3D Numerical Modeling of MICROCYSTIS Distribution in a Water Reservoir

A 3D computational fluid dynamics program was used to calculate the wind-induced accumulation of phytoplankton in Eglwys Nynydd, a water supply reservoir in Wales. The computational fluid dynamics model solved the Navier-Stokes equations for the water velocities using the SIMPLE method to calculate the pressure. Two turbulence models were tested: a zero-equation model and the k-(epsilon) model. An unstructured nonorthogonal 3D grid with hexahedral cells was used. The distribution of the bluegreen algae Microcystis was calculated by solving the transient convection-diffusion equation for phytoplankton concentration, based on the modeled flow field. The numerical model included algorithms for calculating the growth rate of phytoplankton and simulating the response of the algae to changes in underwater light intensity. The model was validated by comparing the horizontal distribution patterns produced by simulation with those recorded during a field survey of surface concentrations. The results demonstrated reasonable agreement, particularly when using the k-(epsilon) turbulence model. The main parameter affecting the results was the effective diameter of the Microcystis colonies.