Research Training Group 2530
RTG 2530Biota-mediated effects on Carbon cycling in Estuaries
Successful Pilot Field Experiment ConductedDeveloping a Modified Dilution Experiment to Investigate Factors Influencing Phytoplankton
21 August 2026, by RTG 2530
Photo: Ailis Koch
Ailis Koch, Luca Blümke and Stina Krings (from left).
On 29 and 30 July 2026, Luca Blümke and Stina Krings, doctoral researchers at RTG 2530, conducted a modified dilution experiment in Bunthaus on the Elbe. They were supported by Ailis Koch, a Master’s student in the Environmental Management programme at Kiel University. The experiment is designed to help us understand how zooplankton and viruses influence phytoplankton in natural water bodies. For the two doctoral researchers, who both started their doctoral projects this year, this first field experiment was also an opportunity to gain hands-on experience with the method and to test the experimental approach under natural conditions. Further experiments are planned for 2027, building on the experience and insights gained during this first trial.
Phytoplankton play a central role in global carbon cycle and ecosystems. These microscopic organisms, which float in water, carry out photosynthesis. Using light, they convert inorganic CO₂ into organic compounds such as sugars, and release oxygen (O₂) in the process. As a result, phytoplankton not only form the foundation of aquatic food webs but also make a major contribution to oxygen production and carbon sequestration.
The factors influencing phytoplankton are one of the key research areas in RTG 2530. Zooplankton are small aquatic animals, such as crustaceans and fish larvae and they feed, among other things, on phytoplankton. As a result, they occupy a higher position in the food chain. This is also known as top-down control. Viruses also affect phytoplankton, although this is a much less studied area.
UHH/RTG2530/Krings
“While the influence of zooplankton on phytoplankton has already been comparatively well studied, the role of viruses in natural freshwater and brackish-water ecosystems has so far been much less researched. A better understanding of these interactions helps us to better predict the dynamics of phytoplankton communities and their response to environmental changes in the Elbe estuary and to assess these responses in an informed way,” explains Stina Krings.
To further investigate these relationships, additional field experiments are planned for next year as part of RTG 2530. These will be carried out systematically during different seasons and at different locations along the Elbe, taking into account different environmental conditions.
UHH/RTG2530/Krings
“The aim of the pilot experiment was to gain practical experience with the methodology and experimental procedures under field conditions. The insights obtained provide an important basis for further optimising the experimental procedures and workflows for future experiments,” says Stina Krings. In particular, the team was able to develop valuable practical knowledge of time management and the coordination of the different workflows.
The steps taken by Luca Blümke and Stina Krings in their initial experiment are outlined briefly and simply here:
Step 1
Collection of Elbe water
20 litres of water are collected from the Elbe.
Step 2
Filtration
The water is passed through a 200 µm mesh to remove larger zooplankton organisms (mesozooplankton), such as small crustaceans and water fleas.
Step 3
Separation
The filtered water is divided into three parts:
- Part 1: Larger zooplankton organisms (mesozooplankton) are removed.
- Part 2: Phytoplankton are removed using a 0.45 µm filter.
- Part 3: Viruses are reduced using 100 kDa filter cartridges (fine filtration).
Step 4
Dilution
A series of dilutions is prepared from the three differently filtered water samples.
Step 5
Return to the river
The diluted samples are placed in dialysis bags, attached to steel cages and suspended in the Elbe for 24 hours. The bags allow water and nutrients to pass through while keeping the organisms inside.
Step 6
Measurements
After 24 hours, the concentration of phytoplankton and other parameters, such as zooplankton, viruses and chlorophyll, are measured in the bags. Comparing the results allows the researchers to estimate how strongly zooplankton and viruses control phytoplankton.
