Introduction
Cyanobacteria, often referred to as blue-green algae, can significantly degrade water quality and pose public health risks when occurring in high concentrations. Cyanobacteria are known to migrate vertically in response to environmental conditions, especially light. Capturing this behavior in water quality models helps improve predictions of surface accumulations and overall bloom dynamics.
In a previous blog, Modeling of Cyanobacteria Vertical Migration, we explored several vertical migration options available in EEMS, including Constant Velocity, Daily Cycle Velocity, Daily Cycle Velocity with Depth-varying Light Effects, and Dynamic Velocity, using a two-dimensional (2D) setup. In this blog, we extend that work to a fully three-dimensional (3D) model, focusing on a comparison between the Constant Velocity and Dynamic Velocity approaches.
Model Description and Scenarios
A 3D model was developed for Lake Mendota in Madison, Wisconsin. Details on the model setup and calibration can be found in this EEMS blog. With a grid of 1,405 horizontal cells and 30 vertical Sigma-Zed layers (Figures 1 and 2), the model can produce a detailed representation of water quality processes both horizontally and vertically.
In this study, two model scenarios of cyanobacteria migration were constructed. The first scenario applies a Constant Velocity scheme, in which cyanobacteria move vertically at a fixed rate of 0.5 m/day. The second scenario uses the Dynamic Velocity scheme, which allows vertical movement to vary in response to light. For this option, we used the default parameter values from Visser et al. (1997) that are readily available within EEMS.
Results
The cyanobacteria bloom is analyzed for the 2020 blooming season (June-August). Animation 1 compares the vertical profiles of cyanobacteria concentration in the two scenarios between 2 and 5 July 2020. The Constant Velocity scheme results in an evenly vertical distribution, with cyanobacteria spreading across the depths above approximately 10 m, which is the thermocline depth. The Dynamic Velocity scheme gives a sharply stratified vertical structure, where cyanobacteria tend to accumulate in thin layers at depths that change over time as cells migrate between the surface and the thermocline. This behavior results in pronounced fluctuations in surface concentrations (Figure 3).
Animation 2 shows the 2D horizontal distribution of cyanobacteria concentration at 3 m depth between 2 and 5 July 2020. Under the Constant Velocity scheme, cyanobacteria are distributed more uniformly across the lake, while the Dynamic Velocity scheme produces pronounced spatial heterogeneity, highlighting areas prone to high concentrations and pronounced sensitivity to harmful algal blooms.
Animation 3 presents a 2D vertical cross-section of cyanobacteria concentrations, supporting the vertical profile analysis. As expected, the Constant Velocity scheme produces a broad and diffuse distribution in the upper water column, whereas the Dynamic Velocity scheme generates thin, highly concentrated layers near the surface. Peak concentrations in the Dynamic Velocity scenario exceed 200 ug/L, which may be unrealistic for Lake Mendota and highlight the need for careful model validation when this option is used.
References
Visser, P. M., Passarge, J., & Mur, L. R. (1997). Modelling vertical migration of the cyanobacterium Microcystis. In Hydrobiologia (Vol. 349). Kluwer Academic Publishers.
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