Longitudinal patterns of stream chemistry in a catchment with forest dieback, Czech Republic

Longitudinal streamwater sampling in a 210-ha forested catchment (Jezeri), Czech Republic, was conducted approximately quarterly from August 1992 through August 1994. The catchment has been severely impacted by atmospheric deposition of pollutants and subsequent landscape manipulation for reforestation. The impact and landscape manipulations decrease with decreasing elevation. The concentration patterns reflect processes and dynamics that are not apparent from long-term monitoring at the basin outlet. Streamwater concentrations of many solutes are highly correlated with elevation, vegetation cover and landscape history. Concentrations of sulphate (SO42−), the dominant anion, increased downstream. Low SO42− concentrations at the highest elevations are attributed to a decrease in dry S deposition due to the loss of the forest canopy from Norway spruce dieback and subsequent logging. Furthermore, liming and higher moisture content and water movement through the soils in the headwaters increases S mobility, resulting in lower SO42− concentrations at higher elevations. In contrast, highest nitrate (NO3−) concentrations (300 μeq liter−1) occurred at the highest elevations, decreased downstream to 80 μeq liter−1 at the outlet, and the rate of change in concentration was the most pronounced in the headwaters. The NO3− pattern is attributed to increased nitrification of the forest floor due to landscape excavation for drainage and reforestation, and liming (dolomitic lime). The N demand by 10–25-year-old reforested vegetation at mid elevations (705–800 m above sea level) is much greater than in the very young regrowth and open areas at the highest elevations, causing concentrations to decrease through this zone. Streamwater NO3− concentrations also vary seasonally due to vegetation demand during the growing season; but high NO3− concentrations (>50 μeq liter−1) persist at all sites, indicating N saturation. High streamwater calcium (Ca2+) and magnesium (Mg2+) concentrations and low to moderate alkalinity (ALK) in the limed, deforested and reforested areas also occur, particularly during low runoff periods. ALK and hydrogen ion (H+), fluoride (F−) and dissolved aluminum (AlDIS) concentrations are highly correlated, particularly H+ and AlDIS. The low streamwater ALK (in 90% of the samples ALK is less than 50 μeq liter−1), moderately low pH (in 45% of the samples pH is less than 5.6) and the correlation with AlDIS suggests that the acid-base status is controlled by inorganic Al and organic complexes of Al and F.