Concentrations

One of the key questions asked by Lennart Lindeström when he set out for the compilation of all the monitoring data from the Falun area was: "Can plants and animals live in these waters under the prevailing environmental conditions ?" The question is pertinent because no other watercourse of any importance nor any other lake of the same size in Sweden exhibits such high concentrations of one or several metals as the Faluan River and Lake Runn.

Almost no plants and animals existed in the smaller Lake Tisken up to the end of the 1980s, when the mine water was first treated. The only specimens of higher life observed in the early 1980s were a few aggregations of rushes, sedges and water moss, as well as two species of chironomid midge larvae (Lindeström, 2003). However, after the mine water treatment came into operation (in 1987), new plants appeared in the lake. By 1996, large populations of several species of aquatic plants were recorded, especially the bulbous rush (Juncus bulbosus), but also, for example, yellow water lily and floating carpets of Sparganium gramineum. The mass occurrence of bulbous rush is interesting: at the turn of the millennium, this usually low-growing species with a bulbous, onion-like base, had completely taken over the lake habitat and covered the entire surface of Lake Tisken. The plant is unusually tolerant of acidic conditions and often occurs in mineralised areas, where - in the old days - ore prospectors utilized it -under the name of "ore grass" - as an indicator of the occurrence of lake ore. By the end of the 20th century, fish, such as pike, perch and roach, were also reported from Lake Tisken. The fish was much smaller than normal and had probably not been hatched in the lake, but migrated in from adjacent bodies of water.

The benthic fauna of Lake Runn was investigated in 1978, in 1982 and in 1996. On all these occasions, bottom-dwelling fauna was found, but at the first two investigations, the number of animals and their biomass were much smaller than normal for a lake of this nutritional status and size. During the 1996 survey, a total of 40 different taxa (species or genera) were found in Lake Runn. This number is somewhat lower than expected and may be due to the fact that animals are virtually absent at a depth of one metre, due to man-made fluctuation of the lake's surface. On deeper bottoms in Lake Runn, the number of individuals and the biomass were rather similar to what is found in other oligotrophic lakes in the region, see Table 4.15.

Table 4.15. Macroscopic benthic fauna in Lake Runn and similar oligotrophic lakes in the region investigated in 1996. Results are shown separately for shallow and deep bottoms (above and below the thermocline). After Lindestrom, 2003.

Lake

Max.

Phosphorus

N:o of

Individuals /m2

Biomass, g /m2

depth, m

fog/l)

taxa

shallow

deep

shallow

deep

Runn

27

9

40

1,000

420

700

860

Venjan

36

10

25

270

2,700

160

6,800

Siljan

130

5

53

1,200

280

1,800

330

Skattungen

48

6

43

790

230

490

320

Rogsjon

45

3

57

2,400

760

2,600

1,400

Individual density of benthic animals and biomass per unit area below the thermocline were lower in the large Lake Siljan than in Lake Runn. In Lake Siljan, however, there are sensitive crustaceans, so-called glacial relict species, which do not exist in Lake Runn. Nor were any mussles found in Lake Runn in 1996 at depths greater than two metres. It was also noted that a large portion of the oligochaete worms were abnormally small in size. Thus, the benthic animal community in Lake Runn is undoubtedly affected by one or more factors that do not have anything to do with the general nutrient status of the lake. Most probably, the presence of high concentrations of metals is the cause of this situation (Lindestrom, 2003).

The most obvious explanation is that one or several of the metals have a direct toxic action on the animals. However, it is also highly likely that the consistency of the bottom substrate (extremely "fluffy" due to the precipitates of iron) makes it impossible for many species to establish themselves on the deeper bottoms of the lake. Moreover, these iron compounds bind e.g. nutrients such as phosphorus, which would lead to a nutrient shortage for certain animals, a possible explanation to the small size of worms.

As far as the phytoplankton community is concerned, it was reported from a study in 1982 that the biomass of algae in central Lake Runn was slightly lower than could be expected in view of the nutrient status of the lake. The opposite was true during the investigations carried out in the 1990s, namely that the phytoplankton biomass in Lake Runn was higher than in other lakes in the region with equivalent nutrient status (Figure 4.3). Also to the contrary of the situation in 1982, no observations of deformations of other visible changes in algal cell morphology were made in the lake. The average number of algal species in August in the waters of Lake Runn was normal during the samplings 1990-2000, judged from the relationship between phosphorus status and number of species recorded in many lakes in the region. However, the total number of species (about 70 species) observed at any time of the year in the lake during the same period was slightly lower than expected. This has been interpreted as a possible sign of a genetical adaptation of the algal community to higher metal concentrations in Lake Runn (Lindestrom, 2003).

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