Abstract

 During the last couple of years, a number of studies of pollutants and effects on the fish fauna have been concentrated on Lake Molnbyggen outside Leksand, Sweden. Interest has mainly been focused on leachate from the municipal landfill (Lindbodarna) which is located above the lake. The aim of the present study was to test the toxicity of sediment from Molnbyggen and some adjacent lakes, in order to see if this could explain the causes of the effects on the fish. The sampling sites were selected in co-operation with the project group at the Swedish EPA to make the results more easily comparable with those from other studies on the fish fauna in the area. Both acute and chronic tests were made with the crustacean Ceriodaphnia dubia, and the sediment samples were equilibrated with standard reference water prior to exposure. The exposures were made under standard conditions in the laboratory to make them more readily comparable between the sampling locations and with previous studies. The effects on survival and reproduction were monitored during eight days. After that a series of TIE (Toxicity Identification Evaluation) manipulations of the tested waters were made in order to identify the cause(s) of the toxicity. The results from the TIE tests pointed towards heavy metals as the cause of toxicity, and the analytical results of heavy metals in the water phase showed that the concentrations of six metals (Cd, Co, Mn, Ni, Pb and Zn) were significantly correlated with toxicity. The concentrations of these metals were also correlated with each other making it hard to separate cause and effect among the metals. However, the concentrations of Cd, Ni, Pb and Zn were below the expected effect concentrations, but the measured concentrations of particularly Co and Mn were high enough to be probable causes of the observed experimental toxicity. Therefore, the cause(s) of the high concentrations Co and Mn (their origins) should be investigated. There was no indication that lipophilic compounds should have caused these effects. Therefore, the result from this study was rather surprising, showing that heavy metals like Co and/or Mn which are not generally considered as environmental problems may be of environmental concern. Potential effects of Co and Mn could be mediated through the olfactory system, because both these metals have been observed to affect this system in fish. However, the link between such effects and those observed on reproduction in Lake Molnbyggen is vague.