Minor but nonetheless measurable changes were also expected in the case of geochemical parameters, as the presence of calcium, magnesium and bicarbonate occurs through the weathering of calcite and dolomite stone in river watersheds. In this process, CO2 is bonded and the bicarbonate (HCO3-) thus formed enters the bodies of water. Some CO2 outgases into the atmosphere or crystallises again into calcite. These basic processes in the geochemical carbon cycle take place in equilibrium, and are dependent on environmental conditions.
Bicarbonate concentrations have increased. One reason for this, Zobrist finds, is climate change: “The air temperature increase of c. 1.5°C makes microorganisms in the ground more active, and so they breathe more and give off more CO2”. In wet ground, CO2 is dissolved as carbonic acid. A higher concentration of carbonic acid means that more stone, above all carbonate-containing minerals, is subjected to weathering and the bicarbonate concentrations increase. This process can be quantified with the help of a classic CaCO3-CO2 equilibrium scheme. Downstream of lakes, the effect of reoligotrophication is seen: because the availability of the nutrient phosphorous sank in the period under investigation, less algae grew in the lakes, so less CO2 was used up in photosynthesis. When more CO2 remains dissolved in the water, less calcite is formed.
Opposing effects overlap
In the Thur near Andelfingen the bicarbonate concentration has shown an opposite trend over the last three decades. For one thing, there is no lake upstream of this river, and so the effect of reoligotrophication is not found here. On the other hand, there are developments that have led to a decrease in weathering, according to Zobrist: “The use of agricultural fertiliser that creates acid in the soil has decreased. Deposits of acid from the atmosphere also decreased – for example through the reduction of sulphur in heating oil and above all the decrease in SO2 emissions in the former Eastern Bloc countries.” These resulted in a decline in calcite weathering.
Long-term trends show that the geochemical carbon cycle is subject to changes and reacts to human influences. “The changes are minimal, but nevertheless statistically significant,” sums up Zobrist, who has now retired from research, having seen through this project.