Poster Presentation Australian Society for Fish Biology & Australian Freshwater Sciences Society Conference 2026

CyanoIBM: An individual-based model explaining the autumn disappearance of deep chlorophyll maxima of the toxin-producing cyanobacterium Planktothrix rubescens in temperate lakes (#79)

Mohammad Hassan Ranjbar 1 , Ferdi L. Hellweger 2 , Jens C. Nejstgaard 3 , Stella A. Berger 3 , Hans-Peter Grossart 3 4 , Sabine Wollrab 3 , Charlotte Schampera 3 5 , Michele Burford 1 , Mark O. Gessner 1 3 6 , David Hamilton 1
  1. Australian Rivers Institute, Griffith University, Nathan, Queensland, Australia
  2. Water Quality Engineering, Berlin Institute of Technology (TU Berlin), Berlin, Germany
  3. Department of Plankton and Microbial Ecology, Leibniz Institute of Freshwater Ecology and Inland Fisheries, Stechlin, Brandenburg, Germany
  4. Institute of Biochemistry and Biology, Potsdam University, Potsdam, Brandenburg, Germany
  5. Federal Environment Agency, Berlin, Germany
  6. Department of Ecology, Berlin Institute of Technology (TU Berlin), Berlin, Germany

Interactions between cyanobacterial physiological processes and lake hydrodynamics are often inadequately represented in conventional models. These models fail to capture how prior environmental conditions influence cyanobacterial physiological responses. To address this limitation, we developed an individual-based model (CyanoIBM) to simulate the growth, mortality, buoyancy regulation, and vertical movement of the filamentous cyanobacterium Planktothrix rubescens in response to light, temperature, and lake hydrodynamics, including stratification and mixing. The buoyancy-regulation module of the model explicitly represented cell density changes driven by the filaments' prior exposure to light. Model calibration involved tuning the optimal photosynthetically active radiation (PAR) and maximum rate of filament density change using data collected during a large-scale, replicated (N=6) enclosure experiment conducted at the IGB LakeLab facility in Lake Stechlin, Germany. The calibrated model showed that P. rubescens regulated its buoyancy to maintain its preferred level of long-term light exposure. During summer stratification, when optimal PAR levels for P. rubescens occurred below the metalimnion, low vertical diffusivity enabled the population to form a persistent deep chlorophyll maximum (DCM). In autumn, however, as the surface mixed layer (SML) deepened, the optimal depth was within the SML. Therefore, positively buoyant filaments entrained into this layer were mixed throughout the SML and the DCM dissolved. Filament exposure to high light levels in the upper SML also caused photoinhibition, reducing the average light limitation term for the population by 58% between August and October, thereby reducing net population growth. Thus, the observed autumn decline in P. rubescens densities was likely driven by the combined effects of enhanced turbulent mixing, which dispersed the population, and reduced growth under unfavourable light conditions. Overall, this study demonstrates the potential of individual-based models to couple physical and physiological processes governing DCM-forming phytoplankton populations, providing a mechanistic framework for improving predictions of cyanobacterial dynamics under changing environmental conditions.