His analyses demonstrate that the quantity of flowing water has scarcely changed, while the temperature has increased by 0.8 to 1.3°C. This increase has not, however, taken place in a gradual, linear fashion, but shot up in the second half of the 1980s – in the Rhone below Geneva the thermometer readings rose by 1.1°C, and in the Thur by 0.4°C. “Similar jumps in temperature were noted Europe-wide in bodies of water, ground water and in the soil”, adds Zobrist.

The concentrations and quantities of nitrogen increased markedly from 1982/83 to 1987/88. This is a somewhat delayed reflection of the great intensification of agriculture that caused the highest nitrogen excess at the beginning of the 1980s. That means that more manure and artificial fertilizers were applied than were taken up by the harvested plants. The nitrogen stored in the ground increased markedly and a corresponding quantity of nitrogen was washed into bodies of water. In addition, the unusual temperature increase in ground and water in the late 1980s led to an increase in biological mobilisation of the nitrogen.

Since the 1990s the total nitrogen load has decreased significantly – by up to 50%. “The change in agricultural fertilisation practices and improved wastewater treatment, especially the introduction of denitrification in several large wastewater-treatment plants, have made an impact,” comments Zobrist.

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.