Labile inorganic manganese — An overlooked reason for fish mortality in acidified streams?

Acid deposition has caused acid episodes and elevated concentrations of metals like Fe, Al and Mn in streams in mountain regions of Sweden. In streams in the area of Lofsdalen, central Sweden, an episode in 1978 during snow melt caused massive downstream migrations of fish. Acid episodes have occurred yearly since then. pH of 4–4.5 and high concentrations of total Fe, Mn and Al were measured during these spates. Liming measures have been carried out in a number of catchments in order to increase the pH and try to decrease the leaching of metals. The lime has been delivered on wetlands, seepage areas and areas covered by meltwater during the spring. Cage experiments were carried out with yearling brown trout in the springs of 1982, and 1985–86. Fe, Al and Mn were analysed in water samples and on/in the gills of dead and live brown trout. There were no significant correlations between mortality or average survival time and total concentrations of Fe, Al or Mn in the water or on/in the gills. Average survival time was positively correlated to pH (p< 0.01). Metal speciation was carried out in 15 experiments in 1985 and ?86. A canonical discriminant analysis showed that pH and the concentration of labile inorganic manganese in the water explained all the observed mortality (can. corr 0.89, p< 0.001). The rate of accumulation of manganese on/in the gills was correlated to the concentration of labile inorganic manganese in the water (p<0.01, r²=0.54, linear regression).