Table 1 The main time periods of Lake Kirkkojärvi ecosystem.

From: Multiple mining impacts induce widespread changes in ecosystem dynamics in a boreal lake

Period

Years

Water characteristics

Sediment chemistry

Cladocera

Diatoms

Interpretation

After the accident

2013–2015

Lake is meromictic. High salinity prevails. Fe and P are released from the sediment to hypolimnion.

Fe and P exhibit lower concentrations, also Ni and Zn decrease due to decreased pollution.

B. longirostris dominates.

D. tenuis dominates

Lake is clearly impacted by pollution

Pond accidents

2012–2013

Strong stratification. Dam accident derived Ni and Zn stay on top of chemocline.

Ni and Zn peak. Si peaks due to the pollution accident.

B. longirostris dominates. Richness and diversity are low.

D. tenuis increases

Lake is clearly impacted by pollution

Mining begins

2008–2012

Onset of meromixis and anoxia in deep water. Mine-related elements increase.

Zn, Ni, S and Na increase

Bosmina longirostris increases, diversity and richness decreases

Diatoma tenuis increases

Saline water and meromixis affects the system

Natural

1971–2008

Not permanently stratified, chemistry resembles natural conditions

K increases due to forestry

Daphnia dominates. Rich and diverse community

Tabellaria and Aulacoseira spp. dominate. Rich and diverse community

Most likely nearly natural boreal lake