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Salinization is a global threat to freshwater habitats that has been intensified by climate change. Monitoring, assessment and management of salinity is therefore essential. The first step is to set criteria that are sufficiently stringent to protect ecosystem health. However, many countries have not yet defined criteria, and there are substantial differences between criteria. This has been noted in the EU, where salinity is a required “supporting element” for ecological status in inland waters but also for implementation of UN Sustainable Development Goal (SDG) indicator 6.3.2. for “good ambient water quality” where different approaches and widely different threshold values were reported for salinity criteria. Much of this information has not been published and is difficult to access, hindering further efforts to address the problem. We first discuss the implications of salinization for freshwater ecological health. We go on to discuss the principles and guidelines on how salinity criteria to protect ecology should be established. Next, we review salinity criteria submitted as part of implementation of SDG indicator 6.3.2 and the EU Water Framework Directive. Finally, we discuss setting salinity thresholds in an already-warming world and the challenges facing anyone trying to develop salinity criteria to protect freshwater ecosystems. This article is categorized under: Water and Life > Stresses and Pressures on Ecosystems Water and Life > Conservation, Management, and Awareness Science of Water > Water Quality [ABSTRACT FROM AUTHOR]

Soft water lakes, or so-called lobelia lakes, which are inhabited by a specific vegetation composed of isoetids, have been subjected to intense research aimed at evaluating their condition and conservation status for many years in Poland. At the time of Poland’s accession to the European Union and the implementation of the EU Habitats Directive, these lakes were classified as natural habitat 3110. In accordance with the provision of the Habitat Directive a comprehensive methodology for monitoring and classification of the state of this habitat has been developed. Using this methodology, two monitoring trials (in 2009–2010 and again in 2016–2017) were carried out at 45 and 43 sites of the 3110 natural habitat, respectively. These studies confirm the high sensitivity of these poorly buffered aquatic ecosystems to all external influences, both natural and anthropogenic. The overall conservation status of the 3110 habitat in Poland showed a relatively high stability, with similar proportions of sites classified as favourable (FV), unfavourable inadequate (U1) and unfavourable bad (U2) between 2009–2010 (35%, 49% and 16%, respectively) and 2016–2017 (33%, 56% and 11%, respectively). Out of 43 sites examined in 2016–2017, 29 remained unchanged compared with the results of the previous survey concerning their overall status. Results of the monitoring research also allow for the observation and evaluation of mechanisms and directions of changes in the functioning of these ecosystems. Based on the experiences from two series of monitoring conducted so far, the methodology has been assessed as appropriate for the assessment of the conservation status of the 3110 natural habitat, however, some modifications and additions have been suggested.

The Water Framework Directive introduces the requirement for ecological status assessment based primarily on aquatic organisms and supporting physico-chemical elements, including oxygen conditions. The 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 stratified lakes, and on the oxygen content at the bottom in the summer in polimictic lakes. The 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. The 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 classification 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 fish 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 effects of remedial actions in lake catchment areas in long time frames.

Charophytes (stoneworts) form a group of macrophytes that are considered sensitive to eutrophication. The high indicator value of charophytes toward eutrophication results in their wide use in the bioassessment systems. I explored the variability of stonewort communities' requirements for trophic conditions in lowland temperate lakes and attempted to determine the role of individual syntaxa in assessing the ecological status of lakes in Poland. The position of charophyte communities' niches along the trophic gradient was analysed using the Outlying Mean Index approach. A few stonewort communities, i.e., Nitelletum opacae, N. mucronatae, N. flexilis and Charetum filiformis appeared to be specialised concerning water quality and may be considered indicators of habitats less eutrophic than the “mean” trophic conditions in the study domain. Most stonewort communities were relatively common in European waters. Four of them, i.e., Charetum tomentosae, C. asperae, C. contrariae and Nitellopsidetum obtusae, can be classified as ‘generalists’ with low marginality and broad ecological tolerance. Most stonewort communities appeared in a broad range of ecological status classes. In the case of 15 communities, 6 to 25% of occurrences were observed in lakes representing a less than good status, and they cannot be considered indicative of good ecological conditions. However, the high taxonomic diversity and extensive phytolittoral zone were most likely to occur when the phytocoenoses of stoneworts were present. In lowland naturally eutrophic lakes inhabited by eurytopic species, the extraordinary role of charophytes in the assessment of the ecological status can be interpreted through their significant contribution to the development of the littoral.