At the beginning of March, the journey started with a long flight across the Atlantic and then the Pacific Ocean. After arriving in New Zealand, I immediately joined the PhD students and technicians of the Coastal Marine Group in the laboratory to assemble and test the measurement instruments.
These instruments were part of a larger field campaign focused on understanding how rising atmospheric CO₂ concentrations affect freshwater systems. The Waikato River is particularly well suited for this research: at 415 km, it is the longest river in New Zealand. Located on the North Island, it flows from Mount Ruapehu through Lake Taupō before emptying into the Tasman Sea. Along its course are beautiful waterfalls, including the famous Huka Falls, as well as several impressive reservoirs.
For our measurements, we used an Eddy Covariance system. This system provides high-frequency measurements of the CO₂ exchange between the water surface and the atmosphere. It allows us to determine whether the river releases CO₂ into the atmosphere or absorbs it from the air. These measurements are important for improving our understanding of the role rivers play in the carbon cycle and their significance for the climate system.
After thoroughly testing the Eddy Covariance system in the laboratory, we installed the measurement station above the river, as close to the water surface as possible. However, the site could not be reached without getting wet. As a result, many of my workdays started in a wetsuit: wading through the river to reach the instruments, checking the equipment, and monitoring the measurements. With the sunny weather, it was a very enjoyable way to begin the day.
In addition to measuring the CO₂ exchange, we also recorded flow velocities and small-scale turbulence in the water. For this purpose, we used a drifter. The drifter was deployed from a boat and then followed the river current downstream together with the measurement instruments. Since turbulence in the water strongly influences the exchange of CO₂ between water and air, combining both measurement systems will provide valuable insights into the physical mechanisms controlling these exchange processes and help us develop improved physical descriptions of them.
After several intensive weeks of fieldwork at the river, laboratory work, and a lot of Real Fruit Ice Cream, it was time to say goodbye to this beautiful country and to the outstanding scientists I had the opportunity to meet there. However, the collaboration between Bonn and Hamilton is far from over. The next step will be the analysis of the collected data, a phase that may involve less ice cream, but promises exciting new insights into the Waikato River ecosystem.