Regional monitoring of lake acidification in Finland

Abstract

The primary objective of this monitoring is to detect long-term Long-Range Transboundary Air Pollution (LRTAP) induced changes in the water quality of small lakes, throughout Finland, with low conductivity. The monitored lakes (n=171, sampled every autumn since 1990 and in 1987) have a smaller watershed (usually headwater or seepage lakes), a larger lake/catchment ratio, and lower base cation concentrations, alkalinity and pH than Finnish lakes on average. The monitoring network provides background data for air pollution dose/response studies, critical load calculations and for the modelling of acidification scenarios. The declining sulphate deposition seems to be reflected in small headwater lakes all over southern and central Finland as a lowering of the sulphate concentration in the waters. Nitrate concentrations in these lakes have been typically low in the autumn. The base cation concentration is not generally declining, as it is in deposition in many areas. The sulphate concentration in lakes has declined more than base cations. Hydrologically, the recent years have been quite variable because of varying annual precipitation. The variation in alkalinity and pH in typical Finnish lakes is dependent on the content of humic material derived from catchments. The monitoring period is too short to reveal consistent trends in major ion chemistry. However, signs of improvement in recent years can be seen; in comparing 1993 to 1987, years with similar organic acidity and base cations, it seems that the sulphate decline in lakes monitored is compensated by a significant rise in both alkalinity and pH.