vegetation season), including, among others, the determination of the thermal and oxygen profle at a lake’s deepest points. At the time, the oxygen indicator used in the four-class classifcation of lake quality (three quality classes and out of class wa- ters) involved the mean hypolimnion saturation with oxygen at the peak of summer stagnation (in stratifed lakes) and oxygen content at the bottom in the summer (in non-stratifed lakes). Te indicators were attributed Hanna Soszka, Agnieszka Pasztaleniec, Agnieszka Kolada