According to the researchers, the previously abundant community of organisms has become poor in its range of species, these days resembling that of a highly polluted lake. At the same time, changes further away from the pipeline and in the surface waters have been minor.
“Contrary to what was thought earlier, our findings clearly demonstrate a collapse in the ecological integrity of benthic species. This clearly indicates that both the bottom and surface zones of the water column should be considered separately when investigating the status of lakes. Such investigations should also cover any changes taking place in species groups both temporally and regionally within individual lakes,” says researcher Tomi Luoto from the Faculty of Biological and Environmental Sciences, University of Helsinki.
According to prior research, the accidental and planned discharges from the Talvivaara/Terrafame mine have caused changes in the lakes close to the mine, such as salination and increased sedimentation.
In this study, the focus was on the effects of a mine water discharge pipeline installed in 2015 on Lake Nuasjärvi, a body of water with a maximum depth of 44 metres and a surface area of 96 km2 located between Vuokatti and Kajaani in northern Finland. Large amounts of sulfates are transported to the lake water through the pipeline, which was installed directly to the bottom of Lake Nuasjärvi. Even though the lake’s condition was found to be good in earlier environmental assessments, a distinct increase in electric conductivity has been observed after the installation of the pipeline, which indicates effects originating in the mine.
The study analysed the community composition of species living in open water areas, such as diatoms and cladocerans (water fleas), as well as those living at the bottom in the surface sediment, such as chironomids (midges), in the various parts of Lake Nuasjärvi before and after the pipeline installation. The study employed palaeolimnological methods, or information sources stored in sedimentary deposits. The development of the concentration of oxygen close to the bottom was modelled with statistical methods based on the species composition of chironomids.
Researchers discovered clear oxygen depletion taking place at sampling sites located close to the bottom after the discharge pipeline installation. In samples collected at sites several kilometres from the pipeline, no significant oxygen depletion was observed. Benthic organism communities have clearly changed and become increasingly narrow at the sampling sites, resembling a chironomid composition typical of polluted lakes. Due to the rapid water cycle in the surface zone of the water column, no significant change was seen even in the diatoms and cladocerans living above the discharge pipeline.
The results demonstrate clear stratification caused by salination close to the pipeline, which explains the differences between water close to the bottom and on the surface in terms of ecological and hydrochemical changes. The nearly complete turnover of chironomid species and the loss of diversity among them close to the pipeline have degraded the ecological integrity of the lake's benthic organisms.
Luoto, T.P., Leppänen, J.J. & Weckström, J. 2019. Waste water discharge from a large Ni-Zn open cast mine degrades benthic integrity of Lake Nuasjärvi (Finland). Environmental Pollution 255 (2): 113268. doi:10.1016/j.envpol.2019.113268