I am formulating a model for the dynamics of phytoplankton in a stratified lake. Stratification separates the water with a horizontal plane called thermocline into two zones: epilimnion and hypolimnion. The epilimnion is the upper zone which is warm (lighter) and well mixed. The hypolimnion is the bottom colder zone. It is usually dark, and relatively undisturbed. Most deep lakes on Earth are stratified. Phytoplankton, also called algae, grow via photosynthesis in use of nutrients such as phosphorus and nitrogen from water and energy from sunlight. Because the hypolimnion is not well mixed, the change in the phytoplankton density and nutrient concentration in it depends on time and depth in the water column. Phytoplankton can be moved from their position by tur- bulent mixing (diffusion) or by sinking (advection). The change in the phytoplankton density in the epilimnion is independent of the depth (since it is well mixed over night). Light absorption in a lake follows Lambert-Beer’s Law. This novel model is the hybrid of highly interconnected nonlinear partial and ordinary differential equations. I will later extend the model to a bacteria-phytoplankton interaction model in a stratified lake.