Lakes
Temporal species turnover appeared to co-vary in the treated and untreated lakes (Figure 4). The untreated lake tended to have a greater temporal turnover, both measured as dissimilarity in taxa abundances and species occurrences throughout most of the study period (Figure 4a and 4d). The highest losses in terms of species abundance occurred in the untreated lake in August 2012 and for the treated lake in August 2015 (Figure 4b). Gains were generally highest for the treated lake (Figures 4c and 4f), especially for abundance data. No significant changes in abundance or species occurrence indices between sampling periods were found in the treated or untreated lake (Appendix 4 and 5).
Figure 4
The total abundances of benthic invertebrates in the lakes showed major variations even before the rotenone treatment, i.e., the abundances decreased by about 50% from in from August 2011 to August 2012 in both lakes (Figure 5a). In October 2012, shortly after the treatment, the total abundance in both lakes decreased, but stronger in the rotenone treated Lake Fustvatnet. In June 2013, eight months after the treatment, a minor decrease occurred in both lakes. From August 2013 and onwards the variations in abundance differed between the lakes, with higher values in the treated lake in August 2013, August and October 2014, October 2015 and August and October 2016.
Figure 5
The Ephemeroptera abundance varied over time both within and between lakes (Figure 5b). There was a high abundance in both lakes in June 2013, compared to pre-treatment abundances mainly caused by the Ephemeroptera Siphlonurus sp. Abundances of all taxa from the lakes not presented in the figures are listed in Appendix 6. High abundances were also registered in the treated lake in August 2014 and August 2016. In October 2012, shortly after the lake treatment the abundances was lower in both lakes, compared to samples taken just before the treatment. The abundance of Ephemeroptera taxa decreased immediately after the lake treatment, especially Centroptilum luteolum in the treated lake and Leptophlebiidae in both the treated and untreated lakes. Ephemeroptera species with increased abundances in the treated lake after the treatment included Centroptilum luteolum , Caenis horaria and Ephemera vulgata . However, the abundance of E. vulgata . decreased strongly in august and October 2015 (73% and 71%, respectively), compared to 2014. Except from Siphlonurus sp. there was no marked temporal change in the abundances of Ephemeroptera in the untreated lake.
The abundance of Plecoptera in the treated lake was generally low except in October 2012 just before the rotenone treatment (Figure 5c). In the untreated lake, abundances varied but were generally much higher than in the treated lake. In the treated lake shortly after the treatment in October 2012, there was a strong decrease in Plecoptera abundances (Figure 5c), with only a few specimens of the genus Nemourapresent. The decline was much smaller in the untreated lake. In June 2013, eight months after the treatment, no Plecoptera were recorded in the treated lake, and only a few specimens were observed in the untreated lake. The genus Capnia had the most marked decrease in abundance in the treated lake following the treatment, whereas the abundance of Capnia peaked in some of the years after the treatment in the untreated lake.
For Trichoptera, the abundances in both lakes varied across sampling occasions (Figure 5d). In October 2012, shortly after the treatment, abundances decreased markedly in the treated lake but did not change in the untreated lake. However, a decreased abundance also occurred before the treatment in both lakes between August 2011 and August 2012. The highest abundances in the treated lake after the treatment were recorded in August 2013 and in August 2014. In the untreated lake, the abundances peaked in 2016. Polycentropodidae had the most pronounced decrease among Trichoptera after the treatment in the treated lake, whereas the family increased slightly during the same period in the untreated lake. A few Trichoptera taxa in the treated lake, all found at low abundances, were only recorded before or after the treatment.
The abundance of Chironomidae showed only minor changes after the treatment in both lakes (Figure 5e). In subsequent years, the abundances of Chironomidae in the treated lake varied compared to pre-treatment levels, whereas it did not change or were higher in the untreated lake.
The abundance of Oligochaeta (Figure 5f) and Hydrachnidia generally varied to a similar extent in both lakes. For Corixidae and Coleoptera, and especially of the genus Callicorixa and dytiscid beetles, the abundances increased in the treated Lake Fustvatnet after the treatment. No Corixidae was recorded in untreated Lake Drevvatnet, while Dytiscidae occurred sporadically.