Coenagrion hastulatum and C. lunulatum – their responses to the liming of acidified lakes and the release of fish

Abstract

The rare and acidification-tolerant Coenagrion lunulatum became extinct in Romundstadtjern, a small acidic lake in southern Norway, at some time between 1950 and 1980. The reason was suspected to be liming of the lake to raise the pH level before releasing fish (trout). To substantiate the hypothesis, in 1998–2001 we experimentally limed two other small acidic lakes, Øynaheia A (pH 4.6) and B (pH 4.8), which were also inhabited by C. lunulatum. Instead of being made extinct by the liming and the rise of the pH to 7.0, the C. lunulatum population at Øynaheia grew strongly during the experimental period. However, when fish (perch) were released later, before 2011, the invertebrate fauna became tremendously impoverished, and C. lunulatum was not observed there in 2012, 2014 or 2016. Therefore, liming of lakes does not seem to be a threat to C. lunulatum, but the release of fish may probably lead to its extinction. A coexisting population of C. hastulatum also grew during the years of liming, but not as much as C. lunulatum. However, it survived the introduction of fish, although in low numbers.