PH-levels in intensively drained and peatland-dominated river basin: Paleolimnological approach to detect impacts of past land use

Abstract

Peatland drainage can have major ecological and biogeochemical impacts on aquatic environments in boreal regions. In the Sanginjoki river catchment in North Ostrobothnia, Finland, peatland drainage is suspected to increase the acidity of recipient waters. Possible reasons are acid sulphate soils (ASS) or black schist deposits, which occur especially in lower parts of the river basin, or increased transport of humic substances. It is not known how long acidification has affected water quality and biota in the river. To study historical pH levels, we examined sediment cores from two ponds in the main channel of the Sanginjoki river and one small lake in the Sanginjoki catchment. Diatom and geochemical analysis and 137Cs-derived chronology were combined to estimate historical pH levels and changes in land use practices. The results suggest that changes in land use in the 1970s have affected runoff acidity. During that period, there was a change in diatom communities and sediment geochemistry, indicating that land use in the catchment area, most likely peatland drainage, had probably changed suspended sediment fluxes and pH conditions. The results also showed that changes in pH levels probably did not originate from ASS or black schist horizons, but rather mainly from humic substances leached from the catchment area. However, ASS and black schist deposits may have had an influence on pH levels in lower parts of the Sanginjoki river during dry/wet periods. Thus, the findings suggest that future changes in land use and land management are likely to cause increased acidity in the Sanginjoki catchment. Therefore actual or potential acidification of river water should be included in risk assessments and future land use practices, and should be avoided by not changing natural humic-rich ecosystems.