GRADIENTS IN SEDIMENT CHEMISTRY AND ARCELLACEAN ASSEMBLAGES IN A MINE WATER-IMPACTED FRESHWATER BAY (PETKELLAHTI BAY, FINLAND)

Metals are naturally occurring substances and many of them are essential to life. The ecological impacts of metal inputs are, therefore, often difficult to assess, especially in areas of high background concentrations.

We employed analyses of sedimentary metals and arcellacean (thecamoebian) faunas to study the effects of metal-rich waters from a previous Cu (later talc) mine on a boreal lake bay. Arcellaceans are microscopic freshwater protozoans that have been shown to be sensitive environmental indicators. Sediment chemistry and arcellacean species compositions were analyzed from both pre-impact and impacted samples in a transect extending away from the source of pollution. In addition, two short cores were studied: one from the most impacted area, the other from a reference site.

Based on the results, there is a spatial geochemical gradient from the inner bay, where mine waters enter the lake, towards outer bay. The inner bay sites were enriched with C, S, and Fe, whereas Mn was depleted and precipitates in the outer bay. Of heavy metals, concentrations of Cu, Al, Zn, and Ni had increased in the inner and mid bay and those of Co and Cd in mid bay sites. The present faunal gradient in the bay differed from the natural state, suggesting that human impact has affected arcellaceans. Ordination and distance measures showed that arcellacean species compositions had changed from pre-disturbance assemblages particularly in the inner bay, with fairly small changes in the outer bay. Numerical methods suggested that metals (Cu, Pb, Al, Zn, Cr), redox-sensitive elements (Fe, Mn), as well as organic carbon and nutrients could be related to changes in arcellaceans.

Geochemical changes in the impacted core started at ~ 20 cm with increases in S alternating with peaks in Cr and Mg, derived from the Cr-rich chlorites in the area. Heavy metal concentrations increased markedly only after the mining period, presumably due to commencing of acid mine drainage. Arcellacean species compositions changed already at 20 cm but the most notable change coincided with this geochemical shift at 10 cm. Numerical methods suggest that mining-related metals Co, Cu, Zn and Ni co vary with arcellaceans and Al appears to have an independent signal in species data. Overall, mine waters have affected arcellaceans especially in the inner bay after the mining period.