Flensjøen in S Norway 2016. Status of water quality, zooplankton, benthic invertebrates and fish after liming since 2005.

Summary

Lake Flensjøen is an oligotrophic lake with very low ionic strength. pH and acid neutralizing capacity (ANC) were low before liming, which started in the autumn of 2005. However, the water chemistry had improved markedly compared to the situation in the 1970ies and the 1980ies. Comparison of water chemical data from 1992 with data from 2005 also showed a significant effect of decline in acid rain. Liming, which was also performed yearly in 2009-2013, has resulted in further improvement of the water quality. In 2016 pH dropped from 7.2 to 6.5, and ANC dropped from 147 to 83 μekv/l compared to 2013 values. Nevertheless, the water quality was still significantly better than in 2005, before limning started. Concentrations of potential toxic Al-forms (LAl) have been very low (4-8 μg Al/l), even before liming. The water chemistry in later years should not cause any negative effects on the populations of brown trout and arctic char in Lake Flensjøen. As liming is stopped, ANC and pH will drop to significant lower levels over some years, especially during spring and autumn floods. Whether the water chemistry in spawning streams for the brown trout population is satisfactory has not been investigated in this project. Based on aquatic chemistry the ecological status of Lake Flensjøen was classified as high in 2106. The communities of crustacean zooplankton and littoral crustaceans of Lake Flensjøen have been dominated by species known to tolerate rather acidic conditions, both before and after liming started. No major changes in the species composition seem to have occurred after liming. However, in 2006 and 2016 the acid sensitive Cladocera species Ophryoxus gracilis and Daphnia galeata were observed for the first time respectively in Lake Flensjøen. The diversity of the benthic invertebrate community in the outlet river of Lake Flensjøen was expressed as the number of EPT taxa (Ephemeroptera + Plecoptera + Trichoptera). In 2005, before liming, we recorded 10 EPT taxa, while in 2006-2013 and in 2016 (three years after liming has stopped) the number has varied in the range 13-18. The Acidification index 2 (“Raddum 2”) indicated a moderate ecological status in 2005 and an improvement to high status in years 2006-2013 and in 2016, after liming. Compared to the fish surveys conducted in 2006/2007, the growth pattern and the annual length increment of brown trout caught in 2012/2013/2016 were fairly at the same level. However, the relative density of the brown trout population has significantly increased during the period 2006-2016. The arctic char population comprised a larger fraction of older and larger individuals, and displayed improved growth patterns in 2012 compared to 2006/2007. However, the size distribution in 2013 and 2016 was similar to the distributions in 2006 and 2007. Even though factors like temperature and changes in the harvest regime may have affected the fish populations, it is likely that liming has positively influenced the brown trout and arctic char populations. Based on different approaches, fish as a quality element, was given high ecological status. A total assessment indicates that the ecological status of Lake Flensjøen was high in 2016, three years after the liming was terminated.