The influence of physical and physiological processes on the spatial heterogeneity of a Microcystis bloom in a stratified reservoir

•We explore the physical and physiological processes of a Microcystis bloom event.•ELCOM–CAEDYM is extended to include buoyancy control dynamics for cyanobacteria.•Thermal stability, phosphorus availability, and buoyancy control are identified as key factors.•Inclusion of the buoyancy control considerably improves the model predictability.•Higher surface concentration occurs when buoyancy control is taken into account. A three-dimensional coupled hydrodynamic and ecological model, ELCOM–CAEDYM, was extended to include buoyancy control dynamics for cyanobacteria, and validated in the stratified Daecheong Reservoir (Korea). Specifically, the model was used to explore the physical and biological processes that determined the temporal and spatial variability of Microcystis aeruginosa (hereafter Microcystis) biomass during an abnormally intense mono-specific bloom event. Inclusion of the buoyancy control function within the coupled model considerably improved the model predictability by capturing the biomass accumulation at the surface during the bloom, and the shift of the dominant group from green algae to cyanobacteria. Results indicated that physical processes, particularly inflow mixing, played a dominant role in determining the spatial heterogeneity of Microcystis biomass through the local control of nutrient availability. In addition, the shallow mixed layer depth (zm) relative to the euphotic depth (zp) under a stable thermal stratification provided a perfect physical habitat for the dominance of this cyanobacteria relative to other species, due to their buoyancy control capability. This work demonstrates that the coupled hydrodynamic and ecological modeling has advanced to a stage where it may be used to interpret field data and subject to a suitable level of validation, the model may be used as a management decision support tool.