Cost/benefit of dinitrogen fixation and the ecosystem consequences

• Applicants: Professor Dr. Ferdinand Hellweger; Professor Dr. Jan Köhler
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 529624036

There is currently debate whether mass developments of potentially harmful cyanobacteria can be prevented by reduced phosphorus inputs alone. Many lakes tend to be nitrogen-limited, but the costly reduction of N inputs only makes sense if it is not compensated by the fixation of atmospheric nitrogen (N2) by cyanobacteria (Nostocales) with specialized cells (heterocysts). Reduced N inputs should then even favor these cyanobacteria. However, a long-term study of IGB at the Berlin lake Müggelsee shows a reduction of the proportion of Nostocales in the total phytoplankton and a shift from Aphanizomenon to Anabaena species under reduced N inputs. We want to test the following hypotheses: 1) Energy-intensive N2 fixation is only worthwhile for Nostocales when the supply of nitrate and ammonium (=DIN) is very low. 2) Climate change increases the duration and frequency of thermal stratification of shallow lakes, resulting in the pulse-like release of dissolved P and N from the sediment. The strain-specific strategies of Nostocales in forming heterocysts and turning on N2 fixation depend on the level and dynamics of DIN supply. 3) These strain-specific traits explain the effect of reduced N inputs and climate change on the plankton community observed in the lake. Laboratory and field experiments, analysis of long-term data, and modeling will be combined to test these hypotheses. In the laboratory, growth rates and N2 fixation of four Aphanizomenon and Anabaena strains are measured as a function of DIN concentration, the latter being constant or pulsed. An Anabaena mutant unable to fix N2 will be studied for comparison. Changes in rates of growth or photosynthesis describe fitness costs or benefits of N2 fixation. Competition experiments are conducted to determine which strain prevails at which DIN concentration. Analogous experiments are conducted in Müggelsee with natural plankton communities. N2 fixation by Nostocales and N inputs by the Spree River will be compared. An existing Anabaena model will be updated by recent literature data and our findings from the described experiments and extended to include Aphanizomenon. This model will then be integrated into a biogeochemical model describing a model lake first in one dimension, and for higher temporal resolution also in three dimensions. The model is calibrated and validated using long-term data from Müggelsee. It will then be used to simulate scenarios with different DIN concentrations and the studied strains for relevant ranges of external P and N loads, water temperatures and stratification conditions. In addition, a Nostocales module will be developed that can be integrated into existing water quality models. This new knowledge can be used for more effective and sustainable lake management. Our models can be applied in predicting cyanobacterial development and setting qualified N input limits accordingly, even under changing climatic conditions.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Wolfgang R. Hess